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Catecholaminergic regulation of ovarian function in mammals: Current concepts
Life Sciences, Vol. 39, pp. 1701-1711 Printed in the U.S.A.
Pergamon Journals
MINIREVIEW CATECHOLAMINERGIC REGULATION OF OVARIAN FUNCTION IN MAMMALS: CURRENT CONCEPTS Leon J. Spicer D i v i s i o n of Endocrinology The Milton S. Hershey Medical Center The Pennsylvania State University Hershey, PA 17033
Summar~c A review of the rapidly accumulating data in the l i t e r a t u r e continues to support the notion that catecholamines regulate ovarian function, and extends the complexity of catecholaminergic effects on the ovary via interactions with pituitary and adrenal hormones. I t is clear that catecholamines affect growth and differentiation of ovarian f o | l i c l e s , but their role in f o l l i c u l a r rupture during ovulation and in corpus luteum function ranains unclear. The effects of catecholamines (mediated by membrane receptors) on ovarian function probably should be considered paracrine but classic endocrine regulation of ovarian function cannot be ruled out. Myogenic tonus of ovarian vasculature appears to be regulated by catecholamines, and estrogens may enhance adrenergic receptors in ovarian ~nooth muscle ceils. Since the early studies of Hill (1), i t has been generally accepted that the autonomic neural connections of the ovary play some role in f o l l i c u l a r growth and ovulation. However, tremendous ambiguities s t i l l exist regarding the local neural mechanisms that regulate ovarian function. This review summarizes important features of local adrenergic innervation and function within the mammalian ovary, u t i l i z i n g several current findings. Anatomical Considerations Information on the anatomy of the autonomic innervation of ovaries has been obtained in a variety of mammals, including rodents, primates and ungulates (reviewed in 2-6). Briefly, i t has been shown that adrenergic (sympathetic) nerves are more abundant than cholinergic nerves (parasympathetic) and that nerve fibers enter the ovary with: I) the vasculature via the ovarian plexus along the ovarian artery, and 2) the nerve in the suspensory ligament (superior ovarian nerve) (7). Within the ovary, catecholamine-containing nerve terminals surround blood vessels of the smooth muscle layers of the theca externa. The membrana granulosa is avascu]ar and c~npletely devoid of nerve f i b e r s . Studies u t i l i z i n g sensitive histochemical techniques further demonstrated that, in all species examined, adrenergic nerves richly supply the theca externa of f o l l i c l e s but few neurons are present in corpora lutea and theca interna (3,5,8). I t has also been reported that the steroidogenically active i n t e r s t i t i a l cells of hamster, rat and guinea pig ovaries are directly innervated by adrenergic nerve
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fibers (3,9,10). I n t e r s t i t i a l ceils are heterogeneous in regard to their history and position within the ovary and, thus, are classified into four classes (11). Their collective and principal function is to synthesize and secrete androgens (11). Thus, adrenergic nerves communicate directly with ovarian i n t e r s t i t i a l cells and with ovarian f o l l i c l e s via the theca externa. Moreover, the neurohormone released by these adrenergic nerves, predominantly norepinephrine, diffuses through the basement membrane into the f o l l i c u l a r antrum and, thus, also may ccxnmunicate with granulosa cells (see later section and Fig. I ) . Functional nerve terminals d i r e c t l y on luteal cells have not been observed in any species to date and the few adrenergic nerves present in corpora lutea are pred~ninantly perivascular (2-6,8). Experimental Manipulations In Vivo Effects of severing the ovarian nerve supply on ovarian function have been reviewed previously (2-6,12). B r i e f l y , i t has been found that loss of ovarian innervation via surgical sympathectomy does not appear to affect estrous c y c l i c i t y in rats or baboons but does in mice (2-4). In rabbits, ovulation rates (induced by hCG) were not affected by ovarian denervation (2,4). Differences among species in the density of ovarian nerves may p a r t i a l l y explain differences reported in the response to denervation (3,4). However, in many of the experiments previously reviewed i t was not demonstrated that all nerves (both adrenergic and cholinergic) were absent after manipulation and(or) that normal ovulatory c y c l i c i t y occurred in the absence of nerves. With these points in mind, a review of the recently published experiments w i l l be presented. Rats. Superior ovarian nerve transection in prepubertal rats by electrocautery had no effect on the onset of puberty (13). In addition, transection of the ovarian suspensory ligament had no effect on ovulation rates in immature rats treated with pregnant mare serum gonadotropin (PMSG) (14). Similarly, denervation by freezin~ the ovarian nerve supply had no effect on ovulation rates in cyclic rats (15). In contrast, abdominal vagotomy in rats has had variable effects (both increases and decreases) on numbers of both healthy and atretic ovarian f o l l i c l e s depending on the period that has elapsed from vagotomy (16). No significant effect of vagotomy on serum estradiol or testosterone was observed in this study (16). Again, complete denervation was not verified in these studies. R a t s given PMSG treatments followed by injections of norepinephrine increased ovulation rates, whereas phentolamine (an ~-adrenergic antagonist) decreased ovulation rates (17). The f a i l u r e of some researchers to find alterations in ovarian function after denervation may be explained, in part, by the recent observation that, after transection of the superior ovarian nerve in rats, an increase in numbers of ovarian B-adrenergic receptors occurs coincident with a decrease in ovarian norepinephrine levels (13). No effect on ovulation rates was observed in this study (13). Thus, in vivo responsiveness to circulating catecholamines ~nay actually increase after ovarian denervation. Several studies have investigated the role of adrenergic neurons on ovarian steroid secretion in vivo. Ovarian mesenteric nerve transection blocked the increase in plasma estradiol concentrations induced by electrical stimulation of the medial basal prechiasmatic area of proestrous rats (18). Plasma progesterone levels were unaltered by ovarian denervation in this study as ~ l l as were average number of ova recovered in oviducts (18). In contrast, ovarian progesterone content was s i g n i f i c a n t l y reduced following electrical stimulation of the superior ovarian nerve in diestrous rats (19). More recently, ablation of the superior ovarian nerve during proestrus in rats resulted in an immediate and significant decrease in both progesterone and estradiol secretion (ng/min) which was maintained for 10 to 20 rain after nerve section (20). Blood flow (see
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later section) was also significantly reduced for a 4-min period after nerve section (20). Collectively, secretion of progesterone and estradiol by non-luteal ovaries during proestrus in rats appears to require an intact ovarian nerve supply. The development of c(~npensatory ovarian hypertrophy in rats also has been shown to require intact adrenergic nerves within the ovary (21,22). Rabbits. In pseudopregnant hypophysectomized rabbits, propranolol injected s.c. every hour for a 24-h period had no effect on concentrations of progesterone in serum (23). Days 7 to 8, 10 to 11 and 13 to 14 of pseudopregnancy were evaluted in this study. Similarly, s.c. propranolol pellets applied during days 13 to 17 of pseudopregnancy in hypophysectomized rabbits had no effect on the timing of luteal regression (23). Effectiveness and functional blockage of luteal (3-adrenergic receptors by propranolol was indirectly verified in these studies. These researchers concluded that endogenous catecholamines were not acutely involved in regulating luteal progesterone secretion or luteal regression in vivo (23). Whether this is true for other species rBnains to be determined. Guinea pigs. Previous experiments in which 6-hydroxydopamine (6-OH-D, a congener of norepinephrine that causes selective destruction of adrenergic nerve endings) was injected into animals systemically and ovarian function subsequently evaluated have been criticized for indirect effects that may have caused decreased ovarian function observed (5). In attempts to overcome this c r i t i cism, researchers have injected 6-OH-D into surgically closed ovarian bursae (24,25). These researchers found that after 10 days of 6-OH-D exposure, the number of large (>700 ~m) antral f o l l i c l e s (but not small f o l l i c l e s ) s i g n i f i cantly decreased in cyclic guinea pigs (24). In contrast, the number of preantra] f o l l i c l e s decreased and the number of large (>700 ~m) antral f o l l i c l e s increased after an 8-day exposure of 6-OH-D in prepubertal guinea pigs (25). This effect in prepubertal guinea pigs was overridden with gonadotropin injections (25). Collectively, these recent studies provide further in vivo support to the hypothesis that adrenergic nerves play a role in ovarian f o l l i c u l a r growth, and that this role may be selective to a specific type or size of the f o ] l i c u l a r population. Physiological Considerations Ovarian Levels of Catecholamines. I f levels of catecholamines within the ovary (0.2 to 2.0 ~g norepinephrine/g ovary for most species, reviewed in 5) were found to change with physiological states, more credence to the hypothesis that nerves are involved in ovarian functions would exist. Indeed, ovarian norepinephrine levels significantly decreased after PMSG injections into immature rats (26) and within 4 h following the preovulatory gonadotropin surge in normal cyclic rats (27). The depletion of f o l l i c u l a r norepinephrine appears to be due to increased FSH rather than increased LH or PRL (27), and is temporally associated f i r s t with increases in serum estradioI and then increases in serum progesterone (26). From these results i t was suggested that depletion of f o l l i c u l a r norepinephrine may be involved with formation of corpora lutea in rats (27). More recently, i t was observed that ovarian levels of norepinephrine (ng/ovary) increased 2- to lO-fold with sexual maturation in rats (28,29) and guinea pigs (30). Innervation of blood vessels within ovarian stromata also increases with age in guinea pigs, and thus may account, in part, for the increase in ovarian levels of catecholamines observed during puberty (30,31). In cyclic rats, concentrations of norepinephrine in ovaries (pg/mg) increase s l i g h t l y at proestrus and are associated with a significant decrease in ovarian norepinephrine turnover (32). Similarly, a 40% increase in ovarian content of norepinephrine (ng/ovary) observed 48 h after PMSG-treatment of immature rats was associated with an 88% decrease in ovarian norepinephrine turnover (29).
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Concentrations of norepinephrine (I-7 ng/m]; 6-41 nM) and epinephrine (.2-.9 ng/mI, i-5 riM) have also been measured in porcine f o l l i c u l a r fluid (33-35). Sinai| (1-3 mm) f o l l i c l e s contained greater concentrations of norepinephrine than medium f o l l i c l e s (33,35). However, significantly greater levels of both norepinephrine and epinephrine (as well as estradiol) were found in fluid of large preovulatory f o l l i c l e s (>6 mm) as compared to medium-sized (2-5 mm) f o l l i c l e s (33,35). In contrast, concentrations of norepinephrine and epinephrine in peripheral serum did not significantly change with stage of the estrous cycle in pigs (34). Data available in humans shows that during the menstrual cycle, norepinephrine levels in peripheral plasma are significantly greater during the luteal than f o l l i c u l a r phase (36,37). In addition, concentrations of norepinephrine and estradiol in plasma were significantly correlated (r=O.40, 36). Collectively, these data suggest that local accumulation of catecholamines occurs within follicles as they mature toward ovulation, and that this increase in catecholamines occurs concurrent with an increase in estradiol. Whether the amounts of catecholamines localized within the ovary are due to local neuronal release of catecholamines or accumulation of circulating catecholamines re~ains unclear. Ovarian Levels o f AdrenerBic Receptors. Binding kinetics of B-adrenergic receptors in granulosa cells was f i r s t reported in 1981 (38). Aguado et al. (39) later showed that adrenergic receptors present in rat granulosa cells were predominantly of the B2-type and that B-receptor content of granulosa cells increased with the onset of puberty. Ovarian B-adrenergic receptors in rats are highest on the day of proestrus and lowest on the day of estrus (ovulation) as compared with metestrus or diestrus (39,40). In rats, concentrations of estradiol in serum peak at mid-day of proestrus and then decline by early estrus (41,42). The LH/FSH surges occur between late proestrus and early estrus (41,42). Thus, in vivo, ovarian content of catecholamines and their receptors increase at proes-trus in rats, a time when ovarian levels of estradiol are highest (12,28). B-adrenergic receptors also have been found in various membrane preparations of corpora lutea from rats (43-45), rabbits (46) and pigs (47). Collectively, the stereospecificity of adrenergic receptors in granulosa cells are predominantly B2-type irregardless of species. It is less certain which subtype of receptor is present in corpora lutea. Adrenergic receptors in corpora lutea appear to be pred~ninantly B2-type in rats (43,45), B1.-type in rabbits (46), and nearly equal proportions of 81- and ;32-types in pigs (47). The physiological significance of these differences in stereospecificity is unknown. Moreover, i t should be pointed out that in the studies mentioned above, preparations of granulosa cell membranes used in the binding assays were essentially homogeneous, whereas membrane preparations from corpora lutea contained membranes from ]uteal cells and vascular cells (e.g., smooth muscle cells). Thus, varying anounts of contaminating vascular cells in preparations of luteal membranes anong the studies cited may be clouding the in'cerpretation of such studies. Experimental Manipulations In Vitro Effects of adrenergic agents on cultured granulosa and luteal cells have been reviewed previously (6,12). Briefly, B-adrenergic agents (preferentially B2-type) have been found to be stimulatory to production of 3',5'cyclic adenosine monophosphate (cAMP) and progesterone in f o l l i c u l a r tissue of rats and pigs (12,13,47), and lutea] tissue of rats, rabbits, sheep, pigs and cows (12,47,48). I t has also been shown that prostaglandin F2~ can inhibit tile stimulatory effect of epinephrine on progesterone production by 3- and 5-day old corpora lutea but not newly formed corpora lutea (l-day-old) isolated from rats (49). Estrogen biosynthesis by cultured granulosa cells does not appear to be affected by catecholami nes (12). Recently, catecholamine- stimulated progesterone
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production and(or) adenylate cyclase a c t i v i t y in luteal tissue of monkeys (50) and humans (51) was reported. Regulation of the B2-adrenergic response system has been f a i r l y well characterized using granuI osa ce] Is fro(n estrogen-primed, hypophysectomized rats (12). Briefly, cultured granulosa cells not exposed to f o l l i c l e stimulating hormone (FSH) do not respond to catecholamines, whereas FSH pretrea~nent dranat i c a l l y increases their responsiveness to catecholamines (12,13,52). How FSH e~lhances catecholamine responsiveness is unclear but does not appear to involve an increase in B-adrenergic receptors (52). Epinephrine and norepinephrine are equally potent in stimulating progesterone production after FSH pretreatment (53). Moreover, the stimulatory effect of adrenergic agents on progesterone production by FSH-pri,ned granulosa cells appears to be due to a stimulation of 3B-hydroxysteroid dehydrogenase (3B-HSD) and an i n h i b i t i o n of 20~-hydroxysteroid dehydrogenase (20~-HSD) (54,55). In vivo evidence u t i l i z i n g intact immature rats suggests that adrenergic responsiveness of granulosa cells in v i t r o is demonstrable only i f rats are exposed to luteinizing hormone (LH) in vivo (56). However, LH (2-100 ng/ml) given in v i t r o had no effect on B-adrenergic-stimulated progesterone production by cultu--red granulosa cells collected from intact immature rats (52). Whereas, human chorionic gonadotropin (hCG; 100 ng/ml) given in vi t ro enhanced epinephrine responsiveness of FSH-primed granulosa cells from hypophysectomized rats (53). More work is needed to c l a r i f y these discrepancies. At present, effects of estradiol on catecholamine responsiveness have not been carefully evaluated in cultured murine or porcine granulosa cells. Recent studies u t i l i z i n g granulosa cells from intact immature rats further characterized hormonal regulation of the B-adrenergic response system (52). Aguado and Ojeda (52) found that, in addition to FSH, corticosterone (200 and 400 ng/ml) and prolactin (PRL, 400 ng/m]) when added with B-adrenergic agonists increased progesterone production (but not cAMP production) above that of any single hormone treat~ent. Furthermore, the FSH-, PRL- and corticosteroneenhanced responsivities were not associated with increases in numbers of B-adrenergic receptors (52). Thus, the f a c i l i t a t o r y effect of these hormones appears to be exerted at a step distal to cAMP generation and may be due to direct stimulation of 3B-HSD and i n h i b i t i o n of 20~-HS0 as previously mentioned (54,55). In contrast, other recent data showed that somatomedin-C dra,natically enhanced B-adrenergic-stimulated progesterone and cAMP production by granulosa ceils pretreated with FSH in v i t r o collected from hypophysectomized, estrogen-treated, immature rats (57). In cultured porcine granulosa cells, epinephrine and norepinephrine are also equipotent in stimulating ornithine decarboxylase (ODC) a c t i v i t y and progesterone production (33,58). Moreover, epinephrine-stimulated ODC a c t i v i t y from small (I-2 mm) immature f o l l i c l e s and large (>6 mm) mature f o l l i c l e s is equivalent (59). Although epinephrine stimulates cAMP production by porcine granulosa cells (47), Ca+2 (60) as well as insulin (61) dramatically enhance epinephrine-stimulated progesterone production by processes distal to, or independent of cAMP generation (60). Although effects of LH and FSH on catecholamine responsiveness have not been thoroughly evaluated in cultured porcine granulosa c e l l s , i t has been shown that LH plus epinephrine is additive to ODC stimulation, whereas FSH plus epinephrine is not (33). More work needs to be conducted to evaluate gonadotropin interactions with catecholamines in porcine granulosa c el ls . Recent preliminary evidence indicates that the estrogen metabolite, 2-hydroxyestradiol, dramatical ly enhances progesterone production stimulated by epinephrine and other B-adrenergic agonists in cultured porcine (62) and rat (63) granulosa cells. These new findings suggest that catecholestrogens and
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Recently, i t was reported that catecholamines directly affected steroidogenesis of thecal cells (64). Specifically, 13-adrenergic agents had no effect on basal steroidogenesis but enhanced hCG-stimulated androgen production in cultured rat thecal cells (61). Norepinephrine and epinephrine were equipotent in stimulating thecal androgen production (64). S i n c e functional adrenergic synapses have been demonstrated in the steroidogenic i n t e r s t i t i a l and thecal cells (3), these results provide new, important infon~ation with regard to catecholamine action within the ovary. T h a t is, catecholamines released by nerve tenninals within the theca externa/interna can directly stimulate androgen production by thecal cells which, after diffusion into the membrana granulosa, may be aromatized (Fig. I ) . Thus, effects of catecholamines on f o l l i c u l a r function in vivo may be mediated, in part, by indirect effects of increased estrogen production. Further research is needed to elucidate such possibilities.
THECA EXTERNA
~
THECA INTERNA I / /
//
Androgens
~
-
GRANULOSA
~'~
F F
,;
FIG. 1 Schematic representation of proposed interrelationships between catecholamines and other trophic hormones of mammalian ovarian f o l l i c l e s . Nerve terminals surrounding the smooth muscle cells of blood vessels (BY) and steroidogenic cells of the theca release norepinephrine (NOREPI). Epinephrine (EPI) enters the theca interna via the blood supply. Both NOREPI and EPI enhances LH-stimulated androgen production by thecal cells. NOREPI and EPI also diffuses through the basement ,nembrane (BM) and into the f o l l i c ular fluid (FF)-containing antrum where both catecholamines enhance PRL-, FSH- and perhaps LH-stimulated progesterone production by granulosa cells (GC). Estradiol production by GC is not directly stimulated by NOREPI and EPI Dut is stimulated indirectly by increased thecal androgen production. The mechanism whereby gonadotropins plus catecholamines (versus either alone) enhance steroidogenesis rBnains unclear.
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I t should be ~nphasized that in vitro cell cultures do not take into account any potential effects catecholamines may nave on the f o l l i c u l a r vasculature and microcirculation. Moreover, cultures of isolated granulosa cells do not take into account major interactions between thecal and granulosa cells nor the potential f o l l i c l e - t o - f o l l i c l e interactions. To elilninate some of these c r i t i cis,ns, researchers have developed in vitro perfusions of whole ovaries. Recent studies u t i l i z i n g perfused rabbit ovaries have shown that both a- and B-adrenergic agonists can significantly increase the number of gonadotropin-induced ovulations (65,66). An increase in ovulation rates by B-adrenergic agents (delivered into the ovarian bursa) also has been observed in vivo in gonadotropin-primed immature rats (17). Schmidt et al. (66) speculated that a-receptor-stimulated events were mediated via some physical inechanism, whereas g-receptor-stimulated events were ~nediated via some endocrine mechanis~n. Evidence for the role of a-receptors in ovarian function is presented in the next section of this review. Catecholamines and Mechanical Processes within the Ovary Steroidogenesis versus blood flow regulation. Although i t is clear fr~n the in vitro studies cited in previous sections of this review that adrenergic agents have direct stimulatory effects on thecal and granulosa cell steroidogenesis, i t is unclear what proportion of adrenergic effects in vivo are mediated by some indirect mechanism on the ovarian vasculature. The tone of the ovarian vasculature is regulated via the smooth ~,luscles surrounding bloed vessels and has been reviewed previously (4,67). Two possibilities exist with regard to adrenergic regulation of mechanical processes within the ovary: i) ovarian blood flow (afferent and efferent) during any given physiological state; and 2) f o l l i c u l a r wall contraction or relaxation during the ovulatory process (see next section). A recent study has reported both acute stimulatory effects of norepinephrine on vascular resistance and inhibitory effects on blood flow of corpora lutea in pseudopregnant rats (68). Norepinephrine also has been shown to increase blood flow and(or) vascular tonus of whole ovaries fr~n non-luteal rabbits (69), pregnant guinea pigs (70), and luteal and estrous rats (71) in vivo. Blood flow to non-luteal rat ovaries has also been tBnporarily reduced following transection of the superior ovarian nerve (20). Secretion (rig/rain) of b o t h progesterone and estradiol was also reduced after ovarian nerve transection in this same study. However, steroid secretion remained depressed 10 to 20 min after blood flow returned to control levels (20). These results give support to the notion that catechola~ninergic stimulation of steroidogenesis is not canpletely mediated by alterations in ovarian blood flow. Adrenergic agonists can also increase vascular resistance in whole ovaries in vitro (72) and isolated ovarian arteries in vitro (73). Hore recently, ovaries (perfused in vitro) fr~n luteal-phase g i l t s exhibited significantly greater vasoconstriction than did ovaries fr~n f o l l i c u lar-phase and pregnant g i l t s in response to electrical stimulation (74). Norepinephrine caused a significant increase in vascular contractility to electrical stimulation for all ovaries in this study (74). Moreover, ovarian vascular c o n t r a c t i l i t y was negatively correlated (r=-.99) with the estrogen to progesterone ratio in systemic blood during the estrous cycle in g i l t s (74). Thus, adrenergic receptors in ovarian vasculature may be hormonal ly regulated as suggested for uterine vasculature in pigs (75) and mesenteric vasculature in rats (76). Density of B-adrenergic receptors in hypothalami and anterior p i t u i t a r y glands also seems to be regulated by ovarian steroids (77). Thus, further studies are needed to determine i f steroids ( i . e . , estradiol and progesterone) regulate ovarian adrenergic receptors as well.
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Role of catecholamines in the ovulatory process. In addition to the potent i a l stimulatory effects of catecholamines on f o l l i c u l a r ceil differentiation that occurs during the preovulatory period, catecholamines may also play a role in f o l l i c u l a r wall contraction/relaxation during the ovulatory process. Potent i a l interactions between catecholamines and the documented hormonal control of biochemical processes associated with f o l l i c u l a r rupture (e.g., gonadotropinstimulated plasminogen activator, see ref. 67) has not been studied. Researchers have concentrated on the role of adrenergic agents in f o l l i c u l a r wall contraction u t i l i z i n g strips of bovine or human Graafian f o l l i c l e s (78-80). Briefly, norepinephrine was slightly more potent than epinephrine in stimulating contraction whereas epinephrine was slightly more potent than norepinephrine in stimulating dilation. With this and other evidence, an involvement of both mand B-adrenergic receptors in the contractility/relaxation process was suggested (78-80). For a better understanding of the pharmacology of ~- and B-adrenergic receptors, the reader is referred to several recent reviews (81-85). Recent studies have evaluated the interaction of prostaglandins (PG) on norepinephrine-induced contractions of bovine f o l l i c u l a r walls (86). Although PGF2: contracted f o l l i c u l a r wails, i t had no effect on norepinephrineinduced contractions (86). In contrast, PGEI and PGE2 dranatically reduced tile contractile response induced by norepinephrine (86). Thus, prostaglandins and catecholamines may interact with smooth ,mscle cells of Graafian f o l l i c l e s during the ovulatory process. Collectively, these studies support the concept that myogenic tonus of the ovarian vasculature may be regulated by catecholamines and that estrogens may enhance adrenergic receptors in mnooth muscle cells. The stereospecificity of these adrenergic effects is unclear at present, but i t does appear that both mand B-type receptors are involved in regulation of ovarian myogenic tonus. Conclusions Concentrations of catecholamines and their receptors within the ovary changed significantly with stage of an estrous cycle and during sexual maturation in all mammals examined to date. The major catecholamines, norepinephrine and epinephrine, are found in nanomolar concentrations within mammalian ovaries. In v i t r o , these catecholamines are stimulatory to gonadotropin-induced steroidogenesis in granulosa and thecal ceils. However, the mechanism by which catecholamines and gonadotropins interact to enhance granulosa and thecal steroidogenesis rBnains unclear. In vivo, catecholamines can alter f o l l i c u l a r growth, ovulation and ovarian Jnyogenic tonus. Potential interactions between adrenergic stimulation of f o l l i c u l a r steroidogenesis and ovarian blood flow have not been evaluated. Future research should also address the role catecholamines may play in selection of tile f o l l i c l e ( s ) destined for ovulation. References I. 2. 3. 4. 5. 6.
R.T. HILL, Exp. Med. Surg. 7_, 86-89 (1949). J. BAHR, L. KAO and A.V. NALBANDOV, Biol. Reprod. I__0_0,273-290 (1974). H.W. BURDEN, The Vertebrate Ovary/, R.E. Jones, ed., pp. 615-638, Plenum Press, NY (1978--~. S. MOHSIN and J.N. PENNEFATHER, Clin. Exp. Pha~. Physiol. _6, 335-354 (197g). A. STEFENSON, CH. OWMAN, N.-O. SJOBERG, B. SPORRONG and B. WALLES, Cell Tissue Res. 215, 47-62 (1981). G. SELSTAM, S. RANI, K. NORDENSTROM, E. NORjAVAARA, S. ROSBERG and K.
AHREN, Regyla_tjgn, o_f_ Ta_r~le_t__Ce]j_Re>pgniv_eness, K.W. McKerns, A. Aakvaag and V. Hansson, eds., pp. 37-53, Plenum Presse, NY (1983).
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7. 8. 9. I0. ii. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21° 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.
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Pergamon Journals
MINIREVIEW CATECHOLAMINERGIC REGULATION OF OVARIAN FUNCTION IN MAMMALS: CURRENT CONCEPTS Leon J. Spicer D i v i s i o n of Endocrinology The Milton S. Hershey Medical Center The Pennsylvania State University Hershey, PA 17033
Summar~c A review of the rapidly accumulating data in the l i t e r a t u r e continues to support the notion that catecholamines regulate ovarian function, and extends the complexity of catecholaminergic effects on the ovary via interactions with pituitary and adrenal hormones. I t is clear that catecholamines affect growth and differentiation of ovarian f o | l i c l e s , but their role in f o l l i c u l a r rupture during ovulation and in corpus luteum function ranains unclear. The effects of catecholamines (mediated by membrane receptors) on ovarian function probably should be considered paracrine but classic endocrine regulation of ovarian function cannot be ruled out. Myogenic tonus of ovarian vasculature appears to be regulated by catecholamines, and estrogens may enhance adrenergic receptors in ovarian ~nooth muscle ceils. Since the early studies of Hill (1), i t has been generally accepted that the autonomic neural connections of the ovary play some role in f o l l i c u l a r growth and ovulation. However, tremendous ambiguities s t i l l exist regarding the local neural mechanisms that regulate ovarian function. This review summarizes important features of local adrenergic innervation and function within the mammalian ovary, u t i l i z i n g several current findings. Anatomical Considerations Information on the anatomy of the autonomic innervation of ovaries has been obtained in a variety of mammals, including rodents, primates and ungulates (reviewed in 2-6). Briefly, i t has been shown that adrenergic (sympathetic) nerves are more abundant than cholinergic nerves (parasympathetic) and that nerve fibers enter the ovary with: I) the vasculature via the ovarian plexus along the ovarian artery, and 2) the nerve in the suspensory ligament (superior ovarian nerve) (7). Within the ovary, catecholamine-containing nerve terminals surround blood vessels of the smooth muscle layers of the theca externa. The membrana granulosa is avascu]ar and c~npletely devoid of nerve f i b e r s . Studies u t i l i z i n g sensitive histochemical techniques further demonstrated that, in all species examined, adrenergic nerves richly supply the theca externa of f o l l i c l e s but few neurons are present in corpora lutea and theca interna (3,5,8). I t has also been reported that the steroidogenically active i n t e r s t i t i a l cells of hamster, rat and guinea pig ovaries are directly innervated by adrenergic nerve
0024-3205/86 $3.00 + .00 Copyright
(c) 1986 Pergamon Journals Ltd.
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fibers (3,9,10). I n t e r s t i t i a l ceils are heterogeneous in regard to their history and position within the ovary and, thus, are classified into four classes (11). Their collective and principal function is to synthesize and secrete androgens (11). Thus, adrenergic nerves communicate directly with ovarian i n t e r s t i t i a l cells and with ovarian f o l l i c l e s via the theca externa. Moreover, the neurohormone released by these adrenergic nerves, predominantly norepinephrine, diffuses through the basement membrane into the f o l l i c u l a r antrum and, thus, also may ccxnmunicate with granulosa cells (see later section and Fig. I ) . Functional nerve terminals d i r e c t l y on luteal cells have not been observed in any species to date and the few adrenergic nerves present in corpora lutea are pred~ninantly perivascular (2-6,8). Experimental Manipulations In Vivo Effects of severing the ovarian nerve supply on ovarian function have been reviewed previously (2-6,12). B r i e f l y , i t has been found that loss of ovarian innervation via surgical sympathectomy does not appear to affect estrous c y c l i c i t y in rats or baboons but does in mice (2-4). In rabbits, ovulation rates (induced by hCG) were not affected by ovarian denervation (2,4). Differences among species in the density of ovarian nerves may p a r t i a l l y explain differences reported in the response to denervation (3,4). However, in many of the experiments previously reviewed i t was not demonstrated that all nerves (both adrenergic and cholinergic) were absent after manipulation and(or) that normal ovulatory c y c l i c i t y occurred in the absence of nerves. With these points in mind, a review of the recently published experiments w i l l be presented. Rats. Superior ovarian nerve transection in prepubertal rats by electrocautery had no effect on the onset of puberty (13). In addition, transection of the ovarian suspensory ligament had no effect on ovulation rates in immature rats treated with pregnant mare serum gonadotropin (PMSG) (14). Similarly, denervation by freezin~ the ovarian nerve supply had no effect on ovulation rates in cyclic rats (15). In contrast, abdominal vagotomy in rats has had variable effects (both increases and decreases) on numbers of both healthy and atretic ovarian f o l l i c l e s depending on the period that has elapsed from vagotomy (16). No significant effect of vagotomy on serum estradiol or testosterone was observed in this study (16). Again, complete denervation was not verified in these studies. R a t s given PMSG treatments followed by injections of norepinephrine increased ovulation rates, whereas phentolamine (an ~-adrenergic antagonist) decreased ovulation rates (17). The f a i l u r e of some researchers to find alterations in ovarian function after denervation may be explained, in part, by the recent observation that, after transection of the superior ovarian nerve in rats, an increase in numbers of ovarian B-adrenergic receptors occurs coincident with a decrease in ovarian norepinephrine levels (13). No effect on ovulation rates was observed in this study (13). Thus, in vivo responsiveness to circulating catecholamines ~nay actually increase after ovarian denervation. Several studies have investigated the role of adrenergic neurons on ovarian steroid secretion in vivo. Ovarian mesenteric nerve transection blocked the increase in plasma estradiol concentrations induced by electrical stimulation of the medial basal prechiasmatic area of proestrous rats (18). Plasma progesterone levels were unaltered by ovarian denervation in this study as ~ l l as were average number of ova recovered in oviducts (18). In contrast, ovarian progesterone content was s i g n i f i c a n t l y reduced following electrical stimulation of the superior ovarian nerve in diestrous rats (19). More recently, ablation of the superior ovarian nerve during proestrus in rats resulted in an immediate and significant decrease in both progesterone and estradiol secretion (ng/min) which was maintained for 10 to 20 rain after nerve section (20). Blood flow (see
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later section) was also significantly reduced for a 4-min period after nerve section (20). Collectively, secretion of progesterone and estradiol by non-luteal ovaries during proestrus in rats appears to require an intact ovarian nerve supply. The development of c(~npensatory ovarian hypertrophy in rats also has been shown to require intact adrenergic nerves within the ovary (21,22). Rabbits. In pseudopregnant hypophysectomized rabbits, propranolol injected s.c. every hour for a 24-h period had no effect on concentrations of progesterone in serum (23). Days 7 to 8, 10 to 11 and 13 to 14 of pseudopregnancy were evaluted in this study. Similarly, s.c. propranolol pellets applied during days 13 to 17 of pseudopregnancy in hypophysectomized rabbits had no effect on the timing of luteal regression (23). Effectiveness and functional blockage of luteal (3-adrenergic receptors by propranolol was indirectly verified in these studies. These researchers concluded that endogenous catecholamines were not acutely involved in regulating luteal progesterone secretion or luteal regression in vivo (23). Whether this is true for other species rBnains to be determined. Guinea pigs. Previous experiments in which 6-hydroxydopamine (6-OH-D, a congener of norepinephrine that causes selective destruction of adrenergic nerve endings) was injected into animals systemically and ovarian function subsequently evaluated have been criticized for indirect effects that may have caused decreased ovarian function observed (5). In attempts to overcome this c r i t i cism, researchers have injected 6-OH-D into surgically closed ovarian bursae (24,25). These researchers found that after 10 days of 6-OH-D exposure, the number of large (>700 ~m) antral f o l l i c l e s (but not small f o l l i c l e s ) s i g n i f i cantly decreased in cyclic guinea pigs (24). In contrast, the number of preantra] f o l l i c l e s decreased and the number of large (>700 ~m) antral f o l l i c l e s increased after an 8-day exposure of 6-OH-D in prepubertal guinea pigs (25). This effect in prepubertal guinea pigs was overridden with gonadotropin injections (25). Collectively, these recent studies provide further in vivo support to the hypothesis that adrenergic nerves play a role in ovarian f o l l i c u l a r growth, and that this role may be selective to a specific type or size of the f o ] l i c u l a r population. Physiological Considerations Ovarian Levels of Catecholamines. I f levels of catecholamines within the ovary (0.2 to 2.0 ~g norepinephrine/g ovary for most species, reviewed in 5) were found to change with physiological states, more credence to the hypothesis that nerves are involved in ovarian functions would exist. Indeed, ovarian norepinephrine levels significantly decreased after PMSG injections into immature rats (26) and within 4 h following the preovulatory gonadotropin surge in normal cyclic rats (27). The depletion of f o l l i c u l a r norepinephrine appears to be due to increased FSH rather than increased LH or PRL (27), and is temporally associated f i r s t with increases in serum estradioI and then increases in serum progesterone (26). From these results i t was suggested that depletion of f o l l i c u l a r norepinephrine may be involved with formation of corpora lutea in rats (27). More recently, i t was observed that ovarian levels of norepinephrine (ng/ovary) increased 2- to lO-fold with sexual maturation in rats (28,29) and guinea pigs (30). Innervation of blood vessels within ovarian stromata also increases with age in guinea pigs, and thus may account, in part, for the increase in ovarian levels of catecholamines observed during puberty (30,31). In cyclic rats, concentrations of norepinephrine in ovaries (pg/mg) increase s l i g h t l y at proestrus and are associated with a significant decrease in ovarian norepinephrine turnover (32). Similarly, a 40% increase in ovarian content of norepinephrine (ng/ovary) observed 48 h after PMSG-treatment of immature rats was associated with an 88% decrease in ovarian norepinephrine turnover (29).
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Concentrations of norepinephrine (I-7 ng/m]; 6-41 nM) and epinephrine (.2-.9 ng/mI, i-5 riM) have also been measured in porcine f o l l i c u l a r fluid (33-35). Sinai| (1-3 mm) f o l l i c l e s contained greater concentrations of norepinephrine than medium f o l l i c l e s (33,35). However, significantly greater levels of both norepinephrine and epinephrine (as well as estradiol) were found in fluid of large preovulatory f o l l i c l e s (>6 mm) as compared to medium-sized (2-5 mm) f o l l i c l e s (33,35). In contrast, concentrations of norepinephrine and epinephrine in peripheral serum did not significantly change with stage of the estrous cycle in pigs (34). Data available in humans shows that during the menstrual cycle, norepinephrine levels in peripheral plasma are significantly greater during the luteal than f o l l i c u l a r phase (36,37). In addition, concentrations of norepinephrine and estradiol in plasma were significantly correlated (r=O.40, 36). Collectively, these data suggest that local accumulation of catecholamines occurs within follicles as they mature toward ovulation, and that this increase in catecholamines occurs concurrent with an increase in estradiol. Whether the amounts of catecholamines localized within the ovary are due to local neuronal release of catecholamines or accumulation of circulating catecholamines re~ains unclear. Ovarian Levels o f AdrenerBic Receptors. Binding kinetics of B-adrenergic receptors in granulosa cells was f i r s t reported in 1981 (38). Aguado et al. (39) later showed that adrenergic receptors present in rat granulosa cells were predominantly of the B2-type and that B-receptor content of granulosa cells increased with the onset of puberty. Ovarian B-adrenergic receptors in rats are highest on the day of proestrus and lowest on the day of estrus (ovulation) as compared with metestrus or diestrus (39,40). In rats, concentrations of estradiol in serum peak at mid-day of proestrus and then decline by early estrus (41,42). The LH/FSH surges occur between late proestrus and early estrus (41,42). Thus, in vivo, ovarian content of catecholamines and their receptors increase at proes-trus in rats, a time when ovarian levels of estradiol are highest (12,28). B-adrenergic receptors also have been found in various membrane preparations of corpora lutea from rats (43-45), rabbits (46) and pigs (47). Collectively, the stereospecificity of adrenergic receptors in granulosa cells are predominantly B2-type irregardless of species. It is less certain which subtype of receptor is present in corpora lutea. Adrenergic receptors in corpora lutea appear to be pred~ninantly B2-type in rats (43,45), B1.-type in rabbits (46), and nearly equal proportions of 81- and ;32-types in pigs (47). The physiological significance of these differences in stereospecificity is unknown. Moreover, i t should be pointed out that in the studies mentioned above, preparations of granulosa cell membranes used in the binding assays were essentially homogeneous, whereas membrane preparations from corpora lutea contained membranes from ]uteal cells and vascular cells (e.g., smooth muscle cells). Thus, varying anounts of contaminating vascular cells in preparations of luteal membranes anong the studies cited may be clouding the in'cerpretation of such studies. Experimental Manipulations In Vitro Effects of adrenergic agents on cultured granulosa and luteal cells have been reviewed previously (6,12). Briefly, B-adrenergic agents (preferentially B2-type) have been found to be stimulatory to production of 3',5'cyclic adenosine monophosphate (cAMP) and progesterone in f o l l i c u l a r tissue of rats and pigs (12,13,47), and lutea] tissue of rats, rabbits, sheep, pigs and cows (12,47,48). I t has also been shown that prostaglandin F2~ can inhibit tile stimulatory effect of epinephrine on progesterone production by 3- and 5-day old corpora lutea but not newly formed corpora lutea (l-day-old) isolated from rats (49). Estrogen biosynthesis by cultured granulosa cells does not appear to be affected by catecholami nes (12). Recently, catecholamine- stimulated progesterone
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production and(or) adenylate cyclase a c t i v i t y in luteal tissue of monkeys (50) and humans (51) was reported. Regulation of the B2-adrenergic response system has been f a i r l y well characterized using granuI osa ce] Is fro(n estrogen-primed, hypophysectomized rats (12). Briefly, cultured granulosa cells not exposed to f o l l i c l e stimulating hormone (FSH) do not respond to catecholamines, whereas FSH pretrea~nent dranat i c a l l y increases their responsiveness to catecholamines (12,13,52). How FSH e~lhances catecholamine responsiveness is unclear but does not appear to involve an increase in B-adrenergic receptors (52). Epinephrine and norepinephrine are equally potent in stimulating progesterone production after FSH pretreatment (53). Moreover, the stimulatory effect of adrenergic agents on progesterone production by FSH-pri,ned granulosa cells appears to be due to a stimulation of 3B-hydroxysteroid dehydrogenase (3B-HSD) and an i n h i b i t i o n of 20~-hydroxysteroid dehydrogenase (20~-HSD) (54,55). In vivo evidence u t i l i z i n g intact immature rats suggests that adrenergic responsiveness of granulosa cells in v i t r o is demonstrable only i f rats are exposed to luteinizing hormone (LH) in vivo (56). However, LH (2-100 ng/ml) given in v i t r o had no effect on B-adrenergic-stimulated progesterone production by cultu--red granulosa cells collected from intact immature rats (52). Whereas, human chorionic gonadotropin (hCG; 100 ng/ml) given in vi t ro enhanced epinephrine responsiveness of FSH-primed granulosa cells from hypophysectomized rats (53). More work is needed to c l a r i f y these discrepancies. At present, effects of estradiol on catecholamine responsiveness have not been carefully evaluated in cultured murine or porcine granulosa cells. Recent studies u t i l i z i n g granulosa cells from intact immature rats further characterized hormonal regulation of the B-adrenergic response system (52). Aguado and Ojeda (52) found that, in addition to FSH, corticosterone (200 and 400 ng/ml) and prolactin (PRL, 400 ng/m]) when added with B-adrenergic agonists increased progesterone production (but not cAMP production) above that of any single hormone treat~ent. Furthermore, the FSH-, PRL- and corticosteroneenhanced responsivities were not associated with increases in numbers of B-adrenergic receptors (52). Thus, the f a c i l i t a t o r y effect of these hormones appears to be exerted at a step distal to cAMP generation and may be due to direct stimulation of 3B-HSD and i n h i b i t i o n of 20~-HS0 as previously mentioned (54,55). In contrast, other recent data showed that somatomedin-C dra,natically enhanced B-adrenergic-stimulated progesterone and cAMP production by granulosa ceils pretreated with FSH in v i t r o collected from hypophysectomized, estrogen-treated, immature rats (57). In cultured porcine granulosa cells, epinephrine and norepinephrine are also equipotent in stimulating ornithine decarboxylase (ODC) a c t i v i t y and progesterone production (33,58). Moreover, epinephrine-stimulated ODC a c t i v i t y from small (I-2 mm) immature f o l l i c l e s and large (>6 mm) mature f o l l i c l e s is equivalent (59). Although epinephrine stimulates cAMP production by porcine granulosa cells (47), Ca+2 (60) as well as insulin (61) dramatically enhance epinephrine-stimulated progesterone production by processes distal to, or independent of cAMP generation (60). Although effects of LH and FSH on catecholamine responsiveness have not been thoroughly evaluated in cultured porcine granulosa c e l l s , i t has been shown that LH plus epinephrine is additive to ODC stimulation, whereas FSH plus epinephrine is not (33). More work needs to be conducted to evaluate gonadotropin interactions with catecholamines in porcine granulosa c el ls . Recent preliminary evidence indicates that the estrogen metabolite, 2-hydroxyestradiol, dramatical ly enhances progesterone production stimulated by epinephrine and other B-adrenergic agonists in cultured porcine (62) and rat (63) granulosa cells. These new findings suggest that catecholestrogens and
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Catecholamines
catechol ami nes ~nay differentiation.
and Ovarian
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fol I i cul ar
Recently, i t was reported that catecholamines directly affected steroidogenesis of thecal cells (64). Specifically, 13-adrenergic agents had no effect on basal steroidogenesis but enhanced hCG-stimulated androgen production in cultured rat thecal cells (61). Norepinephrine and epinephrine were equipotent in stimulating thecal androgen production (64). S i n c e functional adrenergic synapses have been demonstrated in the steroidogenic i n t e r s t i t i a l and thecal cells (3), these results provide new, important infon~ation with regard to catecholamine action within the ovary. T h a t is, catecholamines released by nerve tenninals within the theca externa/interna can directly stimulate androgen production by thecal cells which, after diffusion into the membrana granulosa, may be aromatized (Fig. I ) . Thus, effects of catecholamines on f o l l i c u l a r function in vivo may be mediated, in part, by indirect effects of increased estrogen production. Further research is needed to elucidate such possibilities.
THECA EXTERNA
~
THECA INTERNA I / /
//
Androgens
~
-
GRANULOSA
~'~
F F
,;
FIG. 1 Schematic representation of proposed interrelationships between catecholamines and other trophic hormones of mammalian ovarian f o l l i c l e s . Nerve terminals surrounding the smooth muscle cells of blood vessels (BY) and steroidogenic cells of the theca release norepinephrine (NOREPI). Epinephrine (EPI) enters the theca interna via the blood supply. Both NOREPI and EPI enhances LH-stimulated androgen production by thecal cells. NOREPI and EPI also diffuses through the basement ,nembrane (BM) and into the f o l l i c ular fluid (FF)-containing antrum where both catecholamines enhance PRL-, FSH- and perhaps LH-stimulated progesterone production by granulosa cells (GC). Estradiol production by GC is not directly stimulated by NOREPI and EPI Dut is stimulated indirectly by increased thecal androgen production. The mechanism whereby gonadotropins plus catecholamines (versus either alone) enhance steroidogenesis rBnains unclear.
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Catecholamines and Ovarian Function
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I t should be ~nphasized that in vitro cell cultures do not take into account any potential effects catecholamines may nave on the f o l l i c u l a r vasculature and microcirculation. Moreover, cultures of isolated granulosa cells do not take into account major interactions between thecal and granulosa cells nor the potential f o l l i c l e - t o - f o l l i c l e interactions. To elilninate some of these c r i t i cis,ns, researchers have developed in vitro perfusions of whole ovaries. Recent studies u t i l i z i n g perfused rabbit ovaries have shown that both a- and B-adrenergic agonists can significantly increase the number of gonadotropin-induced ovulations (65,66). An increase in ovulation rates by B-adrenergic agents (delivered into the ovarian bursa) also has been observed in vivo in gonadotropin-primed immature rats (17). Schmidt et al. (66) speculated that a-receptor-stimulated events were mediated via some physical inechanism, whereas g-receptor-stimulated events were ~nediated via some endocrine mechanis~n. Evidence for the role of a-receptors in ovarian function is presented in the next section of this review. Catecholamines and Mechanical Processes within the Ovary Steroidogenesis versus blood flow regulation. Although i t is clear fr~n the in vitro studies cited in previous sections of this review that adrenergic agents have direct stimulatory effects on thecal and granulosa cell steroidogenesis, i t is unclear what proportion of adrenergic effects in vivo are mediated by some indirect mechanism on the ovarian vasculature. The tone of the ovarian vasculature is regulated via the smooth ~,luscles surrounding bloed vessels and has been reviewed previously (4,67). Two possibilities exist with regard to adrenergic regulation of mechanical processes within the ovary: i) ovarian blood flow (afferent and efferent) during any given physiological state; and 2) f o l l i c u l a r wall contraction or relaxation during the ovulatory process (see next section). A recent study has reported both acute stimulatory effects of norepinephrine on vascular resistance and inhibitory effects on blood flow of corpora lutea in pseudopregnant rats (68). Norepinephrine also has been shown to increase blood flow and(or) vascular tonus of whole ovaries fr~n non-luteal rabbits (69), pregnant guinea pigs (70), and luteal and estrous rats (71) in vivo. Blood flow to non-luteal rat ovaries has also been tBnporarily reduced following transection of the superior ovarian nerve (20). Secretion (rig/rain) of b o t h progesterone and estradiol was also reduced after ovarian nerve transection in this same study. However, steroid secretion remained depressed 10 to 20 min after blood flow returned to control levels (20). These results give support to the notion that catechola~ninergic stimulation of steroidogenesis is not canpletely mediated by alterations in ovarian blood flow. Adrenergic agonists can also increase vascular resistance in whole ovaries in vitro (72) and isolated ovarian arteries in vitro (73). Hore recently, ovaries (perfused in vitro) fr~n luteal-phase g i l t s exhibited significantly greater vasoconstriction than did ovaries fr~n f o l l i c u lar-phase and pregnant g i l t s in response to electrical stimulation (74). Norepinephrine caused a significant increase in vascular contractility to electrical stimulation for all ovaries in this study (74). Moreover, ovarian vascular c o n t r a c t i l i t y was negatively correlated (r=-.99) with the estrogen to progesterone ratio in systemic blood during the estrous cycle in g i l t s (74). Thus, adrenergic receptors in ovarian vasculature may be hormonal ly regulated as suggested for uterine vasculature in pigs (75) and mesenteric vasculature in rats (76). Density of B-adrenergic receptors in hypothalami and anterior p i t u i t a r y glands also seems to be regulated by ovarian steroids (77). Thus, further studies are needed to determine i f steroids ( i . e . , estradiol and progesterone) regulate ovarian adrenergic receptors as well.
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Role of catecholamines in the ovulatory process. In addition to the potent i a l stimulatory effects of catecholamines on f o l l i c u l a r ceil differentiation that occurs during the preovulatory period, catecholamines may also play a role in f o l l i c u l a r wall contraction/relaxation during the ovulatory process. Potent i a l interactions between catecholamines and the documented hormonal control of biochemical processes associated with f o l l i c u l a r rupture (e.g., gonadotropinstimulated plasminogen activator, see ref. 67) has not been studied. Researchers have concentrated on the role of adrenergic agents in f o l l i c u l a r wall contraction u t i l i z i n g strips of bovine or human Graafian f o l l i c l e s (78-80). Briefly, norepinephrine was slightly more potent than epinephrine in stimulating contraction whereas epinephrine was slightly more potent than norepinephrine in stimulating dilation. With this and other evidence, an involvement of both mand B-adrenergic receptors in the contractility/relaxation process was suggested (78-80). For a better understanding of the pharmacology of ~- and B-adrenergic receptors, the reader is referred to several recent reviews (81-85). Recent studies have evaluated the interaction of prostaglandins (PG) on norepinephrine-induced contractions of bovine f o l l i c u l a r walls (86). Although PGF2: contracted f o l l i c u l a r wails, i t had no effect on norepinephrineinduced contractions (86). In contrast, PGEI and PGE2 dranatically reduced tile contractile response induced by norepinephrine (86). Thus, prostaglandins and catecholamines may interact with smooth ,mscle cells of Graafian f o l l i c l e s during the ovulatory process. Collectively, these studies support the concept that myogenic tonus of the ovarian vasculature may be regulated by catecholamines and that estrogens may enhance adrenergic receptors in mnooth muscle cells. The stereospecificity of these adrenergic effects is unclear at present, but i t does appear that both mand B-type receptors are involved in regulation of ovarian myogenic tonus. Conclusions Concentrations of catecholamines and their receptors within the ovary changed significantly with stage of an estrous cycle and during sexual maturation in all mammals examined to date. The major catecholamines, norepinephrine and epinephrine, are found in nanomolar concentrations within mammalian ovaries. In v i t r o , these catecholamines are stimulatory to gonadotropin-induced steroidogenesis in granulosa and thecal ceils. However, the mechanism by which catecholamines and gonadotropins interact to enhance granulosa and thecal steroidogenesis rBnains unclear. In vivo, catecholamines can alter f o l l i c u l a r growth, ovulation and ovarian Jnyogenic tonus. Potential interactions between adrenergic stimulation of f o l l i c u l a r steroidogenesis and ovarian blood flow have not been evaluated. Future research should also address the role catecholamines may play in selection of tile f o l l i c l e ( s ) destined for ovulation. References I. 2. 3. 4. 5. 6.
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AHREN, Regyla_tjgn, o_f_ Ta_r~le_t__Ce]j_Re>pgniv_eness, K.W. McKerns, A. Aakvaag and V. Hansson, eds., pp. 37-53, Plenum Presse, NY (1983).
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