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Uterine vascular response to prostacyclin in nonpregnant sheep
Uterine vascular response to prostacyclin in nonpregnant sheep ROBERT
RESNIK,
GARY
W.
Lo ./C&I.
Calijhin
BRINK,
Ms.D. B.A.
The effects of prostacyclin (PGI,) on uterine blood flow were investigated in nonpregnant, castrated ewes with chronicaljy implanted polyvinyl catheters and electromagnetic flow probes. Intra-arterial infusion of PGI, resulted immhdiately in increased uterine blood flow, which returned rapidly to baseline after tenninatibn of the In,fusion. A dose-response curve was constructed and demonstrated that a uterine arterial PGla concentration of 1.2 times 10-W (0.5 &ml) produced flows 50% of those achieved by a deference dose of I pg of 17@estradiil. Increases in uterine blood flow were n@ associated with changes in rriean systemic arterial pressure. These findlngs demonstrate that PGI, has a vasodilatory effect on the uterine circulation and suggest that this substance has a role in the regulation of uterine blood flow. (AM. J. OBSTET. GYNECOL. 137:267, 1980.)
THE prostaglandins are ubiquitous chemical agents that exert a variety of physiologic and pharmacologic effects in numerous organs. Previous studies from our laboratory have shown that prostaglandir+ El and EZ are potent uterine vasodilators, and we have suggested that the local synthesis and release of prostaglandins may be involved in the regulation of uterine blood flow.’ More recently, a new prostaglandin, prostacyclin (PGIp), has been described as a product of the transformation of prostaglandin endoperoxides in blood vessels.2 This relatively unstable substance is generated in the endothelial cell layer of blood vessels in numerous species.5 including man.* Its major properties are the inhibition of platelet aggregation and the initiation of vasodilation in many circulations, including the mesenteric, femoral, renal, and coronary.5* 6 Since little is known about the uterine vascular response to PG12, we used a nonpregnant, chronically instrumented sheep model in order to determine the quantitative characteristics of changes in uterine blood flow after the intra-arterial administration of PG12. From the Department of Reproductive Medicine, Uniwrsit~ of Calzyornia, San Diego Srhwl of Medicin?. This investigation was su#mrted by United States Public Health Service Grant No. I ROI HD 115~4-01AI and a grant from the National Foundation-March of Dimes. Presented by invitation at the Forty-sixth AnnuaE Meeting of the Pa@ Coast Obstetrical and Gynecological Society, Palm Springs, Calijorni& September 25-30, 1979. Reprint requests: Robert Resnik, M.D., Departmat’of Reproductive Medicine, University Hospital H-813, Dickinson St., San Diego, Califoonzia 92103 0002-9378/80/100267+04$00.40/0
0 1980 The C. V. Mosby
Msterfsf and methods Four nonpregnant cross-bred sheep were used during the investigation. The surgical procedure was described in previous reports.‘. s* y After sedation with intravenous sodium pentobarbital, 640 mg, a spinal anesthetic (Pontocaine hydrochloride, 6 mg) was administered. Pelvic laparotomy was performed through a midline incision, and the anterior leaves of the broad ligament were incised so as to expose the uterine arteries and their branches. Polyvinyl tubing (,inner diameter, 0.58 mm; outer diameter, 0.96 mm) was used to catheterize the most lateral branch of the uterine artery, and was advanced retrograde to within 0.5 cm of the bifurcation. Three-millimeter electromagnetic flow probes* were placed around the uterine arteries proximal to the bifurcation. An additional polyvinyl catheter was inserted into the femoral artery in order to measure systemic arterial pressure. After oophorectomy, which was performed to remove the source of most of the endogenous estrogen produced. the catheters and flow probes were brought through a fascial incision and a subcutaneous tunnel and stored in a canvas pouch on the animal’s flank. Postoperatively, the sheep were placed in a rectangular stainless steel stall and kept in the laboratory for the duration of the studies. Uterine arterial flow was measured with Micron R.C. 1000 electromagneric flowmeters.* Prior to daily experiments, the flow signals were calibrated to electrical zero. Flow signals were electronically integrated and displayed on a pen recorder
225 *Micron Co.
Instrument
Company,
Los Angeles,
California.
267
'268
Resnik
and Brink
PGI, 8 +g/min
f2178 IP9
200 MEAN UTEWE BLooD FLOW r&kin
I50 100 W J
s++-v-
0
30
.--J
0
60
90
120
150
180
MINUTES
1. Comparative uterine arterial flow response to intra-arterially 17@estradiol.
Fig.
lOOr
*
OL
10-l
I lo-"
J
10-S
AflTERlALCONCENTRATiON(Mj
Fii. 2. Linear regression of percent maximum uterine blood flow produced by 1 pg of 17&estradiol versus log,, PGIS arterial concentrations (M) determined by method of least squares. Percent maximum flow = 245.6 + 33.0 loglo PGI, concentration (MS. Arterial concentrations were calculated with baseline uterine blood flows in each experiment prior to infusion of PGI,. Each Point represents mean ? SEM of at least 3, and as many as IO, observations. running at 1 mm/minute. Systemic arterial pressures were monitored with Statham P23-Db pressure transducers. Previous studies had demonstrated that maximum uterine vasodilation occurred after a pulsed dose of 1 pg of 17/3-estradiol injected directly into the uterine artery, and that peak flows are reproducible on a daily basis after the fifth postoperative day.g Accordingly, all experiments were performed after this period of postoperative recovery. The 17/S-estradiol* was of reagent grade quality and was prepared for intra-arterial injection by dissolving it in absolute ethanol and diluting the stock solution with 0.9% saline solution. The final concentration of estrogen in the working solution was 1 pg/ml. The final ethanol concentration in the solution was 0.1% by volume. This has been previously shown to have no effect *Sigma Chemical Company, St. Louis, Missouri.
administered
prostacyclin and
on uterine blood how in this model. Prostacyclin” was obtained as the sodium salt. Stock solutions were prepared daily by dissolving the salt in 1 M Tris buffer (pH 8.0) at 3” C, with a final prostacyclin concentration of 1 mg/ml. Working solutions were prepared from aliquots of the stock solution diluted in 50 mM Tris buffer (pH 7.7) on ice. A dose-response curve was constructed to determine what concentrations of prostacyclin would produce minimum and maximum dilatory vascular responses. Since previous studies 8. 9 have shown that a pulsed uterine intra-arterial dose of 1 pg of 17&estradiol produces a maximum dilatory response, the effects of prostacyclin were compared to the maximum blood flows produced by the bolus injection of estrogen. Prostacyclin was infused by means of a constant-infusion pump, and arterial concentrations were calculated on the basis of the control blood Row prior to the uterine intra-arterial infusion, infusate concentration, and rate of administration. PGIy was infused at concentrations ranging from 1 to 64 @g/ml at an infusion rate of 0.247 ml/minute. Infusions of 0.9% saline solution into the contralateral uterine vessels were used as controls.
Results Immediately after intra-arterial infusion of PG12, uterine blood flow began to increase, and reached maximum flows within 2 to 3 minutes, with a rapid return to baseline rate of flow after termination of the infusion. This is contrasted with the uterine vascular response to a pulsed dose of 17@estradiol, which is characterized by a 30-minute delay after the dose, and then the uterine blood flow increases and reaches a maximum at 90 to 100 minutes after injection. Return to baseline flow is linear and is achieved 6 to 8 hours after injection. An example of response in uterine blood flow to PGIE and 17@-estradiol is shown in Fig. 1. A dose response curve relating uterine blood flows *Upjohn
Company, Kalamazoo, Michigan.
Uterine vascular response to arostacyclin
initiated by PG12 to the percent of maximum blood flow resulting from a dose of 1 pg of 1?@estradiol is shown in Fig. 2. A PGI* uterine arterial concentration of 1.2 times IO-” M (0.5 pgiml) resulted in flows 50% of that produced by the reference estrogen dose. No changes in uterine blood How occurred in response to infusions of 50 mM Tris buffer, and no changrs in mean systemic arterial pressure were observed during infusion of PGI,.
Comment The vasodilator effects of PGIz have been demonstrated in many species. Dusting and associates” observed significant decreases in local perfusion pressure after infusion of 0.02 to 2.0 pg of PGIZ into the mesenteric and femoral arteries of anesthetized dogs, doses similar to those which increased uterine blood How in our nonpregnant sheep model. PGIz has also been shown to dilate bovine coronary arteries,‘O and to decrease systemic blood pressure in dogs” and ratsI after intravenous administration. These latter findings suggest that, unlike several other prostaglandins which are rapidlv inactivated by passage through the pulmonaq circulation,“’ PGIz is not. In the sheep, PGIZ is not so potent a uterine vasodilator as prostaglandin E,. We previously showed that a uterint. arterial PGE, concentration of 7.52 times IO-“. a concentration more than one order of magnitude less than that of PGIZ, will increase How to 50% of that produced by a reference dose of 1 PLg of 17pestradiol.’ Armstrong and associates’l have also observed a slightly less potent cardiovascular response to
PGI, than IO PGE, after infusions into the left atria ot’ anesthetized dogs. Although this ma! represent a real difference in biologic activity. it mai’ .tlso rcflcct tht biologic instability of PGI,, which rapidly breaks down in rvaterv solution at 37” C into its stable bur biologicall! less active metabolite. S-oxo-prostaglandin Flcr.” Despite careful pi-eparation of prostacyciin for infusion. at temperatures of 2” or 5” C. one might speculate that exogenouslv administered PGIZ is parti,rlly metabolized in viva to Ii-oxo-PGF,, before it interacts with v:rscular smooth muscle. It is known that PGIs 1%produced in the intimal la)er of blood vessels.:’ Con\equentiv, t hr proximitv of site of‘svnthesis to site 01 action ma\ ah\ PGIp to eiert its physiologic effects on vaaular s’mootlr muscle before significant metabolism occ.ur\. In addition to PGIZ, the cycle-oxygcn~se system metabolizes endoperoxides in platelets I$) thrcmboxane A*. This substance has effects which ale opposite to those of PG12, that is, it promotes vaso~otlstrictiorl and platelet aggregation. These differences in ef‘fects have led Moncada and Vane’ to postulate the presence of a system in which prostacvclin and thromhoxane Ax produce opposite effects in controlling blood How and platelet aggregation within local circulati~)ns. The role which prostaqclin may play in the local regulation of uterine blood How remains to be elucidatctl. but existing evidence provides ample ration& tot- l‘urther StudY. We are grateful to Dr. John Pike. (11 the Uplohn Cornpan\, for strpplying the prosracvclin uwd in this studv.
REFERENCES
1. Resnik, R.. and Brink, C. W.: Effects of prostaglandins E,, E, and Fpa on uterine blood flow in non-pregnant sheep, Am. J. Physiol. 234:H557, 1978. 2. Moncada, S., Gryglewski, R., Bunting, S., et al.: An enzyme isolated from arteries transforms prostaglandin endoperoxides to an unstable substance that inhibits platelet aggregation, Nature 263:663. 1976. 3. Maclntyre, 1). E., Pearson, J. D., and Gordon, J. L.: Localisation and stimulation of prostacyclin production in vascular cells, Nature 271:549. 1978. 4. Moncada, S.. Higgs. E. A., and Vane, J. R.: Human arterial and venous tissues generate prostacyclin (prostaglandin X), a potent inhibitor of platelet aggregation, Lancet 1:18, 1977. 5. Dusting, G. J., Moncada. S.. and Vane, J. R.: Vascular actions of arachidonic acid and its metabolites in perfused mesenteric and femoral beds of the dog, Eur. J, PharmaCdl. 49:65, 1978. 6. Needleman, P., Bronson. S. B., Wyche, A., et al,: Cardiac and renal prostaglandin I,, J. Clin. Invest. 61:839, 1978. 7. Moncada, S.. and Vane, J. R.: The role of prostacyclin in vascular tissue. Fed. Proc. 38:66, 1979. 8. Killam. A. P.. Rosenfeld. C. R., and Battaglia, F. C.: The
269
9. 10.
11. 12. 13. 14. 15.
effect of estrogens on uterine blood How. .I\w. J. hSTE.r. GYNECOI.. 115:1045, 1973. Resnik, R., Killam, A. P.. Battaglia, F. C., ct al.: The stimulation of uterine blood flow bv variotts estrogens, Endocrinology 94: 1192, 1974. Dusting, G. S.. Moncada, S., and Vane, J. R..:Prostacyclin (PGX) is the endogenous metabolite responsible for relaxation of coronary arteries induced by ararhidonic acid, Prostaglandins 13:3, 1977. Fitzpatrick, T. M., Alter, I., Corey, E. J., et al : Cardiovascular responses to PGI, (prostacyclin) in the dog, Circ Rrs. 42:192, 1978. Weeks, J. R.. and Compton, L. C.: The cardiovascular pharmacology of prostacyclin (PG12) in the rat, Prostaglandins 17:501, 1979. Gorman, R. R.. Bunting, S., and Miller, 0. V.: Modulation of human platelet adenylate cyclase by prclstacvclin (PGX), Prostaglandins 13~377, 1977. Armstrong, J. M., Chapple, D., Dusting, C. J .~et al.: Cardiovascular actions of prostacyclin (PGI,) in chloralosc anesthetized dogs, Br. j. Pharmacol. 61: 136P, 1977. .,lohnson, R. A., Morton, D. R., Kinner. I. 11.. et al.: The chemical characterization of prostaglandin X (prostacyclin), Prostaglandins 12: 1915, 1976.
270
Resnik and Brink
May Am. J.
Discussion
DR.
J. NOW, (by invitation) Beaverton, Oregon. Throughout the reproductive cycle the uterine tissues undergo various physiologic and biochemical changes in which prostaglandins (PC) play an important role. There is also much evidence to implicate the PG system in the process of parturition. A wide variety of tissues possesses the ability to synthesize PGs from arachidonic acid. Their capacity to do so, however, is variable, and the type and quantity of PGs produced depend on many factors. In reproductive tissues, hormonal conditions are especially important. Prostaglandins are generally called “local hormones” because they act within or near the tissues in which they are synthesized. Prostacyclin (PG12) may be the one known exception, since it is apparently not metabolized by the lungs and may serve as a circulating hormone. PGIz is the major product of arachidonic acid in most vascular tissues which have been studied. Incubation studies indicate a high formation of PGIz in the fetal ductus ateriosus, which suggests an important role in maintaining patency of the ductus. The production of prostacyclin has also been identified in human placenta and in myometrium from many different aninial species. Amnion is also a major site of PC& production in pregnancy. Thorburn and Challis reasoned that, if PGIz causes relaxation of vascular smooth muscle, it may also relax the myometrium. Recently, it has been shown that PGIz inhibits spontaneous motility of isolated pregnant human myometrium and reduces the tone induced by PGF,,. The relaxing effect of PGIz was associated with an increase in myometrial cyclic AMP. In the nonpregnant human uterus. the production of prostacyclin is confined primarily to the myometrium, with little activity appearing in the endometrium. If the myometrium and endometrium are incubated together, production of PGI, is enhanced. Presumably, the endometrium makes available to the myometrial synthetase an increased supply of endoperoxides for conversion to PGI, at the expense of PGEz and PGF,,. It has been proposed that the onset of labor is initiated by a diversion of arachidonic acid metabolism from the prostacyclin pathway to the PGEz and PGF,a pathways. Although thromboxane A2 and prostacyclin are formed in substantial amounts by intrauterine tissues during pregnancy, there is little evidence to support this hypothesis. Amniotic fluid levels of 6keto-PGF,,, the stable metabolite of PG12, as well as other PGs are elevated in spontaneous labor, in comparison to levels before the onset of labor. Preliminary data from Oxford suggest a role for PGI, in cervical ripening. Current theories of prostaglandin action in parturition have focused on the idea that the fetus or its membranes produce an inhibitor which, during normal pregnancy, maintains PG synthesis at a low level until term. At term, the inhibition is withdrawn and there is a resultant increase in the production of PG. MILES
15, 1980
Obstet. Gyned
Dr. Resnik’s pI-evocative study has directed OUI. attention to the possible role of PGIz in regulating uterine blood flow. Knowledge of the mechanisms which regulate uteroplacental blood flow is important for understanding implantation. fetal growth, and the endocrine function of the placenta. There is a large body of circumstantial evidence which has implicated the various PGs in the regulation of uteroplacental blood flow, but there is a lack of direct proof that the)- are physiologically important. Dr. Resnik has used the well-known preparation in sheep that was developed in Denver, which allows for chronic measurements of uterine blood flow by electromagnetic Howmeter and for specific intra-arterial administration of small doses of various test substances. He has demonstrated an immediate vasodilator response to graded doses of PG12. In this elegant study he has been able to calculate-the dose of PGIz which was administered directly into the blood vessels in question-in this case, the uterine arterioles. He has shown that prostacyclin produces a hyperemic response which is quantitatively similar to that produced by 17/3estradiol. However, the time course is quite different, with estrogens showing a considerable lag period. It is tempting to speculate that the hyperemic effects of estrogens on uteroplacental blood flow are mediated by an increased synthesis of prostaglandin. but neither this study nor others have as yet provided convincing proof of this connection. According to Dr. Kenneth Clark, now at the University of Cincinnati, indomethatin does not inhibit estrogen-induced increases in uterine blood How in either pregnant or nonpregnant sheep. Similarly, at the Oregon Regional Primate Research Center, we were not able to block with large doses of indomethacin the estrogen-induced ovarian hyperemia in rabbits. Finally, I would like to ask Dr. Resnik whether he has made any observations of PGIz effects on uteroplacental blood How in pregnant animals. In 1974. I presented a paper to this Society which showed that uterine contractions in rhesus monkeys produced placental vasoconstriction in association with myometrial vasodilatation, and suggested that PGs could mediate these differential effects of labor on the uterine and placental vascular beds. Since then, Rankin has shown in pregnant sheep that prostacyclin injected into the left ventricle of the pregnant ewe causes placental vasoconstriction and myometrial vasodilatation. It is possible that the placental vasoconstriction after the infusion of PGIz was secondary to the release of catecholamines induced by maternal hypotension. Taken together with earlier observations, Dr. Resnik’s timely study indicates that naturally occurring PGs have pharmacologic effects on the uterine vasculature. Whether PGs, or prostacyclin in particular, play a major physiologic role in regulating uterine blood flow in primates, sheep, or women remains to be seen.
RESNIK,
GARY
W.
Lo ./C&I.
Calijhin
BRINK,
Ms.D. B.A.
The effects of prostacyclin (PGI,) on uterine blood flow were investigated in nonpregnant, castrated ewes with chronicaljy implanted polyvinyl catheters and electromagnetic flow probes. Intra-arterial infusion of PGI, resulted immhdiately in increased uterine blood flow, which returned rapidly to baseline after tenninatibn of the In,fusion. A dose-response curve was constructed and demonstrated that a uterine arterial PGla concentration of 1.2 times 10-W (0.5 &ml) produced flows 50% of those achieved by a deference dose of I pg of 17@estradiil. Increases in uterine blood flow were n@ associated with changes in rriean systemic arterial pressure. These findlngs demonstrate that PGI, has a vasodilatory effect on the uterine circulation and suggest that this substance has a role in the regulation of uterine blood flow. (AM. J. OBSTET. GYNECOL. 137:267, 1980.)
THE prostaglandins are ubiquitous chemical agents that exert a variety of physiologic and pharmacologic effects in numerous organs. Previous studies from our laboratory have shown that prostaglandir+ El and EZ are potent uterine vasodilators, and we have suggested that the local synthesis and release of prostaglandins may be involved in the regulation of uterine blood flow.’ More recently, a new prostaglandin, prostacyclin (PGIp), has been described as a product of the transformation of prostaglandin endoperoxides in blood vessels.2 This relatively unstable substance is generated in the endothelial cell layer of blood vessels in numerous species.5 including man.* Its major properties are the inhibition of platelet aggregation and the initiation of vasodilation in many circulations, including the mesenteric, femoral, renal, and coronary.5* 6 Since little is known about the uterine vascular response to PG12, we used a nonpregnant, chronically instrumented sheep model in order to determine the quantitative characteristics of changes in uterine blood flow after the intra-arterial administration of PG12. From the Department of Reproductive Medicine, Uniwrsit~ of Calzyornia, San Diego Srhwl of Medicin?. This investigation was su#mrted by United States Public Health Service Grant No. I ROI HD 115~4-01AI and a grant from the National Foundation-March of Dimes. Presented by invitation at the Forty-sixth AnnuaE Meeting of the Pa@ Coast Obstetrical and Gynecological Society, Palm Springs, Calijorni& September 25-30, 1979. Reprint requests: Robert Resnik, M.D., Departmat’of Reproductive Medicine, University Hospital H-813, Dickinson St., San Diego, Califoonzia 92103 0002-9378/80/100267+04$00.40/0
0 1980 The C. V. Mosby
Msterfsf and methods Four nonpregnant cross-bred sheep were used during the investigation. The surgical procedure was described in previous reports.‘. s* y After sedation with intravenous sodium pentobarbital, 640 mg, a spinal anesthetic (Pontocaine hydrochloride, 6 mg) was administered. Pelvic laparotomy was performed through a midline incision, and the anterior leaves of the broad ligament were incised so as to expose the uterine arteries and their branches. Polyvinyl tubing (,inner diameter, 0.58 mm; outer diameter, 0.96 mm) was used to catheterize the most lateral branch of the uterine artery, and was advanced retrograde to within 0.5 cm of the bifurcation. Three-millimeter electromagnetic flow probes* were placed around the uterine arteries proximal to the bifurcation. An additional polyvinyl catheter was inserted into the femoral artery in order to measure systemic arterial pressure. After oophorectomy, which was performed to remove the source of most of the endogenous estrogen produced. the catheters and flow probes were brought through a fascial incision and a subcutaneous tunnel and stored in a canvas pouch on the animal’s flank. Postoperatively, the sheep were placed in a rectangular stainless steel stall and kept in the laboratory for the duration of the studies. Uterine arterial flow was measured with Micron R.C. 1000 electromagneric flowmeters.* Prior to daily experiments, the flow signals were calibrated to electrical zero. Flow signals were electronically integrated and displayed on a pen recorder
225 *Micron Co.
Instrument
Company,
Los Angeles,
California.
267
'268
Resnik
and Brink
PGI, 8 +g/min
f2178 IP9
200 MEAN UTEWE BLooD FLOW r&kin
I50 100 W J
s++-v-
0
30
.--J
0
60
90
120
150
180
MINUTES
1. Comparative uterine arterial flow response to intra-arterially 17@estradiol.
Fig.
lOOr
*
OL
10-l
I lo-"
J
10-S
AflTERlALCONCENTRATiON(Mj
Fii. 2. Linear regression of percent maximum uterine blood flow produced by 1 pg of 17&estradiol versus log,, PGIS arterial concentrations (M) determined by method of least squares. Percent maximum flow = 245.6 + 33.0 loglo PGI, concentration (MS. Arterial concentrations were calculated with baseline uterine blood flows in each experiment prior to infusion of PGI,. Each Point represents mean ? SEM of at least 3, and as many as IO, observations. running at 1 mm/minute. Systemic arterial pressures were monitored with Statham P23-Db pressure transducers. Previous studies had demonstrated that maximum uterine vasodilation occurred after a pulsed dose of 1 pg of 17/3-estradiol injected directly into the uterine artery, and that peak flows are reproducible on a daily basis after the fifth postoperative day.g Accordingly, all experiments were performed after this period of postoperative recovery. The 17/S-estradiol* was of reagent grade quality and was prepared for intra-arterial injection by dissolving it in absolute ethanol and diluting the stock solution with 0.9% saline solution. The final concentration of estrogen in the working solution was 1 pg/ml. The final ethanol concentration in the solution was 0.1% by volume. This has been previously shown to have no effect *Sigma Chemical Company, St. Louis, Missouri.
administered
prostacyclin and
on uterine blood how in this model. Prostacyclin” was obtained as the sodium salt. Stock solutions were prepared daily by dissolving the salt in 1 M Tris buffer (pH 8.0) at 3” C, with a final prostacyclin concentration of 1 mg/ml. Working solutions were prepared from aliquots of the stock solution diluted in 50 mM Tris buffer (pH 7.7) on ice. A dose-response curve was constructed to determine what concentrations of prostacyclin would produce minimum and maximum dilatory vascular responses. Since previous studies 8. 9 have shown that a pulsed uterine intra-arterial dose of 1 pg of 17&estradiol produces a maximum dilatory response, the effects of prostacyclin were compared to the maximum blood flows produced by the bolus injection of estrogen. Prostacyclin was infused by means of a constant-infusion pump, and arterial concentrations were calculated on the basis of the control blood Row prior to the uterine intra-arterial infusion, infusate concentration, and rate of administration. PGIy was infused at concentrations ranging from 1 to 64 @g/ml at an infusion rate of 0.247 ml/minute. Infusions of 0.9% saline solution into the contralateral uterine vessels were used as controls.
Results Immediately after intra-arterial infusion of PG12, uterine blood flow began to increase, and reached maximum flows within 2 to 3 minutes, with a rapid return to baseline rate of flow after termination of the infusion. This is contrasted with the uterine vascular response to a pulsed dose of 17@estradiol, which is characterized by a 30-minute delay after the dose, and then the uterine blood flow increases and reaches a maximum at 90 to 100 minutes after injection. Return to baseline flow is linear and is achieved 6 to 8 hours after injection. An example of response in uterine blood flow to PGIE and 17@-estradiol is shown in Fig. 1. A dose response curve relating uterine blood flows *Upjohn
Company, Kalamazoo, Michigan.
Uterine vascular response to arostacyclin
initiated by PG12 to the percent of maximum blood flow resulting from a dose of 1 pg of 1?@estradiol is shown in Fig. 2. A PGI* uterine arterial concentration of 1.2 times IO-” M (0.5 pgiml) resulted in flows 50% of that produced by the reference estrogen dose. No changes in uterine blood How occurred in response to infusions of 50 mM Tris buffer, and no changrs in mean systemic arterial pressure were observed during infusion of PGI,.
Comment The vasodilator effects of PGIz have been demonstrated in many species. Dusting and associates” observed significant decreases in local perfusion pressure after infusion of 0.02 to 2.0 pg of PGIZ into the mesenteric and femoral arteries of anesthetized dogs, doses similar to those which increased uterine blood How in our nonpregnant sheep model. PGIz has also been shown to dilate bovine coronary arteries,‘O and to decrease systemic blood pressure in dogs” and ratsI after intravenous administration. These latter findings suggest that, unlike several other prostaglandins which are rapidlv inactivated by passage through the pulmonaq circulation,“’ PGIz is not. In the sheep, PGIZ is not so potent a uterine vasodilator as prostaglandin E,. We previously showed that a uterint. arterial PGE, concentration of 7.52 times IO-“. a concentration more than one order of magnitude less than that of PGIZ, will increase How to 50% of that produced by a reference dose of 1 PLg of 17pestradiol.’ Armstrong and associates’l have also observed a slightly less potent cardiovascular response to
PGI, than IO PGE, after infusions into the left atria ot’ anesthetized dogs. Although this ma! represent a real difference in biologic activity. it mai’ .tlso rcflcct tht biologic instability of PGI,, which rapidly breaks down in rvaterv solution at 37” C into its stable bur biologicall! less active metabolite. S-oxo-prostaglandin Flcr.” Despite careful pi-eparation of prostacyciin for infusion. at temperatures of 2” or 5” C. one might speculate that exogenouslv administered PGIZ is parti,rlly metabolized in viva to Ii-oxo-PGF,, before it interacts with v:rscular smooth muscle. It is known that PGIs 1%produced in the intimal la)er of blood vessels.:’ Con\equentiv, t hr proximitv of site of‘svnthesis to site 01 action ma\ ah\ PGIp to eiert its physiologic effects on vaaular s’mootlr muscle before significant metabolism occ.ur\. In addition to PGIZ, the cycle-oxygcn~se system metabolizes endoperoxides in platelets I$) thrcmboxane A*. This substance has effects which ale opposite to those of PG12, that is, it promotes vaso~otlstrictiorl and platelet aggregation. These differences in ef‘fects have led Moncada and Vane’ to postulate the presence of a system in which prostacvclin and thromhoxane Ax produce opposite effects in controlling blood How and platelet aggregation within local circulati~)ns. The role which prostaqclin may play in the local regulation of uterine blood How remains to be elucidatctl. but existing evidence provides ample ration& tot- l‘urther StudY. We are grateful to Dr. John Pike. (11 the Uplohn Cornpan\, for strpplying the prosracvclin uwd in this studv.
REFERENCES
1. Resnik, R.. and Brink, C. W.: Effects of prostaglandins E,, E, and Fpa on uterine blood flow in non-pregnant sheep, Am. J. Physiol. 234:H557, 1978. 2. Moncada, S., Gryglewski, R., Bunting, S., et al.: An enzyme isolated from arteries transforms prostaglandin endoperoxides to an unstable substance that inhibits platelet aggregation, Nature 263:663. 1976. 3. Maclntyre, 1). E., Pearson, J. D., and Gordon, J. L.: Localisation and stimulation of prostacyclin production in vascular cells, Nature 271:549. 1978. 4. Moncada, S.. Higgs. E. A., and Vane, J. R.: Human arterial and venous tissues generate prostacyclin (prostaglandin X), a potent inhibitor of platelet aggregation, Lancet 1:18, 1977. 5. Dusting, G. J., Moncada. S.. and Vane, J. R.: Vascular actions of arachidonic acid and its metabolites in perfused mesenteric and femoral beds of the dog, Eur. J, PharmaCdl. 49:65, 1978. 6. Needleman, P., Bronson. S. B., Wyche, A., et al,: Cardiac and renal prostaglandin I,, J. Clin. Invest. 61:839, 1978. 7. Moncada, S.. and Vane, J. R.: The role of prostacyclin in vascular tissue. Fed. Proc. 38:66, 1979. 8. Killam. A. P.. Rosenfeld. C. R., and Battaglia, F. C.: The
269
9. 10.
11. 12. 13. 14. 15.
effect of estrogens on uterine blood How. .I\w. J. hSTE.r. GYNECOI.. 115:1045, 1973. Resnik, R., Killam, A. P.. Battaglia, F. C., ct al.: The stimulation of uterine blood flow bv variotts estrogens, Endocrinology 94: 1192, 1974. Dusting, G. S.. Moncada, S., and Vane, J. R..:Prostacyclin (PGX) is the endogenous metabolite responsible for relaxation of coronary arteries induced by ararhidonic acid, Prostaglandins 13:3, 1977. Fitzpatrick, T. M., Alter, I., Corey, E. J., et al : Cardiovascular responses to PGI, (prostacyclin) in the dog, Circ Rrs. 42:192, 1978. Weeks, J. R.. and Compton, L. C.: The cardiovascular pharmacology of prostacyclin (PG12) in the rat, Prostaglandins 17:501, 1979. Gorman, R. R.. Bunting, S., and Miller, 0. V.: Modulation of human platelet adenylate cyclase by prclstacvclin (PGX), Prostaglandins 13~377, 1977. Armstrong, J. M., Chapple, D., Dusting, C. J .~et al.: Cardiovascular actions of prostacyclin (PGI,) in chloralosc anesthetized dogs, Br. j. Pharmacol. 61: 136P, 1977. .,lohnson, R. A., Morton, D. R., Kinner. I. 11.. et al.: The chemical characterization of prostaglandin X (prostacyclin), Prostaglandins 12: 1915, 1976.
270
Resnik and Brink
May Am. J.
Discussion
DR.
J. NOW, (by invitation) Beaverton, Oregon. Throughout the reproductive cycle the uterine tissues undergo various physiologic and biochemical changes in which prostaglandins (PC) play an important role. There is also much evidence to implicate the PG system in the process of parturition. A wide variety of tissues possesses the ability to synthesize PGs from arachidonic acid. Their capacity to do so, however, is variable, and the type and quantity of PGs produced depend on many factors. In reproductive tissues, hormonal conditions are especially important. Prostaglandins are generally called “local hormones” because they act within or near the tissues in which they are synthesized. Prostacyclin (PG12) may be the one known exception, since it is apparently not metabolized by the lungs and may serve as a circulating hormone. PGIz is the major product of arachidonic acid in most vascular tissues which have been studied. Incubation studies indicate a high formation of PGIz in the fetal ductus ateriosus, which suggests an important role in maintaining patency of the ductus. The production of prostacyclin has also been identified in human placenta and in myometrium from many different aninial species. Amnion is also a major site of PC& production in pregnancy. Thorburn and Challis reasoned that, if PGIz causes relaxation of vascular smooth muscle, it may also relax the myometrium. Recently, it has been shown that PGIz inhibits spontaneous motility of isolated pregnant human myometrium and reduces the tone induced by PGF,,. The relaxing effect of PGIz was associated with an increase in myometrial cyclic AMP. In the nonpregnant human uterus. the production of prostacyclin is confined primarily to the myometrium, with little activity appearing in the endometrium. If the myometrium and endometrium are incubated together, production of PGI, is enhanced. Presumably, the endometrium makes available to the myometrial synthetase an increased supply of endoperoxides for conversion to PGI, at the expense of PGEz and PGF,,. It has been proposed that the onset of labor is initiated by a diversion of arachidonic acid metabolism from the prostacyclin pathway to the PGEz and PGF,a pathways. Although thromboxane A2 and prostacyclin are formed in substantial amounts by intrauterine tissues during pregnancy, there is little evidence to support this hypothesis. Amniotic fluid levels of 6keto-PGF,,, the stable metabolite of PG12, as well as other PGs are elevated in spontaneous labor, in comparison to levels before the onset of labor. Preliminary data from Oxford suggest a role for PGI, in cervical ripening. Current theories of prostaglandin action in parturition have focused on the idea that the fetus or its membranes produce an inhibitor which, during normal pregnancy, maintains PG synthesis at a low level until term. At term, the inhibition is withdrawn and there is a resultant increase in the production of PG. MILES
15, 1980
Obstet. Gyned
Dr. Resnik’s pI-evocative study has directed OUI. attention to the possible role of PGIz in regulating uterine blood flow. Knowledge of the mechanisms which regulate uteroplacental blood flow is important for understanding implantation. fetal growth, and the endocrine function of the placenta. There is a large body of circumstantial evidence which has implicated the various PGs in the regulation of uteroplacental blood flow, but there is a lack of direct proof that the)- are physiologically important. Dr. Resnik has used the well-known preparation in sheep that was developed in Denver, which allows for chronic measurements of uterine blood flow by electromagnetic Howmeter and for specific intra-arterial administration of small doses of various test substances. He has demonstrated an immediate vasodilator response to graded doses of PG12. In this elegant study he has been able to calculate-the dose of PGIz which was administered directly into the blood vessels in question-in this case, the uterine arterioles. He has shown that prostacyclin produces a hyperemic response which is quantitatively similar to that produced by 17/3estradiol. However, the time course is quite different, with estrogens showing a considerable lag period. It is tempting to speculate that the hyperemic effects of estrogens on uteroplacental blood flow are mediated by an increased synthesis of prostaglandin. but neither this study nor others have as yet provided convincing proof of this connection. According to Dr. Kenneth Clark, now at the University of Cincinnati, indomethatin does not inhibit estrogen-induced increases in uterine blood How in either pregnant or nonpregnant sheep. Similarly, at the Oregon Regional Primate Research Center, we were not able to block with large doses of indomethacin the estrogen-induced ovarian hyperemia in rabbits. Finally, I would like to ask Dr. Resnik whether he has made any observations of PGIz effects on uteroplacental blood How in pregnant animals. In 1974. I presented a paper to this Society which showed that uterine contractions in rhesus monkeys produced placental vasoconstriction in association with myometrial vasodilatation, and suggested that PGs could mediate these differential effects of labor on the uterine and placental vascular beds. Since then, Rankin has shown in pregnant sheep that prostacyclin injected into the left ventricle of the pregnant ewe causes placental vasoconstriction and myometrial vasodilatation. It is possible that the placental vasoconstriction after the infusion of PGIz was secondary to the release of catecholamines induced by maternal hypotension. Taken together with earlier observations, Dr. Resnik’s timely study indicates that naturally occurring PGs have pharmacologic effects on the uterine vasculature. Whether PGs, or prostacyclin in particular, play a major physiologic role in regulating uterine blood flow in primates, sheep, or women remains to be seen.