Somatostatin: peripheral venoconstrictive activity and interaction with monoamines in man

Regulatory Peptides, 18 (1987) 267-276 Elsevier

267

RPT 000608

Somatostatin: peripheral venoconstrictive activity and interaction with monoamines in man Alessandro Panconesi, Pier Luigi Del Bianco, Giancarlo Franchi, Bruno Anselmi and Renato Andreini Institute of lnternal Medicine and Clinical Pharmacology, University of Florence, Florence, Italy (Received 29 December 1986; revised version received and accepted 2 April 1987)

Summary The mechanism of somatostatin venoconstriction and tachyphylaxis in the human hand vein in vivo has been investigated. No cross-tachyphylaxis was observed between somatostatin and 5-hydroxytryptamine, noradrenaline, adrenaline, dopamine or tyramine-induced venoconstriction. Somatostatin potentiates the venoconstrictive activity of noradrenaline, adrenaline and dopamine, but not that of 5-hydroxytryptamine and tyramine. Phentolamine antagonizes the somatostatin-induced venoconstriction, whereas methysergide, haloperidol and morphine do not. It is suggested that somatostatin could act on specific receptors in the hand vein, but the mechanism of somatostatin venoconstriction and interaction with vasoactive monoamines remains to be defined. Human vein; Potentiation; Tachyphylaxis

Introduction Somatostatin has been shown to have multiple effects on the circulatory system. Somatostatin decreases splanchnic blood flow and portal pressure [1-7]. Angiographic studies have demonstrated that somatostatin markedly constricts the splanchnic vessels in anesthetized man [6]. Its beneficial effects in esophageal and gastroduodenal bleeding are partially attributed to these actions on the splanchnic vessels [5,8,9]. Correspondence: A. Panconesi, Institute of Internal Medicine and Clinical Pharmacology, University of Florence, V.le Morgagni 85, 50134 Florence, Italy. 0167-0115/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)

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Somatostatin usually does not modify arterial pressure in animal or in man [5,7-13]; however, there may be an increase in arterial blood pressure and systemic vascular resistance in man [4,14~17]. Somatostatin, as well as its analog SMS 201-995, provokes venoconstriction of the human hand vein in vivo: this effect is subject to constant, rapid and long-lasting tachyphylaxis [18-21]. The mechanism of somatostatin venoconstriction and tachyphylaxis in the hand vein is unknown. Somatostatin could constrict the vein by acting on specific receptors or on receptors of monoamines. We investigated (1) if somatostatin interacts with venoconstriction induced by 5-hydroxytryptamine (5-HT), tyramine (TY), noradrenaline (NA), adrenaline (A) and dopamine (DA); (2) if somatostatin tachyphylaxis is selective or crossed with one of these monoamines; (3) if adrenergic, serotonergic or dopaminergic blockers modify somatostatin-induced venoconstriction (since somatostatin receptor blockers are not available).

Materials and Methods

The hand vein was used as substrate to study the activity of somatostatin in man in vivo, with application of the computerized venotest [22]. Briefly, a 0.8 mm caliber needle, provided with a rubber membrane, was orthodromically inserted into the vein. When the test was performed on a different day, a new puncture was made in the same vein. The needle was connected by a Teflon tube to an electric pump and a strain gauge. In order to avoid blood flowing into the needle, the pump infused saline at a constant speed (0.5 ml/min). Venoconstrictive agents, when inoculated through the rubber membrane of the needle, provoked an increase in local vein pressure. The signal of the strain gauge amplified 15 times and computerized by an electronic integrator was recorded. The entity of the venoconstriction was conventionally evaluated in venoconstriction units (VCU): one VCU corresponds to an increase of 1 mm Hg/s. This method is one of the more suitable tools for exploring the neurovascular junction in man and has provided interesting information in some physiological and pathological conditions. This method shows a satisfactory level of sensitivity and reproducibility. It is a typical clinical pharmacological method because it offers little discomfort to the patient while permitting the collection of data from low doses of drugs provoking only local activity.

Participants Forty-seven subjects (28 females, 19 males) ranging in age from 18 to 65 years (mean + S.E.M. age 35 + 3), hospitalized for migraine were studied in the pain-free period (where the vein sensitivity to monoamines is not different from that of healthy subjects). In almost all of these subjects, two investigations were performed. Migraine patients rarely show tachyphylaxis to 5-HT and TY venoconstriction, which is present in healthy subjects [23,24]. The study was then performed in 6 healthy volunteers (4 males, 2 females) ranging in age from 21 to 46 years (mean + S.E.M. age 31 + 2), in which it may be investigated whether or not 5-HT and TY tachyphylaxis are

269 also crossed with that of somatostatin. Procedures and purpose of the experiment were fully explained to all participants and informed consent was obtained. All drugs were discontinued 7 days before the beginning of the study.

Experimental procedure Somatostat& interaction on monoamine-&duced venoconstriction. On the first day venoconstrictive responses to one of the following monoamines were tested in 28 migraine patients: 5-HT (1-2 #g), TY (500 #g) NA (1 #g), A (1 #g), DA (10 #g). The following day, the same amounts of drug were injected 10 min after somatostatin (10 #g) administration in the same hand vein, when somatostatin-induced venoconstriction was terminated. Selective or crossed somatostatin tachyphylaxis. In order to investigate if somatostatin tachyphylaxis is selective or crossed with monoamines, 5-HT, TY, NA, A or DA at the previous doses were injected in the somatostatin tachyphylactic vein of 37 migraine patients. Somatostatin tachyphylaxis was considered developed when the somatostatin-induced venoconstriction (10 #g, administered every 10 min) was reduced by 70-100%. The venoconstrictive responses to each monoamine, injected 10 min following the last somatostatin administration have been compared with those obtained before somatostatin. Furthetrnore, somatostatin (10 #g) was administered to 6 healthy subjects in the hand vein completely desensitized to 5-HT- (1 #g) or TY (500/~g)-induced venoconstriction. Modification of somatostatin-induced venoconstriction by adrenergic, serotonergic and dopaminergic blockers. The phenomenon of tachyphylaxis which rapidly developed when somatostatin was repeatedly injected, represented an obstacle for this study. To overcome this difficulty, in each of 22 migraine patients the test was carried out on different days: first, the venoconstrictive activity of somatostatin (10 #g) was evaluated; on the following days the antagonistic activity of phentolamine (0.1-1 mg), methysergide (10 #g), haloperidol (10 #g) or morphine (2 mg), against the same doses of somatostatin, was tested. Each one of these drugs was inoculated in the same hand vein 2 min before somatostatin administration. These doses of phentolamine, haloperidol and methysergide have been shown to respectively counteract the NA, DA and 5-HT-induced venoconstriction. Morphine was used because it antagonizes 5HT and TY-induced venoconstriction in the hand vein, possibly by inhibiting the release of NA from sympathetic neuronal endings [25,26]. Each experiment was performed from 9.00 to 12.00 h. All drugs were dissolved in saline and administered as a bolus of 0.5 ml. Statistical analysis Student's t-test for paired data was used. Drugs The following drugs were used: somatostatin (Stilamin, Serono), phentolamine (Regitin, Ciba), methysergide (Deserril, Sandoz), haloperidol (Serenase, Lusofarmaco), dopamine (Revivan, Simes), and morphine, 5-HT NA, A, TY.

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Results (1) The venoconstrictive activity of NA, A, DA, but not that of 5-HT and TY, is potentiated by somatostatin (Table I, Fig. 1). This potentiating effect is transitory: venoconstrictive responses obtained 20 rain after somatostatin administration are not different from baseline. (2) In the vein when somatostatin tachyphylaxis developed (after 4-6 somatostatin

TABLE I The venoconstrictive activity of m o n o a m i n e s before and 10 min after somatostatin administration Drug

Cases

Before

5-HT TY NA A DA

6 6 6 5 5

1473 1340 320 351 925

44± 4±

After 459 401 170 256 314

1452 1435 687 976 1751

t-Test 44± ± 4-

335 340 166 261 383

P P P P P

> > < < <

0.05 0.05 0.01 0.001 0.01

The values represented in this and successive tables are expressed in V C U (1 V C U = 1 m m Hg/s; means ± S.E.M.).

0

BASELINE

BASELINE

!

BAS~ELINE

10 min

t

t

t

0

10 m|n

O

I

o

10 min

!

Fig. 1. An example of venoconstriction test: the potentiating effect o f somatostatin (SRIF) on NA, A and D A venoconstriction (upper, middle and lower part of the figure, respectively) is represented. In the upper part of every tracing the vein pressure is reported. It increases when venoconstrictive drugs are locally injected. The lower part of the tracing represents the integral of this enhanced vein pressure, measured conventionally in V C U (1 V C U corresponds to the increase of 1 m m Hg/s). This system is calibrated so that 1 m m corresponds to 10 VCU. The sharp and brief rise in pressure at the beginning of every administration is due to the bolus of the substance locally injected in the vein.

271 a d m i n i s t r a t i o n s ) the 5 - H T or T Y - i n d u c e d v e n o c o n s t r i c t i o n is n o t reduced. Conversely, the N A , A a n d D A - i n d u c e d v e n o c o n s t r i c t i o n is p o t e n t i a t e d (Table II). I n the vein of the 6 healthy subjects, completely desensitized to T Y (3 cases) or 5 - H T (3 cases), the s o m a t o s t a t i n - i n d u c e d v e n o c o n s t r i c t i o n is n o t reduced (respectively from 1890 4- 460 a n d 1645 4- 110 V C U to 2100 + 450 a n d 1637 + 77 V C U o f pretest time). The vein reactivity to s o m a t o s t a t i n in healthy subjects did n o t differ from that in m i g r a i n e patients. (3) O n l y p h e n t o l a m i n e (1 mg) reduces s o m a t o s t a t i n - i n d u c e d v e n o c o n s t r i c t i o n . M o r p h i n e , methysergide a n d haloperidol do n o t c o u n t e r a c t s o m a t o s t a t i n v e n o c o n strict±on (Table III).

TABLE II The venoconstrictive responses of monoamines obtained after somatostatin-tachyphylaxis development were compared with those obtained before somatostatin Drug

Cases

Before

After

5-HT TY NA A DA

10 12 5 5 5

1595 1287 345 443 910

1506 ± 1469 ± 768 ± 1005 ± 1682 ±

± 548 ± 441 ± 175 4- 227 ± 311

t-Test 315 303 144 292 371

P P P P P

> > < < <

0.05 0.05 0.01 0.005 0.01

There is no cross-tachyphylaxis between somatostatin and monoamine-induced venoconstriction. Means + S.E.M.

TABLE III Venoconstrictive responses to somatostatin (10 /~g), administered before and 2 min after monoamine antagonistic drugs, are represented Drug

Dose (mg)

Cases

Before

After

Phentolamine Phentolamine Morphine Methysergide Haloperidol

0.1 1 2 0.01 0.01

4 6 6 3 3

1525 + 179 1931 + 445 1892 + 312 1630 + 197 1578 4- 201

971 440 2011 1835 1626

t-Test + 325 ± 80 ± 559 4- 236 + 172

P P P P P

> < > > >

0.05 0.05 0.05 0.05 0.05

Values are means + S.E.M.

Discussion S o m a t o s t a t i n exhibits circulatory effects, especially at the s p l a n c h n i c level. Som a t o s t a t i n is the only peptide tested which has a p o t e n t venoconstrictive activity o n the h a n d vein [19]. The aim o f the present study was to investigate the m e c h a n i s m of s o m a t o s t a t i n - i n d u c e d v e n o c o n s t r i c t i o n a n d its putative i n t e r a c t i o n s with m o n o a m -

272 ine-induced venoconstriction. This interaction can be hypothesized since somatostatin is costored with catecholamines in some sympathetic neurons [27,28]. In addition, since somatostatin increases the tyrosine hydroxylase activity in the superior cervical ganglia [29], it can influence the NA synthesis in sympathetic neurons. From our study 3 points emerged: (1) Somatostatin potentiates the venoconstriction elicited by NA, A and DA. Conversely, somatostatin does not significantly modify the 5-HT- and TY-induced venoconstriction. Another peptide, the neuropeptide Y, which like somatostatin seems costored with NA in sympathetic neurons, provokes a potent vasoconstriction which is subject to tachyphylaxis [30,31]. Neuropeptide Y potentiates NA and A-induced vasoconstriction but not that provoked by spontaneous or electrically induced release of NA, suggesting that this sensitizing effect is probably due to a postsynaptic mechanism [31,32]. The neuropeptide Y potentiation on amine activity possibly occurs via inhibition of adenylate cyclase or mobilization of intracellular calcium [33,34]. We cannot say how somatostatin sensitizes the monoamine-induced venoconstriction in hand vein. As for the neuropeptide Y, an inhibition of adenylate cyclase could be invoked, since somatostatin inhibits the cyclic AMP formation [35]. The fact that this sensitizing effect is present in the same amount in hand vein fatigued by somatostatin and that it does not concern all monoamines, makes it improbable that an increase in venous tone per se could lead to a relatively non-specific increase in responsiveness. (2) The somatostatin tachyphylaxis is selective, since the venoconstrictive activity of NA, A, DA, 5-HT and TY is not decreased in the somatostatin-fatigued vein. On the other hand, the somatostatin venoconstriction is unchanged in the hand vein completely desensitized to 5-HT or TY, which like somatostatin are subject to tachyphylaxis [23,24]. However, in a few cases previously reported [36], the response of 5-HT and TY was reduced in the somatostatin-desensitized vein: this could be attributed to the interval between administration of these amines that was too brief and therefore insufficient to avoid the development of their tachyphylaxis. Even if somatostatin tachyphylaxis was often described, the mechanism of its development is unknown [3744]. A receptoral desensitization may be involved in the reduction of the ability of somatostatin to inhibit cyclic AMP accumulation and ACTH release from mouse anterior pituitary tumor cells [35]. A receptoral desensitization or saturation is considered to be implicated even in the tachyphylaxis of substance P and angiotensin activity [45,46]. Moreover, brain microvessels rapidly sequester and degrade somatostatin analogues by aminopeptidase action [47]: this mechanism could be important in the somatostatin tachyphylaxis in the hand vein. For the phenomenon of tachyphylaxis of somatostatin, 5-HT and TY, we have previously suggested a common mechanism. A homeostatic opioid mechanism may be concerned, since naloxone is capable of restoring (more or less completely) the venoconstricitive activity of these drugs in the fatigued vein. One can hypothesize that these drugs act by releasing NA from sympathetic neuronal endings: their repeated administration will provoke a progressive stimulus on a release-inhibiting opioid system, with consequent inhibition of NA release and venoconstriction. Naloxone, counteracting this opioid activity, could restore the ve-

273

noconstrictive capacity of these drugs [19,23]. (3) Phentolamine partially reduces the somatostatin-induced venoconstriction, while morphine, methysergide and haloperidol do not. Even the actions of somatostatin on insulin and glucagon release and on gluconeogenesis are reported to be inhibited by phentolamine [16,48-50], stressing an involvement of 0t-adrenergic receptors. However, there is also contrary evidence for an inhibitory action by 0t-antagonists on these and other somatostatin activities [39,41,51 56]. It seems that somatostatin has an inhibiting or facilitating activity on smooth muscle contraction by acting at the presynaptic level [37,39,40,55-57]. Particularly, somatostatin has a stimulatory effect on NA release in the cerebral cortex and adrenal gland [56,57] and an inhibitory effect in vas deferens and ear artery [40,55]. In conclusion, the present data demonstrate that the somatostatin-induced venoconstriction is partially reduced by phentolamine and is subject to selective tachyphylaxis. Moreover, somatostatin has a sensitizing effect on NA, A and DA-induced venoconstriction. These results, therefore, seem to indicate that somatostatin constricts veins with two possible mechanisms: (1) by releasing NA from neuronal endings, reacting on presynaptic receptors which are not inhibited by 5-HT and DA antagonists or opioid agonists; (2) by acting at the postsynaptic level, on one hand stimulating the ~-receptors (an effect blocked by phentolamine), on the other hand acting on different receptors which are neither serotonergic nor dopaminergic. Nevertheless, we are not able to establish the characteristics or the specificity of these receptors.

Acknowledgements This work was supported by a grant from the National Council for Research (CNR), Rome, progetto finalizzato 'Chimica Fine e Secondaria'. We are greatly indebted to Ms. Ann Marchesano for the revision of the English text.

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