Apelin effects in human splanchnic arteries. Role of nitric oxide and prostanoids

Apelin effects were examined in human splanchnic arteries from liver donors (normal arteries) and from liver recipients. Segments 3 mm long were obtained from mesenteric arteries taken from liver donors (normal arteries), and from hepatic arteries taken from cirrhotic patients undergoing liver transplantation (liver recipients), and the segments were mounted in organ baths for isometric tension recording. In arteries under resting conditions, apelin (10(-10)-10(-6) M) caused no effect in any of the arteries tested. In arteries precontracted with the thromboxane A(2) analogue U46619 (10(-7)-10(-6) M), apelin (10(-10)-10(-6) M) produced concentration-dependent relaxation that was lower in hepatic than in mesenteric arteries, whereas sodium nitroprusside (10(-8)-10(-4) M) produced a similar relaxation in both types of arteries. The inhibitor of nitric oxide synthesis N(w)-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) diminished the relaxation to apelin in mesenteric but not in hepatic arteries. The inhibitor of cyclooxygenase meclofenamate (10(-5) M) did not affect the relaxation provoked by apelin in both types of arteries. Therefore, apelin may produce relaxation in normal human splanchnic arteries, and this relaxation may be mediated in part by nitric oxide without involvement of prostanoids. This relaxation as well as the role of nitric oxide may be decreased in splanchnic arteries from cirrhotic patients.     

Effects of prostaglandins and arachidonic acid on baboon cerebral and mesenteric arteries

Effects of prostaglandins (PGs) E1, E2, F2 alpha and I2 in a wide range of concentration were examined in mesenteric and cerebral arteries isolated from mature baboons. PGs E1, E2 and F2 alpha at low concentrations (10(-10) to 10(-7) M) elicited relaxation in helically cut strips of cerebral arteries precontracted with phenylephrine. In contrast, the PGs did not cause relaxation in the mesenteric artery. PGI2 (10(-9) to 10(-6) M) produced marked relaxation in both arteries. The EC25 for PGI2 in the mesenteric artery was significantly lower than that in the cerebral artery. During baseline conditions, cerebral arteries contracted in response to high concentrations (greater than 10(-7) M) of PGs E1, E2 and F2 alpha. In mesenteric arteries, a large contraction was induced by PGs F2 alpha and E2 but not by PGE1. Arachidonic acid (10(-6) M) produced an aspirin-inhibitable relaxation in both arteries to a similar extent, so that the vasodilator PG(s) formed in the two different arterial walls appear to exert a similar relaxant action. Thus, the baboon mesenteric artery was more sensitive to PGI2 for the relaxant effect than was the cerebral artery, while PGs F2 alpha, E1 and E2 caused only a contraction in the mesenteric artery but both relaxation and contraction in the cerebral artery.     

Differential vascular actions of ethanol in feline middle cerebral and mesenteric artery.

The vascular actions of ethanol on feline middle cerebral and mesenteric arteries were investigated in vitro. Ethanol (20-500 mM) caused potent contraction in cerebral arteries, but it contracted the mesenteric arteries only weakly. In the middle cerebral artery (but not in the mesenteric artery) ethanol (300 mM) potentiated the noradrenaline (5.10(-6) M) induced contractions. Antiserum for endothelin (in an appropriate concentration to inhibit endothelin-induced contraction; 0.02 mg/ml) did not inhibit the ethanol-induced contractions. Endothelium-dependent relaxations induced by acetylcholine and ATP were also affected by ethanol (300 mM); in the cerebral artery acetylcholine- but not ATP-induced relaxations, whereas in the mesenteric artery ATP- but not acetylcholine-induced relaxations were inhibited significantly. The results suggest that ethanol causes strong (endothelin-independent) contraction and facilitates the response to noradrenaline in the middle cerebral, but not in the mesenteric artery; and it selectively inhibits endothelium-dependent relaxation. These actions of ethanol may contribute to the development of vascular diseases.     

Role of NPY for vasoregulation in the splanchnic circulation during portal hypertension

Vascular dysfunction in the splanchnic circulation during portal hypertension is characterized by enhanced NO-mediated vasorelaxation and vascular hyporeactivity to norepinephrine that lead to arterial vasodilation. NPY most likely counteracts both of these key features. Firstly, NPY appears to inhibit Ach- and PNS-induced vasorelaxation in mesenteric arteries. This effect is more pronounced in portal hypertensive rats as compared to control, and most likely reflects the inhibition of increased e- and nNOS-derived NO-synthesis during portal hypertensive conditions. Secondly, NPY sensitizes the mesenteric vasculature to alpha(1)-adrenergic vasoconstriction. Most importantly, in portal hypertensive rats but not in sham rats NPY markedly augments vascular contractility and thereby corrects vascular hyporeactivity. Both actions of NPY increase vascular tone and may well act synergistically in the splanchnic circulation during portal hypertension. Moreover, the vasoconstrictive effects of NPY are most pronounced at particularly high levels of alpha(1)-adrenergic activity. Therefore, it appears that NPY becomes increasingly important for optimizing adrenergic vasoconstriction at particularly high adrenergic drive and also for playing a predominant role for vascular homeostasis. Cirrhotic patients present with elevated circulating plasma levels of NPY, which appears to be independent from the severity of liver dysfunction and to correlate with portal pressure. This finding indicates enhanced NPY release during portal hypertension that may represent a compensatory mechanism aimed at counterbalancing arterial vasodilation by restoring the efficacy of endogenous catecholamines and inhibiting vasodilative drive in the splanchnic circulation.     

Inhibitory effects of PGD2, PGJ2 and 15-deoxy-delta12,14-PGJ2 on iNOS induction in rat mesenteric artery

PGD2 and its metabolites PGJ2 and 15-deoxy-delta12,14-PGJ2 have been reported to inhibit iNOS induction in cultured vascular smooth muscle cells. The present study was undertaken to determine whether these prostanoids inhibit iNOS induction in the isolated rat mesenteric artery. The artery without endothelium was incubated with and without lipopolysaccharide (LPS) at 37 degrees C for 6 hrs, then washed and mounted in an organ bath to measure isometric changes in tension. L-arginine but not D-arginine (10(-6) - 10(-3) M) induced concentration-dependent relaxations only in the artery preincubated with LPS, the relaxations of which were attenuated by L-N(G)-nitroarginine methyl ester (LNAME, 10(-4) M), a non-selective iNOS inhibitor, and 1400W (10(-5) and 10(-4) M), a selective iNOS inhibitor. Co-treatment of cycloheximide (10(-5) M), a protein synthesis inhibitor, or actinomycin D (10(-7) M), an RNA synthesis inhibitor with LPS inhibited the development of relaxing ability in response to L-arginine, indicating iNOS induction by LPS. PGD2, PGJ2 and 15-deoxy-delta12,14-PGJ2 but not PGE2, PGI2 or PGF2alpha also inhibited the development of relaxing ability in response to L-arginine when added during incubation with LPS. Incubation of the artery with LPS at 37 degrees C for 6 hrs markedly increased production of nitric oxide (NO), which was abolished by 15-deoxy-delta12,14-PGJ2 (10(-5) M). An imunohistochemical study using antibody against murine iNOS showed that 15-deoxy-delta12,14-PGJ2 (10(-5) M) inhibited the expression of iNOS protein in isolated rat mesenteric arteries. These results demonstrated that PGD2 and its metabolites inhibit iNOS induction by LPS in isolated rat mesenteric arteries, resulting in reduced relaxing ability in response to L-arginine.     

Regional differences in the contractile activity of neuropeptide Y, endothelin, oxytocin and vasopressin: comparison with non-peptidergic constrictors. An in vitro study in the basilar and mesenteric arteries of the rat.

The vasoconstrictor effect of the peptides neuropeptide Y (NPY), endothelin (ENDO), vasopressin (VPR) and oxytocin (OXY) (10(-11)-10(-7) M) was compared in the isolated basilar (BAS) and mesenteric (MES) arteries of rat. The contractile activity of these peptides was compared to that of three nonpeptidergic constrictors: noradrenaline (NA), serotonin (5-hydroxytryptamine, 5-HT) and prostaglandin F2 alpha (PGF2 alpha) (10(-8)-10(-4) M). As regards EC50 values, PGF2 alpha was equally potent in both vessels studied, 5-HT was more potent in BAS and NA was without contractile effect in BAS. Pronounced regional differences were found for the peptides studied. BAS was more sensitive in EC50 values to the peptides in the order ENDO > or = VRP > OXY > NPY. In MES, OXY and NPY caused no and VPR caused weak contraction, whereas the effect of ENDO was pronounced, with a similar EC50 value as in BAS. In conclusion, marked regional differences were found in response to contractile agents in the vascular beds studied. Peptidergic constrictor mechanisms might be of large importance in the regulation of cerebral blood flow during physiological or pathophysiological conditions.     

Heterogeneity in responses of isolated monkey arteries and veins to atrial natriuretic peptide

Regional differences in responses of isolated monkey arteries and veins to atrial natriuretic peptide were investigated by recording isometric tension. Addition of atrial natriuretic peptide (4 X 10(-12) to 4 X 10(-8) M) produced a concentration-dependent relaxation in isolated monkey arteries and veins. No significant difference was observed between the responses to rat and human atrial natriuretic peptides. A marked heterogeneity in responses to rat atrial natriuretic peptide, however, was observed in arterial preparations. The decreasing order of the response was as follows: renal greater than pulmonary greater than femoral = mesenteric greater than coronary greater than middle cerebral greater than basilar arteries. A heterogeneity in the relaxation produced by atrial natriuretic peptide was also observed in monkey veins. The decreasing order of the response was as follows: pulmonary greater than mesenteric = portal greater than femoral greater than renal = inferior caval veins. On the other hand, 10(-5) M sodium nitroprusside caused a maximal relaxation in all monkey arteries and veins used. In the middle cerebral, basilar, and coronary arteries, the relaxant effects of rat atrial natriuretic peptide on KCl-induced contraction were significantly smaller than those on the preparations contracted by an agonist such as prostaglandin F2 alpha. These results suggest that there exist profound regional vasorelaxant selectivities of atrial natriuretic peptide in isolated monkey arteries and veins.     

Neuroeffector actions of prostaglandin D2 on isolated dog mesenteric arteries

Treatment with prostaglandin (PG) D2 in concentrations (10(-8) to 10(-7) M) insufficient to alter the basal tone potentiated the contractile response of helical strips of dog mesenteric arteries to transmural electrical stimulation but did not influence the response to norepinephrine. The potentiating effect of PGD2 was not prevented by treatment with diphloretin phosphate, a PG antagonist, whereas contractions of dog cerebral arteries induced by PGD2 were suppressed. The 3H-overflow evoked by transmural stimulation in superfused mesenteric arterial strips previously soaked in 3H-norepinephrine containing media was significantly increased in PGD2. It is concluded that PGD2 increases the stimulation-evoked release of norepinephrine from adrenergic nerves innervating the arterial wall. PGD2 appears to act differently on receptive sites responsible for increasing the release of norepinephrine and for producing arterial contraction.     

Endothelium-dependent relaxations to histamine in the rat common carotid, renal and cranial mesenteric artery

The effect of histamine on the isolated rat common carotid, renal and cranial mesenteric arteries was examined. Histamine (10(-8)-10(-4) M) caused concentration-dependent relaxations of the arteries during contractions induced with phenylephrine (10(-8)-10(-7) M). Removal of the vascular endothelium inhibited the histamine-induced relaxations. Pyrilamine (6 X 10(-6) M), but not metiamide (10(-6) M), abolished the relaxant effect of histamine. Moreover, pyrilamine (6 X 10(-6) M) did not affect endothelium-dependent relaxations of the arteries produced with acetylcholine. These results indicate that histamine causes endothelium-dependent relaxations of the rat peripheral large conduit arteries, which appeared to be mediated via H1-histaminergic receptors.     

Existence of two types of postjunctional alpha-adrenoceptors in the isolated canine internal carotid artery

The pharmacological characteristics of postjunctional alpha-adrenoceptors in isolated canine internal carotid arteries were investigated by the use of selective agonists and antagonists for alpha 1- and alpha 2-adrenoceptors. Norepinephrine, phenylephrine, and xylazine caused concentration-dependent contractions in the helical strips. The contraction induced by 10(-4)M xylazine was significantly smaller than that produced by 10(-4)M norepinephrine or 10(-4)M phenylephrine. The contraction induced by 10(-4)M phenylephrine was almost the same value as that induced by 10(-4)M norepinephrine. Phentolamine (10(-8) and 10(-7)M) caused a parallel shift to the right of the concentration-response curve to norepinephrine. The contractile responses to low concentrations of norepinephrine were significantly suppressed by pretreatment with an alpha 2-antagonist such as yohimbine (10(-9) and 10(-8)M) or DG5128 (10(-7) and 10(-6)M). On the other hand, the responses to higher concentrations of norepinephrine were mainly reduced by low concentrations of an alpha 1-antagonist, prazosin (3 x 10(-10) and 3 x 10(-9)M). These results suggest that both alpha 1- and alpha 2-adrenoceptors are located on the plasma membrane of smooth muscle cells in canine internal carotid arteries and that the norepinephrine-induced contractions at low and high concentrations are mainly mediated by activation of alpha 2- and alpha 1-adrenoceptors, respectively.     

Essential role of calcium influx in the adrenergic regulation of cAMP and cGMP in rat pinealocytes

The role of Ca2+ in the adrenergic stimulation of pinealocyte cAMP and cGMP was investigated. In this tissue alpha 1-adrenoceptor activation, which by itself is without effect, potentiates beta 1-adrenergic stimulation of cAMP and cGMP 30- to 100-fold. The present results indicate that chelation of extracellular Ca2+ with EGTA or inhibition of Ca2+ influx with inorganic Ca2+ channel blockers (La3+, Co2+, Mn2+) markedly reduces the cyclic nucleotide response to norepinephrine, a mixed alpha 1- and beta-adrenergic agonist, but not to isoproterenol, a beta-adrenergic agonist. In addition, the potentiating effects of alpha 1-adrenergic agonists were mimicked by agents which elevate cytosolic Ca2+, including K+ (EC50 = 2 X 10(-2) M), ouabain (EC50 = 2 X 10(-6) M), ionomycin (EC50 = 3 X 10(-6) M), and A23187 (EC50 = 2 X 10(-6) M); each potentiated the effects of beta-adrenergic stimulation but had no effect alone. Together these results indicate that an alpha 1-adrenoceptor-stimulated Ca2+ influx is essential for norepinephrine to increase pinealocyte cAMP and cGMP.     

Pharmacokinetic analysis of alpha- and beta-adrenoreceptors in the chick embryo amnion

Following a stimulation with acetylcholine, the beta-adrenergic agonists adrenaline (A), noradrenaline (NA), isoproterenol (Iso) and salbutamol (Sal) induced a concentration-dependent decrease in the tone and (or) rate of amnion contraction with EC50 ISO < NA < A < Sal. Metaprolol, a specific beta 1-antagonist, induced a rightward shift in the dose-response curves of Iso, NA and A, whereas beta-antagonist butoxamine was ineffective. pA2 values for beta-antagonists were propranolol 8.3, metoprolol 7.0, butoxamine 5.6. EC50 values of alpha-adrenergic agonists form a sequence: clonidine < NA < methoxamine < phenylephrine. Specific alpha-antagonists yohimbine and idazoxan were found to antagonise competitively the effects of NA. The data obtained characterize the adrenergic receptors mediating stimulation of amniotic contractile activity as alpha 2-adrenergic receptors. Inhibition of contractile receptors in amnion is mainly mediated by beta 1-adrenergic receptor activation.     

Alpha 1 adrenergic receptor function in senescent Fischer 344 rat aorta

There have been numerous conflicting reports concerning alpha 1 adrenergic receptor-mediated blood vessel contraction during aging and possible changes in alpha 1 receptor transduction mechanisms have not been investigated. These studies assess capacity of the aging vascular alpha 1 receptor to stimulate production of inositol phosphates, which are its intracellular second messengers, and to elicit a contractile response via this pathway. Aortic ring segments from mature adult (6 month old) and senescent (24 month old) Fischer 344 rats were incubated with [3H]myo-inositol and then stimulated with the alpha 1 agonist norepinephrine (NE, 10(-7)M-3 x 10(-5)M) in the presence of LiCl (10mM), an inhibitor of inositol phosphate metabolism. There was a substantial increase in inositol phosphate accumulation throughout the dose range in aortic rings from 24 month old rats compared to 6 month old rats. This is an alpha 1 receptor response since it is blocked by the alpha 1 antagonist prazosin but not by the alpha 2 antagonist yohimbine. Aortic inositol phosphate accumulation in response to serotonin did not change with age. To assess second messenger stimulated contraction, aortic ring segments were placed in Ca++ free buffer and then stimulated with NE. Under these conditions Ca++ influx is eliminated and contraction depends on the actions of intracellular second messengers. There is an age-related reduction in aortic contraction in Ca++ free buffer. These results suggest that aortic alpha 1 receptor-mediated formation of inositol phosphate intracellular second messengers is enhanced during aging. Despite this, the capacity of senescent arteries to elicit contraction utilizing second messenger pathways seems to be deficient.     

Higher sensitivity of cerebral arteries isolated from premature and newborn baboons to adrenergic and cholinergic stimulation

To find whether effects of adrenergic and cholinergic agents on cerebral artery were dependent on maturity, we examined responses of isolated cerebral artery strips harvested from premature, term newborn and adult baboons. Although cerebral arteries from many species are only mildly sensitive to norepinephrine, we found the perinatal cerebral arteries to be quite responsive to the amine. Cerebral arteries from premature and newborn baboons were significantly (P less than 0.001) more sensitive to norepinephrine than were arteries from adults; medium effective concentration (EC50) for norepinephrine were 3 X 10(-8), 6 X 10(-8) and 32 X 10(-8)M for prematures, newborns and adults, respectively. Arteries showed a similar age-dependence in the sensitivity of the response to phenylephrine, an alpha 1-adrenoceptor agonist. EC50 values for KC1 did not differ among groups, nor did the maximum response to norepinephrine. Arteries from premature and newborn baboons showed marked contractile response to acetylcholine (maximum tensions 5.9 +/- 0.6 and 6.4 +/- 0.8 g/mm2, respectively), whereas arteries from adult baboons showed little response (0.6 +/- 0.1 g/mm2). Arteries from premature and newborn animals showed a more marked relaxation response to isoproterenol than did arteries from adult animals; the degree of relaxation from an induced contraction was 63% (premature), 72% (newborn) and 10% (adult). There was no age-dependence in the relaxation response to sodium nitrite. We conclude that the events coupling alpha 1, beta or muscarinic receptor activation with cerebral arterial contraction or relaxation are more effective in perinatal than in adult baboons.     

Response to endothelin-1 in arteries from human colorectal tumours: role of endothelin receptors

To examine the reaction of tumour arteries to endothelin-1, we obtained arteries supplying blood flow to colorectal tumours from patients, as well as mesenteric arteries supplying the normal colon tissue from the same patients and mesenteric arteries from patients without a colorectal tumour pathology. The contraction in response to endothelin-1 and the relaxation produced by bradykinin was recorded in each of these arteries. Accordingly, the sensitivity to endothelin-1 but not the maximal response, was higher in the arteries supplying colorectal tumours than in mesenteric arteries supplying normal colon or in mesenteric arteries from patients with no tumour pathology. The contraction produced by endothelin-1 was not modified by exposure to L-NAME or meclofenamate in arteries supplying both the tumour and the normal colon. The endothelin ET(A) andET(B) receptors were expressed similarly in arteries supplying the tumour or normal colon. However, the antagonist of the endothelin ET(B) receptors BQ788 (10(-6) M) decreased the contractions in the arteries supplying the tumour but not in those supplying the normal colon. By contrast, the antagonist of endothelin ET(A) receptors BQ123 (10(-6) M) reduced the contraction equally in both these types of arteries. Likewise, in arteries precontracted with U46619, the relaxation in response to bradykinin was similar in all three types of arteries. Together, these results suggest that the arteries supplying human colorectal tumours are more sensitive to endothelin-1, which could be due to the enhanced activity of endothelin ET(B) receptors in the absence of any change in the modulatory effect of nitric oxide or prostanoids in the arterial response to this peptide.     

Correlation between mRNA levels and functional role of alpha1-adrenoceptor subtypes in arteries: evidence of alpha1L as a functional isoform of the alpha1A-adrenoceptor

The mRNA levels for the three alpha1-adrenoceptor subtypes, alpha1A, alpha1B, and alpha1D, were quantified by real-time RT-PCR in arteries from Wistar rats. The alpha1D-adrenoceptor was prominent in both aorta (79.0%) and mesenteric artery (68.7%), alpha1A predominated in tail (61.7%) and small mesenteric artery (73.3%), and both alpha1A- and alpha1D-subtypes were expressed at similar levels in iliac artery. The mRNA levels of the alpha1B-subtype were a minority in all vessels (1.7-11.1%). Concentration-response curves of contraction in response to phenylephrine or relaxation in response to alpha1-adrenoceptor antagonists on maximal sustained contraction induced by phenylephrine were constructed from control vessels and vessels pretreated with 100 micromol/l chloroethylclonidine (CEC) for 30 min. The significant decrease in the phenylephrine potency observed after CEC treatment together with the inhibitory potency displayed by 8-{2-[4-(2-methoxyphenyl)-1-piperazinyl]-8-azaspiro (4,5) decane-7-dionedihydrochloride} (BMY-7378, an alpha1D-adrenoceptor antagonist) confirm the relevant role of alpha1D-adrenoceptors in aorta and iliac and proximal mesenteric arteries. The potency of 5-methylurapidil (an alpha1A-adrenoceptor antagonist) and the changes in the potency of both BMY-7378 and 5-methylurapidil after CEC treatment provided evidence of a mixed population of alpha1A- and alpha1D-adrenoceptors in iliac and distal mesenteric arteries. The low potency of prazosin (pIC50 < 9) as well as the high 5-methylurapidil potency in tail and small mesenteric arteries suggest the main role of alpha1A/alpha1L-adrenoceptors with minor participation of the alpha1D-subtype. The mRNA levels and CEC treatment corroborated this pattern and confirmed that the alpha1L-adrenoceptor could be a functional isoform of the alpha1A-subtype.     

Vasoconstriction of the isolated communicating cerebral artery induced by field electrical stimulation

The vasoconstrictor effect elicited by field electrical stimulation of the posterior communicating cerebral artery of the goat was analyzed before and after treatment with pharmacological agents to find out if the adrenergic system was involved in this response. For this purpose, trains of 300 square wave pulses (1-32 Hz, 0.5 msec.) at supramaximal voltage were applies to these arteries producing a frequency-dependent increase in tension. The vasoconstrictor response was significantly reduced by tetrodotoxin (3 x 10(-6) M), phentolamine (10(-6) M) and bretylium (5 x 10(-4) M), but it was not modified by cocaine (10(-6) M). The contraction produced by electrical stimulation of arterial segments from goats pretreated with reserpine (0.02 mg/kg/day for three days) and from goats on which a bilateral superior cervical gangliectomy had been performed 12 days previously, was significantly reduced as compared with controls. These results show that a large part of the vasoconstrictor response of the goat cerebral vessels to field electrical stimulation is mediated by an adrenergic mechanism.     

Aldosterone modulates neural vasomotor response in hypertension: role of calcitonin gene-related peptide

Objective: We analyse the effect of aldosterone on vasomotor response induced by electrical field stimulation (EFS) in mesenteric arteries from Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). Results: Aldosterone (0.001–1 μM) reduced vasoconstrictor response to EFS in a dose- and time-dependent manner only in SHR. Thus, the rest of experiments were performed only in SHR. Aldosterone did not affect either noradrenaline response or release. Effect of aldosterone (1 μM) on EFS response was not affected by N^G-nitro-arginine-methyl esther (100 μM), and was abolished by capsaicin (0.5 μM) and the calcitonin gene-related peptide antagonist (CGRP 8–37, 0.5 μM). Calcitonin gene-related peptide (0.1 nM–0.1 μM) induced a concentration-dependent relaxation, which was enhanced by aldosterone (1 μM). Incubation with either spironolactone (1 μM), glibenclamide (10 μM), RU 486 10 μM, ODQ (10 μM) or cycloheximide (10 μM) significantly reduced the enhancement of CGRP-relaxation produced by aldosterone, while remained unmodified by SQ 22,536. Conclusions: Aldosterone decreases the vasoconstrictor response to EFS in mesenteric arteries from SHR but not from WKY. This effect is mediated by an increased response to the sensory neurotransmitter CGRP, substantially, through glucocorticoid receptors activation. Furthermore, this effect is mediated by an increase of cGMP synthesis and ATP-dependent potassium channel activation.     

Inhibition of iloprost of the contractile effect of noradrenaline in mesenteric artery rings: evidence for a possible calcium-dependent mechanism.

Iloprost caused a concentration-dependent decrease in the response to noradrenaline in the rabbit isolated endothelium denuded rings from superior mesenteric artery but not thoracic aorta. Similar inhibition was obtained by verapamil using identical concentrations. In Ca(2+)-free EGTA containing medium noradrenaline both at lower and higher concentrations elicited a reduced contractile response and further addition of Ca2+ (2.5 mM) to the medium produced a second contraction in both mesenteric artery and aortic rings which was significantly and equally inhibited by iloprost and verapamil using identical concentrations in mesenteric artery but not in aortic rings. Prior addition of iloprost to the medium did not protect the inhibitory effect of phenoxybenzamine against noradrenaline-induced contraction. These results were taken as an evidence for the possible Ca2+ entry reducing effect of iloprost in mesenteric artery but not thoracic aorta. These results were also taken as an indirect evidence supporting the hypothesis that increased synthesis of prostacyclin by noradrenaline in the vascular wall may inhibit the contractile effect of the agonist by a (-) feedback mechanism mediated by Ca2+ entry into the vascular smooth muscle.     

Age-related changes in the control of hepatic cyclic AMP levels by alpha 1- and beta 2-adrenergic receptors in male rats

Hepatocytes from juvenile male rats (80-110 g) showed a 12-fold elevation of cAMP in response to epinephrine, which was mediated by beta 2-adrenergic receptors. In these cells, either alpha 1- or beta 2-adrenergic stimulation alone activated phosphorylase and glucose release although the alpha 1-phosphorylase response was 10-fold more sensitive to epinephrine and resulted in more rapid (by 10-20 s) activation of the enzyme. This suggests that the beta 2-adrenergic response is functionally unimportant for glycogenolysis, even in juvenile rats. beta 2-Adrenergic stimulation did, however, produce an increase in the rate of gluconeogenesis from [U-14C] lactate in these cells. Aging in the male rat was associated with attenuation of the beta 2-adrenergic cAMP response coupled with the emergence of an alpha 1-receptor-mediated accumulation of cAMP. The order of potency displayed by the alpha 1-adrenergic/cAMP system to adrenergic agonists and antagonists was identical with that of the alpha 1-adrenergic/Ca2+ system. These data suggest that, in maturity, hepatic alpha 1-receptors become linked to 2 separate transduction mechanisms, namely Ca2+ mobilization and cAMP generation. Calcium depletion of hepatocytes from adult, but not juvenile, male rats increased the alpha 1-component of the cAMP response to epinephrine, but under these conditions, alpha 1-activation of phosphorylase occurred more slowly than in calcium-replete cells. Blockade of alpha 2-adrenergic receptors did not significantly modify catecholamine effects on hepatocyte cAMP or phosphorylase a levels in male rats at any age studied, suggesting a lack of functional significance for these receptors in the regulation of glycogenolysis.