Denervation augments alpha-2 but not alpha-1 adrenergic responses in canine saphenous veins

Experiments were performed in order to determine the influence of sympathetic denervation on alpha-1 and alpha-2 adrenergic responses in canine saphenous veins. In female dogs anesthetized with sodium pentobarbital, the left lumbar sympathetic chain was excised from L1 to L7. After a 3- to 5-week period, the left (denervated) and right (innervated) saphenous veins were removed, cut into rings and suspended for isometric tension recording in organ chambers filled with modified Krebs-Ringer bicarbonate solution. Denervation reduced significantly the norepinephrine content of the venous rings and the contractile responses evoked by the indirect sympathomimetic amine, tyramine. The contractile responses evoked by exogenous norepinephrine were augmented by denervation under control conditions (16.7-fold shift in concentration-effect curve) and also after inhibition of neuronal and extraneuronal uptake and beta adrenoceptors (3.8-fold shift in curve). Denervation increased the contractile responses evoked by the alpha-2 adrenergic agonist, UK 14,304 (5-fold shift in concentration-effect curve), but not those produced by the alpha-1 adrenergic agonist, phenylephrine. The selective augmentation of alpha-2 adrenergic responses by denervation may reflect the preferential innervation of alpha-2 adrenoceptors in the canine saphenous vein.     

Effect of cooling on alpha-1 and alpha-2 adrenergic responses in canine saphenous and femoral veins

Experiments were designed to determine the effects of cooling on alpha-1 and alpha-2 adrenergic responses in isolated canine veins. Rings of saphenous and femoral veins were suspended for isometric tension recording in modified Krebs-Ringer bicarbonate solution, gassed with 95% O2 and 5% CO2. Cooling (from 37-24 degrees C) augmented contractions to norepinephrine in saphenous but caused depression in femoral veins. Cooling (to 24 degrees C) had no effect on alpha-1 adrenergic responses evoked by phenylephrine in saphenous veins but caused depression in femoral veins. Alpha-2 adrenergic responses produced by UK 14,304 were augmented by cooling in the saphenous but were virtually abolished by cooling in femoral veins. Cooling decreased the dissociation constant (i.e., increased affinity) of corynanthine for alpha-1 adrenoceptors in saphenous and femoral veins (approximately 3-fold), and the dissociation constant of rauwolscine for alpha-2 adrenoceptors in saphenous veins (approximately 7.5-fold). The influence of cooling on alpha adrenoceptor responsiveness was analyzed using computer-generated receptor-models. The results suggest that the differential sensitivity of cutaneous and deep blood vessels to cooling results from differences in efficiency of alpha-1 and alpha-2 adrenoceptor response coupling. In the saphenous vein, there is a large alpha-1 adrenoceptor reserve which buffers the alpha-1 adrenergic response from the inhibitory influence of cooling. This coupled with a cooling-induced increase in alpha-2 adrenoceptor affinity ensures that cooling augments the response to norepinephrine. In the femoral vein, there is no alpha-1 adrenoceptor reserve and cooling therefore depresses alpha-1 adrenergic responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcium utilization coupled to stimulation of postjunctional alpha-1 and alpha-2 adrenoceptors in isolated human resistance arteries.

Human s.c. resistance arteries (internal diameters 158-353 microns) were mounted in a microvascular myograph, and experiments were designed to examine the calcium pools utilized by selective stimulation of alpha-1 and alpha-2 adrenoceptors. In a concentration-dependent manner, phenylephrine and B-HT 933 evoked contractions mediated by alpha-1 and alpha-2 adrenoceptors, respectively, both in calcium-containing and in calcium-free saline. With respect to the maximum response to potassium in calcium-containing saline, the maximum responses to phenylephrine and B-HT 933 were 96 +/- 6 and 85 +/- 8%, respectively, in calcium-containing saline, and 79 +/- 4 and 14 +/- 2%, respectively, in calcium-free saline. A qualitatively similar difference in maximum responses to alpha-1 vs. alpha-2 adrenoceptor stimulation in calcium-free saline was demonstrated for norepinephrine in the presence of antagonists selective for the two alpha adrenoceptor subtypes. The maximum relaxation in calcium-containing saline produced by the calcium antagonist nitrendipine was 52 +/- 3% in vessels precontracted with phenylephrine, but 80 +/- 5% in vessels precontracted with B-HT 933. A quantitative difference in receptor reserves was demonstrated between alpha-1 and alpha-2 adrenoceptors; 90% of the maximum response was obtained at 34 +/- 5 and 57 +/- 8% receptor occupation, respectively. These data suggest that compared to responses mediated by stimulation of postjunctional alpha-1 adrenoceptors, stimulation of postjunctional alpha-2 adrenoceptors relies heavily on calcium influx. Stimulation of postjunctional alpha-2 adrenoceptors is, however, also coupled to intracellular release of calcium in isolated human s.c. resistance arteries.     

Ouabain inhibits endothelium-dependent relaxations to arachidonic acid in canine coronary arteries

Experiments were designed to analyze the effects of ouabain on the actions of exogenous arachidonic acid on endothelial and vascular smooth muscle cells. Rings or strips were prepared from left circumflex canine coronary arteries and suspended for isometric tension recording in organ chambers filled with oxygenated modified Krebs-Ringer-bicarbonate solution. During contractions evoked by prostaglandin F2 alpha, arachidonic acid caused relaxations both in the presence and the absence of endothelium. However, removal of the endothelium reduced its inhibitory action. Indomethacin prevented the relaxations in rings without endothelium, but did not affect the response to high doses (10(-6) to 10(-5) M) of arachidonic acid in preparations with endothelium. The inhibitor of lipoxygenase, nordihydroguaiaretic acid, had no effect on the inhibitory responses to arachidonic acid in rings with or without endothelium. Ouabain abolished both the endothelium-dependent and the direct relaxations to arachidonic acid. Endothelium-dependent relaxations in response to oleic acid, elaidic acid, adenosine diphosphate and thrombin were not affected by ouabain. In the presence of indomethacin, coronary artery strips without endothelium were relaxed by arachidonic acid only when layered (intimal surface against intimal surface) with a longitudinal strip with endothelium. In layered preparations, treatment of the intact longitudinal strip with ouabain before layering prevented the relaxation, whereas pretreatment of the strip without endothelium had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiovascular responses to the stimulation of alpha-1 and alpha-2 adrenoceptors in the conscious dog

Hemodynamic responses to the selective stimulation of alpha-1 and alpha-2 adrenoceptors were examined in chronically instrumented, conscious dogs. Norepinephrine (0.02-0.1 micrograms/kg/min), a mixed alpha-1/alpha-2 adrenoceptor agonist, phenylephrine (0.2-1.0 micrograms/kg/min), a selective alpha-adrenoceptor agonist and B-HT 920 (0.5-2.0 micrograms/kg/min), a selective alpha-2 adrenoceptor agonist, were infused i.v. after ganglionic (hexamethonium, 30 mg/kg i.v.), beta adrenoceptor (propranolol, 1, mg/kg i.v.) and muscarinic receptor (atropine methylbromide, 0.1 mg/kg i.v.) antagonism. Each of the alpha adrenoceptor agonists increased mean arterial pressure and total peripheral resistance but had no significant effect on cardiac output, stroke volume or heart rate. Equipressor doses of the alpha adrenoceptor agonists caused similar increases in left ventricular systolic and end-diastolic pressure, but there were no significant changes in left ventricular dP/dt or heart rate with any of the alpha adrenoceptor agonists. Selective antagonism of alpha-1 adrenoceptors with prazosin (1 mg/kg i.v.) abolished the pressor and vasoconstrictor responses to phenylephrine but had a lesser effect on the response to B-HT 920. Antagonism of alpha-2 adrenoceptors with rauwolscine (0.1 mg/kg i.v.) caused a significantly greater attenuation of the pressor and vasoconstrictor responses to B-HT 920 than to phenylephrine. The responses to norepinephrine were significantly attenuated by antagonism of either alpha-1 or alpha-2 adrenoceptors. Thus, in the conscious dog with reflex pathways blocked, selective stimulation of either postsynaptic alpha-1 or alpha-2 adrenoceptors increases arterial pressure and total peripheral resistance but does not significantly change heart rate, left ventricular dP/dt, stroke volume or cardiac output.     

Interaction between alpha adrenergic and serotonergic activation of canine saphenous veins.

Serotonin and norepinephrine produced concentration-dependent contractions of helical strips of canine saphenous veins. The contractile responses to both agonists were inhibited by the alpha adrenergic receptor blocking agent phentolamine. Tolazoline inhibited the contractile responses of canine saphenous veins to norepinephrine but augmented those to serotonin. Blockade of adrenergic neuronal reuptake with cocaine enhanced the sensitivity of the canine saphenous vein to serotonin, but did not suppress the inhibition by phentolamine of the contractile responses to this indolealkylamine. Serotonin-mediated venoconstriction was not secondary to release of norepinephrine since it was not accompanied by an increased release of [7-3H]-norepinephrine. These findings suggest that serotonin does not contract canine saphenous veins by stimulation of typical serotonergic receptors. The binding sites for serotonin and norepinephrine in cutaneous venous smooth muscle may share part of a common receptor complex, which triggers the contractile process. Alternatively, serotonin and norepinephrine may act at two different receptors to elicit contraction of canine saphenous veins.     

Differential inhibition of alpha-1 vs. alpha-2 adrenoceptor-mediated responses in canine saphenous vein by nitroglycerin

In the present investigation, the subtype of alpha adrenoceptor that is preferentially antagonized by the noncalcium entry blocker, nitroglycerin (GTN), was studied in vitro using rings prepared from isolated canine saphenous vein (CSV). The calcium entry blocker, diltiazem (DZ), was used for purposes of comparison. Contractions were produced by the following alpha adrenergic agonists: norepinephrine (NE), a nonselective alpha adrenoceptor agonist, phenylephrine (PE), a selective alpha-1 adrenoceptor agonist and B-HT 920, a selective alpha-2 adrenoceptor agonist. The inhibitory effects of GTN on contractions produced by submaximal concentrations (EC65 to EC75) of NE, PE and B-HT 920 were determined. GTN was found to inhibit preferentially the postsynaptic alpha-2 adrenoceptor-mediated responses to B-HT 920 while producing minimal effects on those produced by NE or PE. Similar results were found with DZ. However, when a portion of the alpha-1 adrenoceptor pool was inactivated by phenoxybenzamine (5 X 10(-8) to 1 X 10(-7) M), GTN and DZ both produced a significant depression of responses to PE, a full alpha-1 adrenoceptor agonist. In addition, contractions produced by l-dobutamine, a selective partial alpha-1 adrenoceptor agonist (no alpha receptor reserve), were highly sensitive to inhibition by GTN. These results suggest that the selective inhibition of contractions mediated by postjunctional alpha-2 vs. alpha-1 adrenoceptors in CSV by GTN and DZ may be due partially to the presence of a large alpha-1 adrenoceptor reserve that conceals an underlying functional antagonism to alpha-1 adrenoceptor-mediated responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Alpha-1 and alpha-2 adrenoceptor: response coupling in canine saphenous and femoral veins

The aim of the present study was to analyze alpha-1 and alpha-2 adrenoceptor response coupling in isolated canine blood vessels. Rings of saphenous and femoral veins and of femoral arteries were suspended for isometric tension recording in modified Krebs-Ringer bicarbonate solution, gassed with 95% O2-5% CO2 and maintained at 37 degrees C. Dissociation constants for the alpha-1 adrenergic agonists, phenylephrine and cirazoline, and the alpha-2 adrenergic agonist, UK 14,304, were determined by analysis of concentration-effect curves to the agonists under control conditions and after partial inactivation of alpha adrenoceptors by phenoxybenzamine. The dissociation constant of phenylephrine for alpha-1 adrenoceptors in saphenous veins was approximately 10-fold higher than that obtained for the agonist in femoral arteries or femoral veins. Similarly the dissociation constant for cirazoline in the saphenous vein was higher than that obtained in other alpha-1 adrenergic systems. Dissociation constants were used to determine alpha adrenoceptor occupancy-response relationships. The alpha-1 adrenergic responses evoked by high intrinsic-efficacy agonists (cirazoline and phenylephrine) were associated with a very large receptor-reserve in the saphenous vein, but no, or only a limited receptor-reserve in the femoral vein. The dissociation constant for UK 14,304 in saphenous veins was significantly lower than that obtained for alpha-2 adrenergic stimulation by norepinephrine. There was no alpha-2 adrenoceptor reserve in the saphenous vein for these putative high intrinsic-efficacy agonists. The differences in receptor-reserve between alpha-1 adrenoceptors in canine saphenous and femoral veins and between alpha-1 and alpha-2 adrenoceptors in saphenous veins may help to explain the differential modulation of adrenergic responses in these blood vessels.     

Endothelium-independent and endothelium-dependent contractions mediated by P2X- and P2Y-purinoceptors in canine basilar arteries

Both alpha, beta-methylene ATP and beta, gamma-methylene ATP (P2X selective agonists) were shown to induce transient contraction in intact and endothelium-removed preparations of canine basilar arteries. 2-Methylthio ATP (a P2Y selective agonist) caused transient contraction of intact arteries and this response was nearly abolished by removal of the endothelium. In the presence of alpha, beta-methylene ATP (10(-6) M), the endothelium-independent contractions induced by alpha, beta-methylene ATP itself (10(-6) M) and by beta, gamma-methylene ATP (10(-5) M) were both abolished. The endothelium-dependent contraction induced by 2-methylthio ATP (10(-7) M) was not attenuated by alpha, beta-methylene ATP. The contraction induced by 2-methylthio ATP (10(-7) M) was attenuated markedly by reactive blue 2 (a P2Y antagonist) (3 x 10(-6) M), aspirin (5 x 10(-5) M), OKY-046 (thromboxane A2 synthetase inhibitor) (10(-5) M) and ONO-3708 (thromboxane A2 antagonist) (10(-8) M). However, these agents did not affect the endothelium-independent contraction induced by alpha, beta-methylene ATP (10(-6) M). Neither TMK-777 (a 5-lipoxygenase inhibitor) (10(-7) M) nor superoxide dismutase (100 U/ml) plus catalase (1,000 U/ml) affected either contraction. The present experiments demonstrate that P2X-purinoceptors mediate endothelium-dependent contraction in the canine basilar artery, and that the endothelium-derived contracting factor in this system is probably thromboxane A2.     

Synergistic alpha-1 and alpha-2 adrenergic stimulation of rat proximal nephron Na+/H+ exchange

Both alpha-1 and alpha-2 adrenoceptors have been localized to the renal cortex, with the majority of binding sites on the proximal tubule. Because the major regulator of Na+ uptake into the proximal tubule is the Na+/H+ exchanger, and because alpha-1 and alpha-2 adrenoceptors stimulate it in other tissues, we tested the hypothesis that both alpha adrenoceptor subtypes can increase Na+ uptake into the proximal nephron by stimulating the Na+/H+ antiporter. Enhancement of Na+ transport by agonists was studied in isolated rat proximal tubules by determining the uptake of 22Na that was suppressible by the Na+/H+ inhibitor, 5-(N-ethyl-N-isopropyl)amiloride (EIPA). The phorbol ester, phorbol-12-myristate-13-acetate, (0.1 microM), directly stimulated the antiporter through protein kinase C and increased EIPA-suppressible 22Na uptake 250% above control. The alpha-1 adrenoceptor agonists, cirazoline and phenylephrine, in addition to the mixed agonist, norepinephrine, maximally stimulated uptake by 226 to 232% at 1 microM concentrations. alpha-2 agonists produced a range of maximal stimulations at 1 microM from 65% with guanabenz to 251% with B-HT 933. Increases in 22Na uptake by agonists were inhibited by selective adrenergic antagonists and by EIPA. The drugs did not change the EIPA-resistant component of 22Na uptake. Inasmuch as the adrenoceptor subtypes likely stimulated Na+/H+ exchange by differing intracellular pathways impinging upon common transport steps, we examined whether simultaneous stimulation of both pathways was additive. Submaximal concentrations (5 nM each) of alpha-1 and alpha-2 adrenoceptor agonists in combination synergistically enhanced 22Na uptake to a level similar to 1 microM concentrations of adrenoceptor agonists alone or in combination.(ABSTRACT TRUNCATED AT 250 WORDS)     

Receptor interactions of imidazolines. IX. Cirazoline is an alpha-1 adrenergic agonist and an alpha-2 adrenergic antagonist

The alpha-1 and alpha-2 adrenergic effects of cirazoline were evaluated in guinea-pig aorta and field-stimulated guinea-pig ileum, respectively. Cirazoline was found to be a full agonist at alpha-1 receptors having an ED50, dissociation constant (KA) and relative efficacy similar to that of (-)-norepinephrine. In contrast, cirazoline does not possess agonist activity at presynaptic alpha-2 receptors in the guinea-pig ileum. Thus, whereas norepinephrine and cirazoline both inhibited the twitch response of the field-stimulated ileum, only the response to norepinephrine was blocked by the selective alpha-2 antagonist, yohimbine. The nonadrenergic inhibition of the twitch response observed in the ileum with cirazoline resulted from weak anticholinergic activity (antimuscarinic) at the level of the postsynaptic effector organ and was observed only at high concentrations. At concentrations far below the level required to inhibit the twitch response, cirazoline was found to competitively antagonize the alpha-2-mediated inhibition of the twitch response elicited by norepinephrine. A Schild plot analysis indicated that cirazoline is a potent competitive alpha-2 receptor antagonist characterized by a pA2 value (i.e., -log KB) of 7.56. These results indicate that cirazoline is unique among imidazolines in that it is a potent alpha-1 adrenergic receptor agonist and an even more potent alpha-2 receptor antagonist. This unusual combination of activities could make cirazoline a particularly effective vasoconstricting agent.     

Time-dependent enhancement of xylazine-induced, alpha-2 adrenoceptor-mediated vasoconstriction in isolated and perfused canine pulmonary veins

By using the cannula inserting method, vasoconstrictor responses to norepinephrine (a mixed alpha-1 and alpha-2 adrenoceptor agonist), phenylephrine (a selective alpha-1 adrenoceptor agonist), clonidine and xylazine (selective alpha-2 adrenoceptor agonists) were investigated in the isolated and perfused canine pulmonary vein. The segment of vessels was perfused by Krebs-Ringer bicarbonate solution at a constant flow rate at 37 degrees C. Two hours after setting up the perfusion preparation, norepinephrine and phenylephrine induced dose-dependent increases in perfusion pressure. Xylazine and clonidine, however, did not induce any significant responses at this time. Although the vasoconstrictor responses to norepinephrine and phenylephrine did not change statistically during 11 hr after setting up, xylazine-induced responses were perfusion-time-dependently and significantly enhanced 5, 8 and 11 hr after setting up. On the other hand, clonidine induced no significant vascular responses during 11 hr. Xylazine-induced responses were antagonized by DG-5128 (a selective alpha-2 adrenoceptor antagonist), but not by bunazosin (a selective alpha-1 adrenoceptor antagonist). Furthermore, even in the endothelium-removed preparation by treatment with saponin, the enhancement of xylazine-induced responses were similarly observed in a time-dependent manner. These findings suggest that, in the isolated and perfused canine pulmonary vein, xylazine-induced alpha-2 adrenoceptor-mediated vasoconstrictor response was time-dependently enhanced with the presence and absence of the endothelium. In contrast with xylazine, clonidine did not induce significant vasoconstriction during the experiments. This result suggests that clonidine has little activity at the alpha-2 adrenoceptors which mediate latent xylazine-induced vasoconstriction in the isolated, long-time-perfused canine pulmonary veins.     

Pertussis toxin inhibits contractions but not endothelium-dependent relaxations of rabbit pulmonary artery in response to acetylcholine and other agonists

Guanine nucleotide binding proteins (G proteins) sensitive to pertussis toxin (PTX) mediate the muscarinic receptor responses in several tissues. Therefore, the present study sought to investigate whether smooth muscle contractions and/or endothelium-dependent relaxations in response to acetylcholine (ACh) and other agonists were sensitive to PTX. In endothelium-denuded rabbit pulmonary artery rings, ACh, clonidine and serotonin produced concentration-dependent contractions which were markedly inhibited in nominally Ca+(+)-free medium and abolished in the presence of ethylene glycol bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (0.2 mM). In endothelium-denuded arterial rings obtained from rabbits treated in vivo with PTX (5 micrograms/kg i.v., 5 days before sacrifice) maximum contractions to ACh, clonidine and serotonin were inhibited by 77, 67 and 35%, respectively. Contractions induced with KCl (10-40 mM) were also abolished in Ca+(+)-free medium, but they were not affected by PTX. Endothelium-dependent relaxations of phenylephrine-contracted pulmonary arteries in response to ACh adenosine triphosphate and substance P were also reduced or abolished upon removal of extracellular Ca++. However, the endothelium-dependent relaxations were not affected by PTX. These data demonstrate that contractions of pulmonary arterial smooth muscle cells after stimulation through muscarinic receptors, alpha adrenoceptors and serotonin receptors require the influx of extracellular Ca++. This receptor-stimulated Ca++ influx is likely to be regulated by a PTX-sensitive G protein. Also, the induction of release of relaxing factor from endothelial cells of the pulmonary artery via muscarinic, purinergic or substance P receptors requires extracellular Ca++. However, in these cells, a different mode of signal transduction, insensitive to PTX, seems to be involved.     

Pertussis toxin inhibits hormonal stimulation of bone resorption in fetal rat limb bones.

The cellular basis for hormonal control of bone resorption is poorly understood. As the identifiable receptors for bone resorbing agents such as parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] are located on osteoblasts rather than osteoclasts, the nature of cellular signaling is obscure. Here it is reported that exposure of fetal rat limb bones to pertussis toxin, a bacterial protein that inhibits certain GTP binding proteins (G-proteins) involved in signal transduction, markedly inhibits bone resorption elicited by PTH, 1,25(OH)2D3 and prostaglandin E2. Pertussis toxin does not block the inhibition of alkaline phosphatase activity by PTH or 1,25(OH)2D3, and it potentiates the cyclic AMP response to PTH. These data support the existence of a pertussis toxin-sensitive G-protein that participates in regulation of bone resorption. The putative G-protein is apparently not involved in the initial transduction of hormonal signals, but it may be part of a final common pathway through which the osteoclast is activated by agents with widely divergent initial actions.     

Endothelium-dependent and independent responses to prostaglandin H2 and arachidonic acid in isolated dog cerebral arteries

The addition of prostaglandin (PG) H2 produced a transient contraction followed by a relaxation in helical strips of dog cerebral arteries partially contracted with PGF2 alpha or K+. The contraction was abolished by removal of endothelium, and the relaxation was potentiated. Relaxation induced by PGI2 was not influenced by endothelium denudation. The PGH2-induced contraction in strips with intact endothelium was not influenced by OKY-046, a thromboxane A2 synthesis inhibitor, but was abolished by treatment with ONO3708, an antagonist of vasoconstrictor PGs, whereas the relaxation was inhibited by tranylcypromine or diphloretin phosphate, a nonselective PG antagonist. Contraction induced by arachidonic acid (AA) was reversed to relaxation by removal of endothelium or treatment with ONO3708. Treatment with indomethacin attenuated the AA-induced contraction in the intact strips and also the relaxation in the strips treated with ONO3708 or denuded of endothelium. It may be concluded that vasoconstrictor PGs are synthesized from PGH2 or AA mainly in endothelium, and the production of PGI2 from PGH2 is not dependent on endothelium. Thromboxane A2 in concentrations sufficient to elicit significant contractions does not appear to be liberated from the cerebroarterial wall stimulated by PGH2.     

Smooth muscle alpha-2 adrenoceptors mediate vasoconstrictor responses to exogenous norepinephrine and to sympathetic stimulation to a greater extent in spontaneously hypertensive than in Wistar Kyoto rat tail arteries

The alpha adrenoceptor-mediated vasoconstriction in isolated perfused tail arteries from spontaneously hypertensive (SHR) and age matched Wistar Kyoto (WKY) normotensive rats has been examined. Responses induced by periarterial field stimulation, exogenous norepinephrine or the selective alpha-1 adrenoceptor agonist methoxamine were preferentially antagonized by prazosin in both SHR or WKY tail arteries. However, in SHR only, the alpha-2 adrenoceptor antagonist idazoxan (RX 781094) at low concentrations, significantly antagonized responses to periarterial field stimulation and to exogenous norepinephrine. Except at rather high concentrations, idazoxan was inactive as an antagonist of responses induced by methoxamine. The alpha-1 adrenoceptor blocking agent prazosin was a very potent antagonist of the responses induced by periarterial field stimulation and by methoxamine. These results indicate that alpha-2 adrenoceptors predominate in both SHR and WKY tail arteries, but a significant subpopulation of smooth muscle alpha-2 adrenoceptors is present in tail arteries of SHR but not of WKY rats. In contrast to WKY normotensive rats, postjunctional alpha-2 adrenoceptors may also be involved in the vasoconstrictor responses to sympathetic nerve stimulation in tail arteries of SHR.     

A WB 4101-sensitive alpha-1 adrenergic receptor subtype modulates repolarization in canine Purkinje fibers.

We used standard microelectrode techniques to study alpha-1 adrenergic modulation of repolarization in canine Purkinje fibers. Our objectives were to subtype this alpha-1 receptor response pharmacologically, to determine whether alpha-1 adrenergic modulation of repolarization is dependent on the function of a pertussis toxin-sensitive G protein and to identify developmental changes in this alpha-1 response. Phenylephrine (Phe) induced a dose-dependent increase in transmembrane action potential duration at 50% (APD50) and 90% (APD90) repolarization. For the adult fibers, control APD50 and APD90 were 310 +/- 5 and 407 +/- 5 msec; after superfusion with Phe, 1 x 10(-6) M, the values were 350 +/- 6 and 468 +/- 8 msec, respectively (P less than .05). In 2- to 3-week-old dogs, control APD50 and APD90 were 170 +/- 14 and 255 +/- 10 msec; after superfusion with Phe, the values were 228 +/- 10 and 305 +/- 16 msec, respectively (P less than .05). Propranolol, 2 x 10(-7) M, did not affect the response to Phe. The alpha-1 blocker prazosin, 1 x 10(-7) M, and the alpha-1 receptor subtype selective antagonist, WB 4101, 1 x 10(-7) M, suppressed the response to Phe, but no effect on the response to Phe was seen with the subtype selective antagonist, chloroethylclonidine. In vivo pretreatment of dogs with pertussis toxin, 30 micrograms/kg i.v., decreased markedly the amount of G protein substrate available for subsequent in vitro ADP-ribosylation by pertussis toxin and [32P]NAD (from 7039 +/- 713 to 537 +/- 50 fmol/mg of protein in adult fibers and from 1134 to 62 fmol/mg of proteins in pooled young fibers). Pertussis toxin pretreatment increased the Phe-induced prolongation of APD50 and APD90 in the young and adult fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Postjunctional alpha-1 and alpha-2 adrenoceptors in human skin arteries. An in vitro study

Experiments were performed to characterize the postjunctional alpha adrenoceptors that mediate adrenergic constriction in human skin arteries. Abdominal s.c. arteries were obtained from patients who died 3 to 12 hr before, and vascular segments 2 mm in length and 600 to 1050 microns in external diameter were prepared for isometric tension recording. On application of norepinephrine, phenylephrine (alpha-1 adrenergic agonist) or clonidine (alpha-2 adrenergic agonist) the arteries contracted in a dose-dependent manner and, in terms of the mean EC50 values, the order of potencies was clonidine greater than norepinephrine greater than phenylephrine. With regard to their ability to induce maximal contraction, the order was norepinephrine = phenylephrine greater than clonidine. In the presence of phentolamine (nonspecific alpha adrenergic antagonist) or yohimbine (selective alpha-2 adrenergic antagonist) the control curve for norepinephrine was displaced to the right in a parallel way. Prazosin (selective alpha-1 adrenergic antagonist) depressed both the slope and maximal response of the control curve for norepinephrine but the shift was not significant. Prazosin and yohimbine produced a parallel rightward shift in the control curve for phenylephrine and clonidine, respectively. These results suggest that skin arteries of humans have a mixed population of postjunctional alpha-1 and alpha-2 adrenoceptors and that alpha-2 adrenoceptors are more prominent. They also suggest that the alpha-2 adrenergic component of the response to norepinephrine is a low-maximum effect compared to the alpha-1 adrenergic component. This could be of significance in regulating skin blood flow and thermoregulatory function.     

Prostaglandin synthesis elicited by adrenergic stimuli in rabbit aorta is mediated via alpha-1 and alpha-2 adrenergic receptors

The purpose of this study was to characterize the type of adrenergic receptor(s) involved in both prostaglandin (PG) synthesis and the contractile response elicited by adrenergic receptor agonists in the rabbit aorta. The synthesis of prostacyclin as measured by the production of 6-keto-PGF1 alpha was assessed in vitro after exposing the aortic rings to different adrenergic agonists. Norepinephrine (NE), selective alpha 1 adrenergic receptor agonists methoxamine (MET), phenylephrine (PHE) and cirazoline (CIR) and the alpha 2 adrenergic receptor agonists UK 14304 (UK) and xylazine (XYL), but not the beta adrenergic receptor agonist isoproterenol (ISP), enhanced 6-keto-PGF1 alpha synthesis in a concentration-dependent manner with following order of potency: NE greater than UK 14304 greater than XYL greater than PHE greater than MET greater than CIR. The NE-induced increased in 6-keto-PGF1 alpha synthesis was attenuated by the alpha 1 adrenergic receptor antagonists prazosin (PZ) and corynanthine (COR) and by the alpha 2 adrenergic receptor antagonists rauwolscine (RW) and yohimbine (YOH). MET-induced 6-keto-PGF1 alpha synthesis was reduced by PZ and COR but not by RW. UK-induced 6-keto-PGF1 alpha synthesis was reduced by RW, YOH, and PZ, which also acts as alpha-2B receptor antagonist, but not by COR. In rabbit aortic rings, adrenergic agonists produced contraction with the following order of potency: NE greater than PHE greater than MET greater than CIR greater than UK greater than XYL.(ABSTRACT TRUNCATED AT 250 WORDS)