Characteristics of adrenoceptors and [3H]nitrendipine receptors of porcine vascular smooth muscle: differences between coronary artery and aorta

Celiprolol induces β(3)-adrenoceptors-dependent relaxation in isolated porcine coronary arteries

In porcine coronary arteries (PCAs), celiprolol, a selective β(1)-adrenoceptors antagonist, induces vasodilatation by an endothelium- and nitric oxide (NO)-dependent pathway. However, the mechanisms of that vascular effect have not been precisely established. β(3)-Adrenoceptors have been shown to be involved in the relaxation per se of various vascular beds, including coronary vessels. Thus, we evaluated (i) the presence of β(3)-adrenoceptors in the PCA and (ii) their role in celiprolol-induced vasodilatation. PCA rings were placed in organ baths and preconstricted with KCl. All experiments were performed in the presence of nadolol (a β(1)/β(2)-adrenoceptor antagonist). Cumulative concentration-response curves to SR 58611A and ICI 215001 (2 β(3)-adrenoceptor agonists) and to celiprolol were constructed. We also used semiquantitative reverse transcription - polymerase chain reaction, which clearly showed the presence of β(3)-adrenoceptor transcripts. SR 58611A, ICI 215001, and celiprolol induced concentration-dependent relaxations in PCA rings. SR 58611A-induced relaxation was almost abolished after removal of endothelium or pretreatment with L-NAME (a NO synthase inhibitor). The vasorelaxations induced by SR 58611A and celiprolol were inhibited in the presence of SR 59230A and L-748337 (2 selective β(3)-adrenoceptor antagonists). We showed (i) that PCAs possess functional β(3)-adrenoceptors mediating endothelium- and NO-dependent relaxation, and (ii) that celiprolol exerts a β(3)-adrenoceptor agonistic activity in this vascular bed.

A full pharmacological analysis of the three turkey β-adrenoceptors and comparison with the human β-adrenoceptors

BACKGROUND

There are three turkey β-adrenoceptors: the original turkey β-adrenoceptor from erythrocytes (tβtrunc, for which the X-ray crystal structure has recently been determined), tβ3C and tβ4C-receptors. This study examined the similarities and differences between these avian receptors and mammalian receptors with regards to binding characteristics and functional high and low affinity agonist conformations.

METHODOLOGY/PRINCIPAL FINDINGS

Stable cell lines were constructed with each of the turkey β-adrenoceptors and 3H-CGP12177 whole cell binding, CRE-SPAP production and (3)H-cAMP accumulation assays performed. It was confirmed that the three turkey β-adrenoceptors are distinct from each other in terms of amino acid sequence and binding characteristics. The greatest similarity of any of the turkey β-adrenoceptors to human β-adrenoceptors is between the turkey β3C-receptor and the human β2-adrenoceptor. There are pharmacologically distinct differences between the binding of ligands for the tβtrunc and tβ4C and the human β-adrenoceptors (e.g. with CGP20712A and ICI118551). The tβtrunc and tβ4C-adrenoceptors appear to exist in at least two different agonist conformations in a similar manner to that seen at both the human and rat β1-adrenoceptor and human β3-adrenoceptors. The tβ3C-receptor, similar to the human β2-adrenoceptor, does not, at least so far, appear to exist in more than one agonist conformation.

CONCLUSIONS/SIGNIFICANCE

There are several similarities, but also several important differences, between the recently crystallised turkey β-adrenoceptor and the human β-adrenoceptors. These findings are important for those the field of drug discovery using the recently structural information from crystallised receptors to aid drug design. Furthermore, comparison of the amino-acid sequence for the turkey and human adrenoceptors may therefore shed more light on the residues involved in the existence of the secondary β-adrenoceptor conformation.

The vasorelaxant effect of caffeic acid phenethyl ester on porcine coronary artery ring segments

Caffeic acid phenethyl ester (CAPE) is a naturally occurring compound isolated from honeybee propolis whose cardiovascular properties remain uncertain. The purpose of this study was to investigate the possible mechanisms of CAPE-induced vasorelaxation in porcine coronary artery rings. It was found that both the quiescent and precontracted coronary artery ring segments were relaxed by CAPE (10(-7)-10(-)(4) M). N(omega)-nitro-l-arginine (L-NNA), methylene blue and removal of endothelium significantly attenuated CAPE-induced relaxation of both quiescent and precontracted artery rings. This relaxing effect of CAPE on coronary arteries was also significantly reduced by propranolol, and SQ22536, but not by indomethacin. In addition, the dose-response curves of KCl (2.5-100 mM) and CaCl(2) (10(-5)-10(-2) M) were displaced downwards in the presence of CAPE. These results suggest that the relaxant effect of CAPE on porcine coronary artery rings might involve the action of nitric oxide (NO) and adrenergic beta-receptor, together with their second messenger, cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP), respectively, but not involve the synthesis of prostaglandin.

Involvement of Na+-Ca2+ exchanger in cAMP-mediated relaxation in mice aorta: evaluation using transgenic mice

BACKGROUND AND PURPOSE

Although vascular smooth muscle cells are known to express the Na+-Ca2+ exchanger (NCX), its functional role has remained unclear, mainly because of its relatively low expression. We thus investigated the involvement of NCX in the mechanism for the forskolin-induced vaso-relaxation, using wild type (WT) and transgenic (TG) mice that specifically over-express NCX1.3 in smooth muscle.

EXPERIMENTAL APPROACH

We examined the relaxing effect of forskolin during the pre-contraction induced by 100 nM U46619, a thromboxane A2 analogue in the mouse isolated thoracic aorta. We also measured the intracellular Ca2+ concentration ([Ca2+]i) in fura-PE3-loaded aortic strips.

KEY RESULTS

The forskolin-induced decreases in [Ca2+]i and tension were much greater in aortas from TG mice than in those from WT mice. In a low Na+ solution, forskolin-induced decreases in [Ca2+]i and tension were greatly inhibited in both groups of aortas. In WT aortas, the presence of 100 nM SEA0400, an NCX inhibitor, had only a little effect on the forskolin-induced decreases in [Ca2+]i, but inhibited the forskolin-induced relaxation. However, in TG aortas, the presence of SEA0400 greatly inhibited the forskolin-induced decreases in [Ca2+]i and tension.

CONCLUSIONS AND IMPLICATIONS

The NCX was involved in the forskolin-induced reduction of [Ca2+]i and tension in the mouse thoracic aorta. Measurement of [Ca2+]i and tension in aortas of the TG mouse is thus considered to be a useful tool for evaluating the role of NCX in vascular tissue.

Interactions of sildenafil with various coronary vasodilators in isolated porcine coronary artery

There are reports of serious hypotension or circulatory shock when sildenafil citrate, a selective cyclic nucleotide phosphodiesterase type 5 inhibitor, which was developed for the treatment of erectile dysfunction, is given to patients taking certain coronary vasodilators. We thus examined the interaction of sildenafil with various coronary vasodilators including nitric oxide (NO) donors in isolated porcine coronary artery. Sildenafil caused concentration-dependent relaxations of the artery precontracted with U46619 (9,11-dideoxy-9 alpha,11 alpha-methanoepoxy-prostaglandin F(2alpha)). Incubation with the NO synthase inhibitor NG-nitro-L-arginine or the soluble guanylate cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one) significantly shifted the concentration-response curve for sildenafil to the right without affecting the maximum response, indicating that some part of the relaxant response to sildenafil may be the result of the inhibition of phosphodiestrase type 5-induced degradation of cyclic GMP (cGMP) that is produced through guanylate cyclase activation by NO released spontaneously. The relaxant effects of the vasodilators with an NO donor property, isosorbide dinitrate, sodium nitroprusside, nicorandil and nipradilol, were significantly enhanced by sildenafil, as shown by a significant leftward shift of their concentration-response curves. In contrast, the relaxant responses to the drugs without a property as an NO donor, diltiazem, celiprolol and pinacidil, were not affected by sildenafil. The cGMP level of the tissue was elevated after adding sildenafil, and the cGMP-generating effect of a combination of sildenafil and sodium nitroprusside was higher than that of each drug alone. The cyclic AMP level determined simultaneously was not changed by sildenafil. These results suggest that sildenafil potentiates specifically the relaxant responses of porcine coronary artery to the drugs which behave as an NO donor, providing basic evidence that the benefit of sildenafil in the treatment of erectile dysfunction can be limited by a risk of marked vasodilation when used together with NO-related coronary vasodilators.

Comparison of adrenoceptor subtype expression in porcine and human bladder and prostate

We have quantified and characterized alpha 1-, alpha 2- and beta-adrenoceptor subtypes in porcine bladder detrusor and bladder neck, human bladder detrusor, and porcine and human prostate. alpha 1-, alpha 2- and beta-adrenoceptor were identified in radioligand binding studies using [3H]prazosin, [3H]RX 821002 and [125I]iodocyanopindolol, respectively, as the radioligands. In porcine male and female detrusor and bladder neck and male prostate, adrenoceptors were detected in the order of abundance beta > alpha 2 >> alpha 1 (not detectable), with no major difference between the sexes or between detrusor and bladder neck. In human detrusor and prostate the order of abundance was beta > alpha 2 >> alpha 1 (not detectable) and beta >> alpha 1 > alpha 2, respectively. The alpha 2-adrenoceptors in all tissues were homogeneously of the alpha 2A-subtype as evidenced by competition binding studies with yohimbine, prazosin, ARC 239 and oxymetazoline. The beta-adrenoceptors represented a mixed population with a dominance of the beta 2-subtype in all tissues as demonstrated by competition binding with ICI 118,551 and CGP 20,712A. We conclude that pigs may be a suitable model for studies of detrusor function with respect to adrenoceptor expression. They may be less suitable for studies of bladder neck or prostate function.

Some effects of nipradilol, a beta-antagonist possessing a nitroxy group, on smooth muscle of the pig coronary artery

1. The effects of nipradilol, a beta-adrenoceptor antagonist which possesses a nitroxy group, on cytosolic Ca2+ concentration ([Ca2+]i), and on tension development were simultaneously measured by front-surface fluorometry and fura-2-loaded strips in the proximal portion of pig coronary arteries. 2. Nipradilol reduced in a concentration-dependent manner both the [Ca2+]i and tension, irrespective of whether the strips were unstimulated or exposed to either high K+ or histamine containing solutions. However, both in the case of contractions induced by high K+-depolarization and histamine stimulation, for a given [Ca2+]i elevation the tension which developed in the presence of nipradilol was smaller than that generated in its absence, so that the [Ca2+]i-tension curves during the contraction were shifted to the right. 3. In the absence of extracellular Ca2+, the [Ca2+]i elevation due to the release of Ca2+ from histamine-sensitive store was inhibited by nipradilol. Nipradilol had no effect on the [Ca2+]i elevation due to the release of Ca2+ from caffeine-sensitive stores; however, it did inhibit the caffeine-induced increase in tension. A derivative of nipradilol, which lacked a nitroxy molecule (Nip(-N)), had no effect on the [Ca2+]i and tension elevated by histamine or caffeine in the absence of extracellular Ca2+. 4. The beta-adrenoceptor agonist, isoprenaline, reduced [Ca2+]i tension when applied to steady state contractions induced by high K+, or at the peak level of tension to histamine. The reduction of [Ca2+]i and tension induced by isoprenaline was inhibited by Nip(-N) in a concentration-dependent manner and nipradilol inhibited the isoprenaline-induced relaxation with bell-shaped concentration-response curves. At lower concentrations, nipradilol acted as a beta-blocker, the IC50- value being smaller than that of Nip(-N), and at higher concentrations, it acted as a nitrovasodilator. 5. Thus, it is suggested that, at lower concentrations, nipradilol, an antianginal drug, acts as a beta-adrenoceptor antagonist. At higher concentrations, it relaxes the proximal portion of the coronary artery by directly reducing [Ca2+]i and the Ca2+-sensitivity of the myofilaments, apparently due to the presence of the nitroxy molecule.

The relationship between density of alpha-adrenoceptor binding sites and contractile responses in several porcine isolated blood vessels

1. The aim of this study was to investigate constrictor alpha-adrenoceptors in three isolated blood vessels of the pig, the thoracic aorta (TA), the splenic artery (SA) and marginal ear vein (MEV) and then compare the functional response with the densities of alpha 1- and alpha 2-adrenoceptor binding sites in these and several other porcine vascular tissues, palmar common digital artery (PCDA), palmar lateral vein (PLV) and ear artery (EA). 2. Noradrenaline (NA), phenylephrine (PE) and UK14304 (all at 0.03-10 microM) elicited concentration-dependent contractions in the TA and MEV, with a rank order of potency of UK14304 > NA > PE. UK14304 produced maximal responses which were 58% (TA) and 65% (MEV) of that of NA. In the SA, UK14304 and PE produced maximal responses which were less than 10% and 50% of the NA-induced maximal response respectively, with an order of potency of NA > PE. In the SA, NA-induced contractions were competitively antagonized by prazosin (pA2 = 8.60 +/- 0.15). Further, rauwolscine (1-10 microM) antagonized NA-induced contractions with an apparent pKB of 6.09 +/- 0.11 (n = 6), indicating an action at alpha 1-adrenoceptors. The combination of the two antagonists at concentrations selective for alpha 1- (0.1 microM) and alpha 2-adrenoceptors (1 microM) had no greater effect than either antagonist alone. This suggests that the SA expresses only post-junctional alpha 1-adrenoceptors. 3. In the TA, prazosin produced non-parallel shifts in the NA-induced CRC and this was also observed with rauwolscine, where reductions in the maximal responses were also observed. In the MEV, prazosin was largely inactive in antagonizing NA-induced contractions. In both these vessels a combination of these two antagonists had a greater effect than either alone, indicating the presence of functional alpha 1- and alpha 2-adrenoceptors. The post-junctional alpha 2-adrenoceptors in all of these vessels were resistant to prazosin, suggesting the alpha 2-adrenoceptor to be of the alpha 2A/2D subtype. The expression of functional alpha 2-adrenoceptors was MEV > TA > PLV > PCDA > SA. 4. In radioligand binding studies using TA P2 pellet membranes, [3H]-prazosin and [3H]-RX821002 ([1,4-[6,7(n)-3H] benzodioxan-2-methoxy-2-yl)-2-imidazole) labelled different high affinity sites, and in competition studies using identical membranes corynanthine displaced [3H]-prazosin with 10 fold higher affinity than rauwolscine, indicating that [3H]-prazosin was selectively binding to alpha 1-adrenoceptor sites. Further, rauwolscine displaced [3H]-RX821002 with approximately 100 fold greater affinity compared to corynanthine, which is indicative of selective alpha2-adrenoceptor binding.5. Separation of the P2 pellet into plasma membrane and mitochondrial fractions was carried out using a differential sucrose density gradient. [3H]-prazosin and [3H]-RX821002 binding sites were found in both the plasma membrane and mitochondrial fractions.6. In saturation studies all tissues produced single site saturation curves with no difference in the Kd(range 0.13-0.20nM) of the alpha1-adrenoceptor sites for [3H]-prazosin. However, there was considerable variation in Bmax of alpha 1-adrenoceptor sites; the highest density was found in the TA (397.9 =/- 52.7 fmol mg-1, n = 4), followed by the PCDA (256.7 +/- 22.7 fmol mg-1, n = 4), the PLV and SA having approximately equal density (143.6 +/- 3.9 and 159.1 +/- 7.0 fmol mg-1 respectively, n = 4 for both), followed bythe EA (91.3 +/- 10.5 fmol mg-1, n = 3) and the MEV had the lowest density (48.9 +/- 11.4 fmol mg-1,n = 3).7. In saturation studies using [3H]-RX821002, all tissues produced single site saturation curves with no differences in the Kd values (range 1.31 +/- 2.16 nM) but the highest densities were found in the TA and MEV (545.3 +/- 36.2 and 531.0 +/- 40.9 fmol mg-1 respectively), followed by the PLV (418.4 +/- 39.4 fmol mg-1), then the EA (266.3 +/- 40.0 fmol mg-1), and low densities of [3H]-RX821002 binding being found in the PCDA and SA (155.9 +/- 18.1 and 117.5 +/- 19.3 fmol mg-1 respectively).8. The pattern of binding site distribution for alpha l- and alpha 2-adrenoceptors is in reasonable agreement with functional studies carried out in these porcine vascular tissues; the TA has the highest densities of alpha 1-and alpha2-adrenoceptors; in the SA and PCDA there is a predominance (although small) of alpha l-adrenoceptor binding sites, the reverse of which is observed both in the PLV and MEV (i.e. greater density of alpha2-adrenoceptor sites). Thus, it would appear that alpha 1- and alpha2-adrenoceptor densities play a role in the expression of functional responses via these receptor subtypes; although it is interesting to note that the SA did have a small density of alpha 2-adrenoceptor binding sites, no functional response was observed after alpha2-adrenoceptor activation.

Pharmacological characterization of noradrenaline-induced contractions of the porcine isolated palmar lateral vein and palmar common digital artery

1. The aim of this study was to examine the pharmacological characteristics of alpha-adrenoceptor-mediated contractions in two porcine isolated blood vessels, the palmar lateral vein (PLV) and the palmar common digital artery (PCDA). This was carried out with noradrenaline used as the agonist throughout, and either phentolamine (non-selective alpha-adrenoceptor antagonist), prazosin and YM-12617 (selective alpha 1-adrenoceptor antagonists) or rauwolscine and CH-38083 (selective alpha 2-adrenoceptor antagonists). 2. Noradrenaline (0.003-10 microM) produced concentration-dependent contractions in both vessels, with the PCDA (pD2 = 6.33 +/- 0.07, n = 10) being approximately 10 fold less sensitive to noradrenaline compared to the PLV (pD2 = 7.39 +/- 0.09, n = 8). Also, the maximal response to noradrenaline was greater in the PCDA compared to the PLV. Phentolamine (0.03-30 microM) produced parallel rightward shifts in the CRC to noradrenaline in both tissue preparations. The pA2 values were similar and slopes of the Schild plots were not significantly different from unity, indicating an interaction between phentolamine and a single receptor in each preparation. 3. In the PCDA the alpha 1-adrenoceptor antagonists, prazosin (0.01-1 microM) and YM-12617 (0.01-1 microM) produced non-parallel rightwards shifts in the CRC to noradrenaline, with the lower 10-15% of the CRC exhibiting greater resistance to the effects of these antagonists compared to the upper part. In contrast, rauwolscine (1-10 microM) and CH-38083 (10 microM) produced parallel displacement of the CRC to noradrenaline. In the PLV, low concentrations of either alpha l- (0.01 microM) or alpha2-adrenoceptor antagonists(0.1-1 microM) produced a large shift in the CRC, but subsequent higher concentrations had only small additional effects. Based upon pKB values estimated from the effects of the lower concentrations of antagonists, the results are consistent with a large population of alpha1-adrenoceptors in the PCDA and a mixture of alpha l- and alpha2-adrenoceptors in the PLV.4. In both tissues, when an ac,- and an a2-adrenoceptor antagonist were used in combination the effect produced was greater than that with either agent alone. In contrast, the combination of the alpha1-adrenoceptor antagonists (prazosin and YM-12617 together) or the alpha2-adrenoceptor antagonists (CH-38083 and rauwolscine together) were no more effective than that produced by the individual antagonists. These findings suggest the presence of functional alpha l- and alpha2-adrenoceptors in the PLV andPCDA.5. Phenoxybenzamine (0.3-3 microM, 60min exposure) produced a concentration-dependent reduction in the maximal response to noradrenaline which was more pronounced in the PCDA than the PLV. After a 60 min exposure to a combination of phenoxybenzamine (1 microM) and rauwolscine (1 microM), the remaining NA-induced contraction after washout was resistant to prazosin (0.1 microM) and sensitive to rauwolscine(1 microM) in both tissue preparations, indicating the existence of functional alpha2-adrenoceptors in both vessels.6. Evidence suggests that post-junctional alpha l- and alpha2-adrenoceptors contribute to noradrenaline-induced contractions in the PCDA and PLV, with the latter possessing a larger population of functional alpha2-adrenoceptors.

Guanine nucleotide binding regulatory proteins: their characteristics and identification

Many biological signals are processed by the binding of chemicals to cell surface receptors. Signals are switched to intracellular language via guanine nucleotide binding regulatory proteins (G-proteins) which are present in all eukaryotic cells. Thus, G-proteins serve as interfaces between receptor-response coupling. Two forms of G-proteins have been reported: conventional G-proteins which are heterotrimeric and consist of alpha, beta, and gamma subunits, and monomeric small molecular weight G-proteins which are generally found as single polypeptides. Recently, high molecular weight G-proteins have also been described. The family of G-proteins contains multiple genes that encode the alpha, beta, or gamma subunits. G-proteins play a pivotal role in excitation-contraction coupling in smooth muscle function and control metabolic and secretory processes. In this review article, we have given a brief overview on the characteristics and methodology for the identification of G-proteins. The heterotrimeric G-proteins are generally identified by Western blotting and ADP-ribosylation with bacterial toxins. The monomeric and high molecular weight G-proteins have been identified by [35S]GTP delta S overlay technique and photoaffinity labeling, respectively. Recently, the use of molecular genetic probes has made it possible to investigate the expression of the message for various G-proteins.

cAMP induces up-regulation of ETA receptor mRNA and increases responsiveness to endothelin-1 of rat aortic smooth muscle cells in primary culture

The effects of cAMP on the expression of ETA (ET-1 selective type) receptor mRNA and on the response to endothelin-1 (ET-1) were investigated in rat aortic smooth muscle cells in primary culture, using reverse transcription polymerase chain reaction (RT-PCR) and fura-2 microfluorometry of cytosolic Ca2+ concentrations ([Ca2+]i). Incubation of the smooth muscle cells with 10 microM forskolin and 10 microM 3-isobutyl-1-methyl-xanthine (IBMX) induced an increase in ETA receptor mRNA by 420% of control after 24 hours. ET-1 induced a biphasic increase in [Ca2+]i, first transient and second sustained phases. Incubation of the smooth muscle cells with forskolin and IBMX for 24 hours increased the [Ca2+]i response by 849% in the first phase and 286% in the second phase, compared with time-matched controls. From these results, we conclude that cAMP induces an up-regulation of the ETA receptor mRNA and increases responsiveness to ET-1 of rat aortic smooth muscle cells in primary culture.

Inhibitory role of the coronary arterial endothelium to alpha-adrenergic stimulation in experimental heart failure

The role of the endothelium in regulating coronary alpha-adrenergic tone was evaluated in isolated coronary arterial rings from dogs with and without pacing-induced congestive heart failure (CHF). The maximal contractile response to methoxamine was attenuated approximately 43% (p less than 0.05) in both intact and denuded CHF rings compared with control. Conversely, norepinephrine-induced contractions were diminished 58% in intact CHF vessels and 39% in denuded CHF vessels (p less than 0.05). Denudation did not alter responses to methoxamine but significantly (p less than 0.05) augmented the tension generated by norepinephrine in both control (1.7-fold) and CHF (2.4-fold) arteries. In both intact control and CHF coronary arteries, norepinephrine elicited rapid, transient relaxations that preceded slow, sustained contractions; the initial relaxation phase was endothelium dependent, because denudation eliminated the response. Relaxations to the selective alpha 2-adrenoceptor agonist BHT 920 were also dependent on the presence of an endothelium. At peak CHF, endothelium-dependent relaxations to norepinephrine and BHT 920 were enhanced, whereas relaxations to nitroglycerin and acetylcholine were unaltered. The data suggest that alpha-adrenergic tone in canine coronary arteries is diminished by pacing-induced CHF because of a decrease in alpha 1-adrenoceptor-mediated constriction and an enhanced capacity of the endothelium to antagonize the direct vascular smooth muscle response of norepinephrine through endothelium-dependent, alpha 2-adrenoceptor-mediated relaxations.

Cytosolic calcium transients differ between porcine coronary arterial and aortic smooth muscle cells in primary culture

Using quin 2 microfluorometry of porcine vascular smooth muscle cells in primary culture at 25 degrees C, we investigated differences in cytosolic calcium transients between epicardial coronary artery and aorta. Both in coronary arterial and aortic smooth muscle cells, histamine induced transient and dose-dependent elevations of cytosolic calcium concentrations, with a similar time course and EC50 (coronary artery, 1.4 x 10(-7) M; aorta, 1.8 x 10(7) M). However, a transient and dose-dependent elevation of cytosolic calcium concentrations was induced by norepinephrine in aortic smooth muscle cells (EC50 = 2.5 x 10(-7) M) but not in coronary arterial smooth muscle cells. Isoproterenol, which produced no change in cytosolic calcium concentrations in aortic vascular smooth muscle cells, significantly and dose dependently decreased concentrations of calcium in coronary arterial smooth muscle cells (EC50 = 1.5 x 10(7) M). Dibutyryl cAMP decreased the concentration of cytosolic calcium both in the coronary arterial and aortic vascular smooth muscle cells with a similar time course and EC50 (coronary artery, 9.8 x 10(-6) M; aorta, 1.1 x 10(-5) M). Intracellular concentration of cAMP was increased in response to isoproterenol, as determined with radioimmunoassay of the coronary arterial smooth muscle cells but not in the aortic cells. Thus, the characteristics of receptors on the sarcolemma may play a key role in the regulation of responsiveness of vascular smooth muscle cells to various vasoactive substances. Aortic smooth muscle cells are alpha-receptor dominant, and activation results in a transient elevation of cytosolic calcium concentrations. The epicardial coronary arterial smooth muscle cells are beta-receptor dominant, and activation results in an increase in cAMP and a reduction of cytosolic calcium concentrations. These results may account for the poor contraction, or relaxation, of epicardial coronary artery induced by sympathetic stimulation and exogenously applied catecholamines.

Identification of an alpha 2-adrenoceptor in human coronary arteries by radioligand binding assay

A single high affinity binding site for an alpha 2-adrenoceptor in human coronary arteries was identified by radioligand binding assay. Human coronary arteries were obtained at autopsy within 6 hours of death. A crude membrane solution was incubated with (3H)-rauwolscine at 25 degrees C for 30 min. The binding of (3H)-rauwolscine was rapidly saturable and reversible. Kd was 1.2 +/- 0.2 (SE) nM and Bmax 22 +/- 3 fmol/mg protein. This is the first study which has shown the presence of an alpha 2-adrenoceptor in human coronary arteries using a radioligand binding assay method.

Alpha- and beta-adrenergic control of large coronary arteries in conscious calves

Large and small coronary arteries are subject to control by alpha- and beta-adrenergic mechanisms. However, controversy exists as to the distribution and physiological effects of alpha- and beta-adrenergic receptor subtypes in large coronary arteries. Studies in our laboratory have addressed these questions in conscious calves, chronically instrumented to measure large coronary artery diameter and coronary blood flow. Additionally, adrenergic receptor subtype distribution was determined using ligand binding assays in membrane preparations isolated from large coronary arteries of calves. Physiological results demonstrate, in contrast to the results of most previous studies, that both alpha 1- and alpha 2-adrenergic receptors elicit constriction of the large coronary artery. Studies with ganglionic blockade indicate that the constriction was unaltered by autonomic reflexes or presynaptic release of neurotransmitters. Selective beta-adrenergic receptor activation demonstrated that both beta 1- and beta 2-adrenergic receptors elicit dilation of large coronary arteries, and that the vasodilation was direct, i.e., it was not mediated by increases in coronary blood flow. Biochemical characterization of adrenergic subtype density indicated the presence of both alpha 1- and alpha 2-, as well as beta 1- and beta 2-adrenergic receptor subtypes. Thus, both biochemical and physiological data support the concept that large coronary arteries are regulated by both alpha 1- and alpha 2-, as well as beta 1- and beta 2-adrenergic receptor subtypes.

Minimal alpha 1- and alpha 2-adrenoceptor-mediated coronary vasoconstriction in the anaesthetized swine

alpha-Adrenoceptor-mediated coronary vasoconstriction contributes to the initiation and aggravation of experimental and clinical myocardial ischaemia. However, the extent of alpha 1- and alpha 2-adrenoceptor-mediated constriction has not been characterized in the porcine coronary circulation despite the frequent use of this experimental model. Fifteen swine were anaesthetized with either alpha-chloralose, enflurane or isoflurane to determine the amount of alpha-adrenoceptor-mediated coronary constriction elicited by either the selective alpha 1-adrenoceptor agonist methoxamine or the selective alpha 2-adrenoceptor agonist azepexole. The left anterior descending coronary artery was cannulated and perfused by an external pump delivering constant blood flow from the carotid artery. Following bilateral cervical vagotomy and beta-adrenoceptor blockade with propranolol (2 mg kg-1), graded dosages of either one of the alpha-adrenoceptor agonists (9-45 micrograms kg-1 min-1) were infused into the coronary perfusion line while coronary arterial pressure (CAP) was measured through a distal side arm of the cannula to detect changes in coronary vascular resistance. Infusion of the alpha-adrenoceptor agonists was terminated when systemic arterial pressure increased. Sonomicrometers were used to measure anterior left ventricular wall thickening for the assessment of regional contractile function. During methoxamine infusion, no increase in vascular resistance was observed during alpha-chloralose, enflurane or isoflurane anaesthesia, whereas the infusion of azepexole increased CAP from 103 +/- 31 mmHg to 120 +/- 35 mmHg (alpha-chloralose), from 101 +/- 16 mmHg to 122 +/- 11 mmHg (enflurane) and from 84 +/- 20 mmHg to 94 +/- 19 mmHg (isoflurane), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of calcium channel antagonist binding sites labeled by [3H]nitrendipine in porcine coronary artery and aorta

The receptor sites for dihydropyridine calcium channel antagonists in porcine coronary artery were characterized with [3H]nitrendipine (NTD), and were compared with those in the thoracic aorta. Specific [3H]NTD binding in the coronary artery and aorta was saturable, reversible and of high affinity. These sites showed a pharmacological specificity and stereoselectivity characteristic of the receptor sites for dihydropyridine calcium channel antagonists. The pharmacological potencies of nitrendipine, nifedipine, nisoldipine, (+)-PN 200-110 and (-)-PN 200-110 in the isolated porcine coronary artery correlated well with their potencies in competing for [3H]NTD binding sites. The Hill coefficients for the competition curves for these antagonists in the porcine coronary artery and aorta were close to one. Verapamil partially inhibited specific [3H]NTD binding in both tissues. The [3H]NTD binding was increased by Mg2+ and Ca2+ but was markedly reduced by EDTA. Thus, the present study has shown that [3H]NTD selectively labels the pharmacologically relevant dihydropyridine receptors in the porcine coronary artery and that there is no significant difference between the binding characteristics of the receptor sites in the coronary artery and aorta of pigs.

Demonstration of beta 1-adrenoceptor mediating relaxation of porcine coronary artery by radioligand binding and pharmacological methods

beta-adrenoceptors in the porcine coronary artery were characterized by a radioligand binding assay using (-)-[3H]dihydroalprenolol (DHA) and also by measuring the relaxant response of isolated coronary artery to norepinephrine. Specific (-)-[3H]DHA binding in the porcine coronary artery was saturable, reversible and of high affinity (Kd = 1.6 nM) with a maximal number of binding sites of 63 fmol/mg protein, and it showed a pharmacological specificity as well as stereoselectivity which characterized beta-adrenoceptors. The Hofstee analysis of inhibition of (-)-[3H]DHA binding by atenolol, practolol and ICI 118551 has shown that the averaged concentration of beta 1 and beta 2-adrenoceptors in this tissue was 68% and 32% respectively. The relaxant response of isolated coronary artery to norepinephrine was competitively antagonized by (-)propranolol, (+)propranolol, atenolol, practolol and ICI 118551. The pA2 values of these adrenoceptor antagonists were significantly correlated with the Ki values for beta 1 but not beta 2-adrenoceptors determined by the (-)-[3H]DHA binding assay. Thus, the present study demonstrates that the relaxant response of porcine coronary artery to norepinephrine is predominantly mediated through the stimulation of beta 1-adrenoceptors on vascular smooth muscles.