Comparative study of postsynaptic alpha-adrenoceptors in aorta obtained from human and other mammalian species

Regulation of alpha(2)-adrenoceptors in human vascular smooth muscle cells

This study analyzed the regulation of alpha2-adrenoceptors (alpha2-ARs) in human vascular smooth muscle cells (VSMs). Saphenous veins and dermal arterioles or VSMs cultured from them expressed high levels of alpha2-ARs (alpha2C > alpha2A, via RNase protection assay) and responded to alpha2-AR stimulation [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14,304, 1 microM)] with constriction or calcium mobilization. In contrast, VSMs cultured from aorta did not express alpha2-ARs and neither cultured cells nor intact aorta responded to UK-14,304. Although alpha2-ARs (alpha2C >> alpha2A) were detected in aortas, alpha2C-ARs were localized by immunohistochemistry to VSMs of adventitial arterioles and not aortic media. In contrast with aortas, aortic arterioles constricted in response to alpha2-AR stimulation. Reporter constructs demonstrated higher activities for alpha2A- and alpha2C-AR gene promoters in arteriolar compared with aortic VSMs. In arteriolar VSMs, serum increased expression of alpha2C-AR mRNA and protein but decreased expression of alpha2A-ARs. Serum induction of alpha2C-ARs was reduced by inhibition of p38 mitogen-activated protein kinase (MAPK) with 2 microM SB-202190 or dominant-negative p38 MAPK. UK-14,304 (1 microM) caused calcium mobilization in control and serum-stimulated cells: in control VSMs, the response was inhibited by the alpha2A-AR antagonist BRL-44408 (100 nM) but not by the alpha2C-AR antagonist MK-912 (1 nM), whereas after serum stimulation, MK-912 (1 nM) but not BRL-44408 (100 nM) inhibited the response. These results demonstrate site-specific expression of alpha2-ARs in human VSMs that reflects differential activity of alpha2-AR gene promoters; namely, high expression and function in venous and arteriolar VSMs but no detectable expression or function in aortic VSMs. We found that alpha2C-ARs can be dramatically and selectively induced via a p38 MAPK-dependent pathway. Therefore, altered expression of alpha2C-ARs may contribute to pathological changes in vascular function.

A modified aortic multiple-ring preparation for functional studies

A comparison is made of four rat aortic preparations, that is, single ring, spiral strip, zig-zag strip, and multiple-ring, on the basis of the responses to norepinephrine (NE) and acetylcholine (ACh). The single ring preparation was suitable under isometric, but not under isotonic conditions, because of the small isotonic tension which developed in response to the contractile agonist. The spiral and the zig-zag strips showed a discontinuity in the relaxant activity of the ACh, probably because of the removal, or not, of the endothelium from the preparation. The reproducibility of both contracturant and relaxing responses in the multiple-ring preparation makes this the most suitable of all four for the study of vasoactive drugs.

Analysis of the activity of alpha 1-adrenoceptor antagonists in rat aorta

1. In this study, the effect of seven alpha 1-adrenoceptor antagonists (tamsulosin, phentolamine, prazosin, WB-4101, 5-methylurapidil, spiperone and HV723) have been examined on the contractile response to noradrenaline (NA) and phenylephrine (PE) in rat isolated aorta. 2. NA and PE, when administered using a cumulative dosing schedule, both produced concentration-dependent contraction of aortic rings. It was possible to fit the individual concentration-effect (E/[A]) curve data to the Hill equation to provide estimates of the curve midpoint location (p[A]50 = 7.74 +/- 0.10 and 7.14 +/- 0.18), midpoint slope (nH = 0.82 +/- 0.03 and 0.99 +/- 0.10) and upper asymptote (alpha = 3.2 +/- 0.3 and 3.1 +/- 0.2 g) parameters for NA and PE, respectively. However, the Hill equation provided a better fit to the E/[A] curve data obtained with another contractile agent, 5-hydroxytryptamine (5-HT) (p[A50] = 6.09 +/- 0.08, nH = 1.49 +/- 0.09, alpha = 2.6 +/- 0.3 g), as judged by calculation of the mean sum of squares of the differences between the observed and predicted values. 3. All of the antagonists investigated produced concentration-dependent inhibition of the contractile responses of the aorta to NA and PE. Although no significant effects on the upper asymptotes of the E/[A] curves of any of the antagonists tested were detected, only tamsulosin and 5-methylurapidil did not have a significant effect on the slope (nH) of the NA and PE E/[A] curves. The other antagonists produced significant steepening of the curves obtained with NA and/or PE. 4. Notwithstanding the fact that one of the basic criteria for simple competitive antagonism at a single receptor class was not always satisfied, the individual log [A]50 values estimated in the absence and presence of antagonist within each experiment were fitted to the competitive model. The Schild plot slope parameters for the antagonism of NA and PE by phentolamine and HV723 were found to be significantly less than unity. The Schild plot slope parameters for the other antagonists were not significantly different from unity. 5. In the absence of evidence to suggest that the deviations from simple competitive antagonism were due to failure to satisfy basic experimental conditions for quantitative analysis, an attempt was made to see whether the data could be accounted for by an existing two-receptor model (Furchgott, 1981). The goodness-of-fit obtained with the two-receptor model was significantly better than that obtained with the one-receptor model. Furthermore, with the exception of the data obtained with phentolamine, the pKB estimates for the two receptors were independent of whether NA or PE was used as agonist. 6. To determine which alpha 1-adrenoceptor subtypes may be associated with those defined by the two receptor model, the mean pKB estimates obtained from the two-receptor model fit were compared with affinities measured by Laz et al. (1994) for rat cloned alpha 1-adrenoceptor subtypes expressed in COS-7 cells. The sum of squared differences of the data points from the line of identity was smallest for both pKB1 and pKB2 in the case of the alpha 1a/d-adrenoceptor (now referred to as alpha 1d-adrenoceptor; Hieble et al., 1995). Therefore, the complexity exposed in this study may be due to the expression of closely-related forms of the alpha 1d-adrenoceptor. However, relatively good matches were also found between pKB1 and alpha 1c and between pKB2 and alpha 1b. Therefore, on the basis of these data, it is not possible to rule out the involvement of all three alpha 1-adrenoceptors. The conflicting reports concerning the characteristics of the alpha 1-adrenoceptor population mediating contraction of the rat aorta may, at least in part, be due to the lack of highly selective ligands and to between-assay variation in the expression of multiple alpha 1-adrenoceptors.

Vascular alpha-1 adrenergic receptor subtypes in the regulation of arterial pressure

Alpha 1 (alpha 1)-adrenoceptors can be found at numerous end organs in the autonomic nervous system, especially vascular smooth muscle. The tonic sympathetic activation of vascular alpha 1-adrenoceptors maintains vascular resistance and is vital to the regulation of arterial pressure. Recent evidence clearly demonstrates that alpha 1-adrenoceptors are a heterogenous class of receptors and that each subtype may subserve specific cardiovascular functions. Elucidation of the physiological role of each subtype in the regulation of vascular resistance and arterial pressure will enhance our understanding of the cardiovascular system and may facilitate the development of therapeutics with improved efficacy and tolerability.

Mediation of noradrenaline-induced contractions of rat aorta by the alpha 1B-adrenoceptor subtype

1. The subtypes of alpha 1-adrenoceptor mediating contractions to exogenous noradrenaline (NA) in rat aorta have been examined in both biochemical and functional studies. 2. Incubation of rat aortic membranes with the irreversible alpha 1B-adrenoceptor antagonist, chloroethylclonidine (CEC: 10 microM) did not change the KD of [3H]-prazosin binding in comparison to untreated membranes, but reduced by 88% the total number of binding sites (Bmax). 3. Contractions of rat aortic strips to NA after CEC (50 microM for 30 min) incubation followed by repetitive washing, showed a marked shift in the potency of NA and a partial reduction in the maximum response. The residual contractions to NA after CEC incubation were not affected by prazosin (10 nM). 4. The competitive antagonists prazosin, terazosin, (R)-YM-12617, phentolamine, 5-methylurapidil and spiperone inhibited contractions to NA with estimated pA2 values of 9.85, 8.54, 9.34, 7.71, 7.64 and 8.41, respectively. 5. The affinity of the same antagonists for the alpha 1A- and alpha 1B- adrenoceptors was evaluated by utilizing membranes from rat hippocampus pretreated with CEC, and rat liver, respectively. 5-Methylurapidil and phentolamine were confirmed as selective for the alpha 1A-adrenoceptors, whereas spiperone was alpha 1B-selective. 6. A significant correlation was found between the pA2 values of the alpha 1-adrenoceptor antagonists tested and their affinity for the alpha 1B-adrenoceptor subtype, but not for the alpha 1A-subtype. 7. In conclusion, these findings indicate that in rat aorta most of the contraction is mediated by alpha 1B-adrenoceptors, and that the potency (pA2) of an antagonist in this tissue should be related to its antagonistic effect on this subtype of the alpha 1-adrenoceptor population.