Michael T. Piascik

Central Hypotensive Effects of the α2a-Adrenergic Receptor Subtype

α2-Adrenergic receptors (α2ARs) present in the brainstem decrease blood pressure and are targets for clinically effective antihypertensive drugs. The existence of three α2AR subtypes, the lack of subtype-specific ligands, and the cross-reactivity of α2AR agonists with imidazoline receptors has precluded an understanding of the role of individual α2AR subtypes in the hypotensive response. Gene targeting was used to introduce a point mutation into the α2aAR subtype in the mouse genome. The hypotensive response to α2AR agonists was lost in the mutant mice, demonstrating that the α2aAR subtype plays a principal role in this response.

α1-Adrenergic Receptor Subtypes Molecular Structure, Function, and Signaling

The α1ARs are important mediators of sympathetic nervous system responses, particularly those involved in cardiovascular homeostasis, such as arteriolar smooth muscle constriction and cardiac contraction.1 2 In addition, α1ARs have more recently been implicated in the pathogenesis of cardiac hypertrophy, in ischemia-induced cardiac arrhythmias, and in ischemic preconditioning.1 3 Like other ARs, α1ARs are activated by the catecholamines, norepinephrine and epinephrine. They are intrinsic membrane glycoproteins and are members of the GPCR superfamily. Over the past 10 to 15 years, data initially based on functional, radioligand, and biochemical studies have accumulated, indicating that the α1ARs are a heterogeneous group of distinct but related proteins. This conclusion has been confirmed with the molecular cloning of three distinct α1-receptor subtypes, although until recently discrepancies between the properties of the cloned expressed receptors and those characterized pharmacologically and biochemically have led to confusion in the classification of α1-receptor subtypes and their coupled effector responses. As detailed in the present review, much of this confusion has now been clarified for the three cloned α1ARs. These and other recent insights into the molecular structure, function, and signaling of α1ARs, the control of α1AR-gene expression, and pharmacological evidence for additional α1AR subtypes will be reviewed here. For additional information, the reader is also referred to several previous reviews of α1ARs.4 5 6 7 Functional studies of AR responses, particularly from the laboratories of McGrath8 and Ruffolo,9 provided the initial evidence that there may be subtypes of α1ARs. These studies indicated that postjunctional responses mediated by α1ARs could not be explained adequately on the basis of a single population of receptors. This concept was further advanced …

α-Adrenoceptors and vascular regulation: Molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

Hypotension, Autonomic Failure, and Cardiac Hypertrophy in Transgenic Mice Overexpressing the α1B-Adrenergic Receptor

α1-Adrenergic receptors (α1A, α1B, and α1D) are regulators of systemic arterial blood pressure and blood flow. Whereas vasoconstrictory action of the α1A and α1D subtypes is thought to be mainly responsible for this activity, the role of the α1B-adrenergic receptor (α1BAR) in this process is controversial. We have generated transgenic mice that overexpress either wild type or constitutively active α1BARs. Transgenic expression was under the control of the isogenic promoter, thus assuring appropriate developmental and tissue-specific expression. Cardiovascular phenotypes displayed by transgenic mice included myocardial hypertrophy and hypotension. Indicative of cardiac hypertrophy, transgenic mice displayed an increased heart to body weight ratio, which was confirmed by the echocardiographic finding of an increased thickness of the interventricular septum and posterior wall. Functional deficits included an increased isovolumetric relaxation time, a decreased heart rate, and cardiac output. Transgenic mice were hypotensive and exhibited a decreased pressor response. Vasoconstrictory regulation by α1BAR was absent as shown by the lack of phenylephrine-induced contractile differences between ex vivo mesenteric artery preparations. Plasma epinephrine, norepinephrine, and cortisol levels were also reduced in transgenic mice, suggesting a loss of sympathetic nerve activity. Reduced catecholamine levels together with basal hypotension, bradycardia, reproductive problems, and weight loss suggest autonomic failure, a phenotype that is consistent with the multiple system atrophy-like neurodegeneration that has been reported previously in these mice. These results also suggest that this receptor subtype is not involved in the classic vasoconstrictory action of α1ARs that is important in systemic regulation of blood pressure.

Recent advances in the molecular pharmacology of the α1-adrenergic receptors

Abstract This review is intended to discuss recent developments in the molecular pharmacology of the α 1 -adrenergic receptor ( α 1 -AR) subtypes. After a brief historical development will focus on the more contemporary issues having to do with this receptor family. Emphasis will be put on recent data regarding the cloning, nomenclature, signalling mechanisms, and genomic organization of the α 1 -AR subtypes. We will also highlight recent mutational studies that identify key amino acid residues involved in ligand binding, as well as the role of the α 1 -AR subtypes in regulating physiologic processes.

Evidence for a complex interaction between the subtypes of the α1-adrenoceptor

Abstract Ligand binding studies with WB 4101 revealed that the rat aorta contains both the α 1a - and α 1b -adrenoceptor subtypes. Results obtained following treatment with the irreversible antagonists phenoxybenzamine, chlorethylclonidine or SZL-49 (4-amino-6,7-dimethoxy-2-quinazolinyl-4-(2-bicyclo[2,2,2]octa-2,5-dienylcarbonyl-2-piperazine) suggest that there is a complex interaction between the α 1 -adrenoceptor subtypes in the aorta. Chlorethylclonidine affects only the α 1b -adrenoceptor, whereas the predominant action of SZL-49 is on the α 1a -subtype. Chlorethylclonidine significantly inhibited the response to either methoxamine or phenylephrine, agents which are selective α 1a -adrenoceptor agonists. Following inactivation with either chlorethylclonidine or SZL-49, the response of the rat aorta to phenylephrine was only partially antagonized by either prazosin or WB 4101. SZL-49 also inhibited the response of the rat tail artery to electrical stimulation. The response of the tail artery obtained following inactivation with SZL-49 was effectively antagonized by prazosin. Phenylephrine, prazosin or WB 4101 afforded complete protection from chlorethylclonidine adrenoceptor inactivation, while these same ligands were only partially effective against SZL-49. Either SZL-49 or chlorethylclonidine significantly impaired the irreversible adrenoceptor blocking actions of phenoxybenzamine. These results suggest: (1) only the α 1a -adrenoceptor subtype appears to be associated with nerve terminals in the tail artery, (2) there may be a complex interaction between the α 1 -adrenoceptor subtypes such that both receptors must be iniact and functional to observe normal agonist and antagonist interactions, (3) there may be three sites of action for agonists associated with the rat aorta.

CALCIUM DEPENDENT REGULATION OF BRAIN AND CARDIAC MUSCLE ADENYLATE CYCLASE

The very close interdependence of Ca2+ and hormones in the overall metabolism of cyclic nucleotides has recently been emphasized by Cheung. Clearly the results presented here show that [Ca2+] in the physiological range (less than 10(-7) M to greater than 10(-6) M) has profound effects on the activity of adenylate cyclase from both brain and cardiac muscle. Whereas both brain and cardiac cyclase exhibit a Ca2+ dependent inhibition (perhaps mediated by calmodulin), only the brain cyclase is activated by Ca2+ via calmodulin. With both cyclases there is an inverse relationship between the inhibition of cyclase and the activation of calmodulin dependent (cAMP and cGMP) phosphodiesterase as a function of Ca2+ concentration. Because the IC50s for Ca2+ are the same in both heart and brain, the possibility exists that the Ca2+ inhibitory site of both cyclases is similar and perhaps identical. Considering the ability of Ca2+ to both stimulate and inhibit cyclase, one could imagine that in different species, tissues, or regions of the same tissue, there could exist multiple populations of cyclase, that is a cyclase which would only show Ca2+ dependent inhibition, Ca2+ dependent stimulation, or the biphasic response to Ca2+ (FIGURE 7). The fact that Ca2+ still regulates adenylate cyclase after various stimuli (histamine, NaF, etc.) suggests that Ca2+ may function to regulate the cyclase over shorter time periods (regardless of its state of stimulation) and that other affectors of cyclase (e.g., hormones) would serve to regulate the cyclase over longer time periods.

α1-adrenoceptor subtypes and the regulation of peripheral hemodynamics in the conscious rat

The peripheral hemodynamic effects of SZL-49, a prazosin analog capable of selectively inactivating the alpha 1a-adrenoceptor subtype, was evaluated in the conscious rat. One hour after SZL-49 administration, total peripheral vascular resistance and arterial blood pressure significantly decreased and cardiac output and heart rate increased. Twenty-four hours after SZL-49, blood pressure returned to control preinjection levels while peripheral resistance remained decreased and cardiac output and heart rate were elevated. The phenylephrine dose-response curves for mean arterial blood pressure and total peripheral vascular resistance were shifted to the right but the maximal responses were not decreased. These data show that the alpha 1a receptor plays a role in the tonic maintenance of arterial blood pressure. The alpha 1b receptor appears to participate in the response to exogenously administered agonists.

Gene expression profiling of α1b-adrenergic receptor-induced cardiac hypertrophy by oligonucleotide arrays

OBJECTIVE Cardiac hypertrophy is closely associated with the development of cardiomyopathies that lead to heart failure. The alpha(1B) adrenergic receptor (alpha(1)-AR) is an important regulator of the hypertrophic process. Cardiac hypertrophy induced by systemic overexpression of the alpha(1b)-AR in a mouse model does not progress to heart failure. We wanted to explore potential gene expression differences that characterize this type of hypertrophy that may identify genes that prevent progression to heart failure. METHODS Transgenic and normal mice (B6CBA) representing two time points were compared; one at 2-3 months of age before disease manifests and the other at 12 months when the hypertrophy is significant. Age-matched hearts were removed, cRNA prepared and biotinylated. Aliquots of the cRNA was subjected to hybridization with Affymetrix chips representing 12,656 murine genes. Gene expression profiles were compared with normal age-matched controls as the baseline and confirmed by Northern and Western analysis. RESULTS The non-EST genes could be grouped into five functional classifications: embryonic, proliferative, inflammatory, cardiac-related, and apoptotic. Growth response genes involved primarily Src-related receptors and signaling pathways. Transgenic hearts also had a 60% higher Src protein content. There was an inflammatory response that was verified by an increase in IgG and kappa-chained immunoglobulins by western analysis. Apoptosis may be regulated by cell cycle arrest through a p53-dependent mechanism. Cardiac gene expression was decreased for common hypertrophy-inducing proteins such as actin, collagen and GP130 pathways. CONCLUSIONS Our results suggest a profile of gene expression in a case of atypical cardiac hypertrophy that does not progress to heart failure. Since many of these altered gene expressions have not been linked to heart failure models, our findings may provide a novel insight into the particular role that the alpha(1B)AR plays in its overall progression or regression.

The role of α1-adrenoceptor subtypes in the regulation of arterial blood pressure

The effect of chlorethylclonidine on α1-adrenoceptor subtypes and arterial blood pressure has been evaluated. Chlorethylclonidine significantly reduced the α1-adrenoceptor population. Chlorethylclonidine treatment had no significant effect on resting systematic arterial blood pressure or heart rate and shifted the phenylephrine pressor dose-response curve only 2.4-fold to the right. These data suggest that only one of the α1-adrenoceptor subtypes plays a major role in the regulation of arterial blood pressure.