Effects of age on alpha 1-adrenoceptor subtypes in the heart ventricular muscle of the rat

Impaired alpha1-adrenergic responses in aged rat hearts

To determine age-related changes in the cardiac effect of alpha1-adrenergic stimulation, both cardiomyocyte Ca2+-transient and cardiac protein kinase C (PKC) activity were measured in 3-month- (3MO) and 24-month- (24MO) old Wistar rats. Ca2+ transients obtained under 1 Hz pacing by microfluorimetry of cardiomyocyte loaded with indo-1 (405/480 nm fluorescence ratio) were compared in control conditions (Kreb's solution alone) and after alpha1-adrenergic stimulation (phenylephrine or cirazoline, an alpha1-specific agonist). PKC activity and PKC translocation index (particulate/total activity) were also assayed before and after alpha1-adrenergic stimulation. In 3MO, cirazoline induced a significant increase in Ca2+ transient for a 10(-9) M concentration which returned to control values for larger concentrations. In contrast, in 24MO, we observed a constant negative effect of cirazoline on the Ca2+ transient with a significant decrease at 10(-6) M compared with both baseline and Kreb's solution. Preliminary experiments showed that, in a dose-response curve to phenylephrine, the response of Ca2+ transient was maximal at 10(-7) M. This concentration induced a significant increase in Ca2+ transient in 3MO and a significant decrease in 24MO. The same concentration was chosen to perform PKC activity measurements under alpha1-adrenergic stimulation. In the basal state, PKC particulate activity was higher in 24MO than that in 3MO but was not different in cytosolic fractions; so that the translocation index was higher in 24MO (P < 0.01). After phenylephrine, a translocation of PKC toward the particulate fraction was observed in 3MO but not in 24MO. In conclusion, cardiac alpha1-adrenoceptor response was found to be impaired in aged hearts. The negative effect of alpha1-adrenergic stimulation on Ca2+ transient in cardiomyocytes obtained from old rats can be related to an absence of alpha1-adrenergic-induced PKC translocation.

Diminished alpha1-adrenergic-mediated contraction and translocation of PKC in senescent rat heart

Myocardial reserve function declines with aging due in part to reduced alpha- and beta-adrenergic receptor (AR)-mediated contractile augmentation. Whereas specific age-associated deficits in beta-AR signaling have been identified, it is not known which components of the alpha1-AR signaling cascade, e.g., protein kinase C (PKC) and associated anchoring proteins (receptors for activated C kinase; RACKs), underlie deficits in alpha1-AR contractile function with aging. We therefore assessed cardiac contraction (dP/dt) in Langendorff perfused hearts isolated from adult (5 mo) and senescent (24 mo) Wistar rats following maximal alpha1-AR stimulation with phenylephrine (PE), and we measured the subcellular distribution of PKCalpha and PKCepsilon, and their respective anchoring proteins RACK1 and RACK2 by Western blotting. The maximum dP/dt response to PE (10(-5) M) was significantly reduced by 41% in 24-mo-old vs. 5-mo-old (P < 0.01). Inhibitory effects of PKC blockade (chelerythrine; 10 microM) on dP/dt following alpha1-AR stimulation with PE observed in adult hearts were absent in 24-mo-old hearts (P < 0.01). In 5-mo-old hearts, PE elicited reductions in soluble PKCalpha and PKCepsilon levels, while increasing particulate PKCalpha and PKCepsilon levels to a similar extent. In contrast, soluble PKCalpha and PKCepsilon levels in 24-mo-old hearts were increased in response to PE; particulate PKCepsilon and PKCalpha were unchanged or reduced and associated with significant reductions in particulate RACK1 and RACK2. The results indicate, for the first time, that selective translocation of PKCalpha and PKCepsilon in response to alpha1-AR stimulation is disrupted in the senescent myocardium. That age-related reductions in particulate RACK1 and RACK2 levels were also observed provide evidence that alterations in PKC-anchoring proteins may contribute to impaired PKC translocation and defective alpha1-AR contraction in the aged rat heart.

Alpha 1-adrenergic receptor regulation: basic science and clinical implications

Adrenergic receptors (ARs) are members of the G-protein-coupled receptor family, which includes alpha 1ARs, alpha 2ARs, beta 1ARs, beta 2ARs, beta 3ARs, adenosine, muscarinic, angiotensin, endothelin receptors, and many others that are responsible for a large variety of physiologic effects through G-protein coupling. This review focuses on alpha 1ARs and their regulation at both the mRNA and protein levels. Currently, three alpha 1AR subtypes have been characterized both pharmacologically and at the gene level: alpha 1aAR, alpha 1bAR, and alpha 1dAR. These are expressed in a species- and tissue-dependent manner. Mutagenesis approaches have been extremely valuable in the identification of key residues that govern alpha 1AR ligand binding and signaling. These studies reveal that alpha 1ARs have evolved an exquisitely sensitive regulation of their activity in which any disruption of the native structure has profound effects on subsequent function and effector coupling. Significant advances have also been made in the elucidation of signaling pathway components, resulting in the identification of novel pathways that can lead to pathologic conditions. Specific topics include mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and G-protein-coupled receptor cross-talk pathways. Within this context, recent studies identifying underlying transcriptional mechanisms involved in the regulation of the alpha 1AR subtypes are also discussed. Finally, given the potentially important role of alpha 1ARs in the vasculature, as well as in the pathology of many diseases, such as myocardial hypertrophy and benign prostatic hyperplasia, the clinical relevance of alpha 1AR distribution, pharmacology, and therapeutic intervention is reviewed.

Alpha 1D-adrenoceptors play little role in the positive inotropic action of phenylephrine

This study was designed to determine if the positive inotropic action of alpha 1-adrenergic stimulation in rat heart is mediated via alpha 1D-adrenoceptors. Isolated left atrial and papillary muscle were suspended in oxygenated Krebs-Henseleit buffer (37 degrees C) containing 1 microM nadolol and paced at 3.0 Hz. Isometric tension was continuously monitored. Cumulative concentration-response curves for phenylephrine were obtained in the presence and absence of BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4.5] decane-7,9-dione), a selective competitive alpha 1D-adrenoceptor antagonist. BMY 7378 at concentrations up to 30 nM did not significantly affect the positive inotropic response to phenylephrine. In contrast, as reported by other investigators, alpha 1D-adrenoceptor-selective concentrations of this antagonist (3 and 10 nM) did elicit a concentration-dependent right-ward shift in the vasoconstrictor response to phenylephrine in rat abdominal aorta. These data suggest that alpha 1D-adrenoceptors do not play a major role in the positive inotropic action of alpha-adrenoceptor stimulation in rat cardiac muscle.

Expression of alpha 1 adrenergic receptor subtype mRNAs in the rat cardiovascular system with aging

Alpha 1 adrenoreceptors mediate inotropic and chronotropic responses in the heart and vaso-constriction in blood vessels. Pharmacological studies using selective antagonists for subtypes of alpha 1 receptor suggest that different receptor subtypes may mediate different physiological effects. Prior studies have demonstrated that alpha 1-mediated cardiac responses are altered in aging and that alpha 1 receptor number declines with age in the rat heart. The distribution of alpha 1 receptor subtypes in cardiac tissue and subtype-specific changes with aging, however, have not been established. Using R Nase protection assays and in situ hybridization techniques, we have detected message for alpha 1B, alpha 1A, and alpha 1D in all four chambers of the rat heart and in multiple blood vessels. We found no significant changes in alpha 1 receptor subtype mRNA levels in hearts from young (4 months) and old (25 months) Sprague-Dawley rats. Also, there was expression of these three subtypes in all blood vessels assayed. We conclude that transcriptional regulation of alpha 1 adrenoreceptor subtypes does not account for age-related changes in cardiac alpha-adrenergic responsiveness.

Sustained increase in rat myocardial alpha 1A-adrenoceptors induced by 6-hydroxydopamine treatment involves a decelerated receptor turnover

The biochemical mechanisms involved in the alpha 1-adrenoceptor up-regulation and possible changes in subtypes of adrenoceptors in the rat heart after chemical denervation were investigated. The effects of acute 6-hydroxydopamine treatment (two increasing doses 24 h apart) on the pseudo-steady state densities and turnover rates of alpha 1-adrenoceptors were studied in ventricular myocardium of the rat. We have assessed the repopulation kinetics of [3H]prazosin binding sites after irreversible inactivation of alpha 1-adrenoceptors induced by a single dose of phenoxybenzamine (1 mg/kg i.p.) in rats acutely treated either with 6-hydroxy-dopamine or with vehicle (control animals). Seven days after the last administration of 6-hydroxydopamine an enhanced density of [3H]prazosin binding sites (Bmax 58.7 +/- 3.6 fmol/mg protein vehicle-treated rats versus 82.6 +/- 5.3 fmol/mg protein 6-hydroxydopamine-treated rats) was observed. This was not accompanied by changes in the dissociation constant value. Furthermore, the proportion of high affinity sites for WB-4101 was altered (21 +/- 2% versus 72 +/- 3% for animals treated with vehicle and 6-hydroxydopamine, respectively). In rat myocardium, alpha 1-adrenoceptor turnover, evaluated during the 6-hydroxydopamine-induced up-regulation (7-19 days after the completion of treatment with 6-hydroxydopamine) revealed an increase in the half-life of the alpha 1-adrenoceptor (t1/2 of 67.2 h versus 38.7 h in control animals). The present study confirms an increase in alpha 1-adrenoceptors in rat myocardium after chemical denervation and reveals that the effect is almost completely confined to the alpha 1A-adrenoceptor subtype. Furthermore, the up-regulation of alpha 1A-adrenoceptors is the result of a decrease in the cellular processes that control the rate of receptor degradation.

In vivo recovery of alpha 1-adrenoceptors in rat myocardial tissue after alkylation with phenoxybenzamine

The rate of recovery of rat myocardial alpha 1-adrenoceptor density and responsiveness after in vivo block with phenoxybenzamine (1 mg/kg, i.p.) have been investigated by measuring [3H]prazosin binding, and noradrenaline-stimulated [3H]inositol phosphate production. Repopulation of alpha 1-adrenoceptors was monoexponential, with a t1/2 of 33 h; functional recovery was also monoexponential, with t1/2 of 28 h. Furthermore, our results clearly demonstrate the absence of a receptor reserve for alpha 1-adrenoceptors mediating noradrenaline-stimulated phosphoinositide breakdown in rat myocardial tissue. These observations indicate a close relationship between the density of [3H]prazosin binding sites and the ability of alpha 1-adrenoceptors to respond to noradrenaline. Moreover, based on competition curves for inhibition of specific [3H]prazosin by WB-4101 to rat myocardial membranes 48 h and 7 days after the administration of phenoxybenzamine, the results suggest that rat myocardial membranes contain both alpha 1-adrenoceptors subtypes, i.e., alpha 1A and alpha 1B, in an approximate ratio of 20:80, and this relative ratio does not seem to be altered during the recovery process.