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

Distribution of alpha1-adrenergic receptor mRNA species in rat heart

Radioligand binding studies have detected alpha1A- and alpha1B-adrenergic receptors (AR) in rat heart, but the ligands available for these studies lack the sensitivity and specificity needed to map possible differences in alpha1-AR subtype expression. We therefore used competitive reverse transcriptase-polymerase chain reaction (RT-PCR) techniques to measure steady-state amounts of alpha1-AR messenger RNA (mRNA) subtypes in tissue dissected from several regions of rat heart. We detected mRNA for alpha1A-, alpha1B-, and alpha1D-AR in each region. Irrespective of the alphaAR subtype, the total number of alpha1-AR transcripts has the following regional rank order: left ventricular papillary muscle > left ventricle > left atrium > apex > right ventricle > ventricular septum > right atria. Among the regions, the fractional contribution of alpha1A-, alpha1B-, and alpha1D-AR mRNA to the total amount of alpha1-AR displays considerable variability. The alpha1B-AR mRNA accounts for >50% of the total alpha1-AR mRNA in all regions except the ventricular septum. There are also significant percentages of alpha1A-AR in each region, especially in the papillary muscle (48%) and ventricular septum (48%). The alpha1D-AR mRNA transcripts are found in comparatively low numbers; their highest levels (18% of total) were found in the right ventricle. These differences in alpha1-AR mRNA expression may contribute to the observed regional differences in myocardial responses to alpha1-AR agonists and antagonists.

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.

Cardiac alpha(1)-adrenoceptors that regulate contractile function: subtypes and subcellular signal transduction mechanisms

Activation of alpha(1)-adrenoceptors as well as endothelin (ET) and angiotensin II (Ang II) receptors in cardiac muscle is coupled to acceleration of the hydrolysis of phosphoinositide (PI), with resultant production of inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol. There is an excellent correlation between the extent of acceleration of the PI hydrolysis and the positive inotropic effect (PIE) under most experimental conditions after the administration of a-adrenoceptor agonists, ET and Ang II in the rabbit ventricular muscle. The PIE of the alpha-adrenoceptor agonists, ET and Ang II is associated with a negative lusitropic effect and an increase in the sensitivity of myofilaments to Ca(2)+ ions. The PIE can be selectively inhibited by inhibitors of protein kinase C (PKC) such as staurosporine, NA 0345 and H-7, with little effect on the PI hydrolysis and the PIE of isoproterenol and Bay k 8644. Surprisingly, an activator of PKC, phorbol 12,13-dibutyrate (PDBu), selectively and more completely inhibited the PIE and acceleration of PI hydrolysis induced by the alpha-adrenoceptor agonists as well as by ET and Ang II in the rabbit. These receptor agonists consistently cause intracellular alkalinization by activation of Na+-H+ exchange, while the effects on membrane ion channel activities are divergent. For example, alpha-adrenoceptor agonists cause monophasic prolongation of the action potential, the time course of which coincides well with that of the PIE, while ET and Ang II produce a biphasic change in action potential duration, i.e., the long-lasting prolongation preceded by a transient abbreviation. Alpha-adrenoceptor agonists scarcely affect I(ca), whereas ET elicits a biphasic alteration of the current. In addition, the potassium current, I(K1), is markedly suppressed by alpha-adrenoceptor agonists, but this effect is not revealed with Ang II under the same experimental condition. These results indicate that the effects of alpha(1)-adrenoceptor stimulation are partially shared by those of FT and Ang II receptor activation in the heart. Approximately 60% of the total population of alpha(1)-adrenoceptors in the rabbit ventricle are composed of alpha(1A) subtype, which is susceptible to chlorethylclonidine (CEC) and is predominantly responsible for the alpha(1)-mediated PIE and PI hydrolysis. The remaining fraction that belongs to alpha(1A) subtype is further subclassified into the WB 4101-sensitive (partly coupled to PI hydrolysis) and the niguldipine-sensitive (PI hydrolysis-unrelated) subtypes.

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.