Sustained increase in rat myocardial α1A-adrenoceptors induced by 6-hydroxydopamine treatment involves a decelerated receptor turnover

The biochemical mechanisms involved in the α1-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 α1-adrenoccptors were studied in ventricular myocardium of the rat. We have assessed the repopulation kinetics of [3H]prazosin binding sites after irreversible inactivation of α1-adrenoceptors induced by a single dose of phenoxybenzamine (1 mg/kg i.p.) in rats acutely treated either with 6-hydroxydopamine 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, α1-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 α1-adrenoceptor (t1/2 of 67.2 h versus 38.7 h in control animals). The present study confirms an increase in α1-adrenoceptors in rat myocardium after chemical denervation and reveals that the effect is almost completely confined to the α1A-adrenoceptor subtype. Furthermore, the up-regulation of α1A-adrenoceptors is the result of a decrease in the cellular processes that control the rate of receptor degradation.