LOCATION OF ALPHA(1)-ADRENOCEPTORS RELATIVE TO BETA-ADRENOCEPTORS IN RAT MYOCARDIUM

Electrically driven rat papillary muscles (1 Hz) were examined for the location of their alpha(1)-adrenoceptor and beta-adrenoceptor populat ions relative to each other. We determined the horizontal position of the dose-response curves for the positive inotropic effects exerted by noradrenaline in the absence and presence of the neuronal uptake bloc ker cocaine and in the absence and presence of the beta-adrenoceptor a ntagonist timolol and of the alpha(1)-adrenoceptor antagonist prazosin . Cocaine slightly shifted the dose-response curves for alpha(1)-adren oceptor stimulation to a lower concentration of agonist. In contrast, the dose-response curve for beta-adrenoceptor stimulation was markedly shifted by cocaine to a lower concentration of agonist. Experiments w ith corticosterone (an extraneuronal uptake blocker) revealed no diffe rential shift of either of the dose-response curves. Together, these d ata indicate that the alpha(1)-adrenoceptor population is located more distantly from the adrenergic nerve terminals than the beta(1)-adreno ceptor population in rat myocardium.