Characterization of Mechanical Effects and β-Adrenoceptor Binding of Prenalterol in Rat Myocardium

Publisher Website
Google Scholar
Abstract
The partial .beta.-agonist prenalterol had been found to differ from the full agonist isoprenaline in some aspects of its cardiac action. This study examined in rat myocardium whether prenalterol elicited the same qualitative changes of the contraction-relaxation cycle as was previously found for isoprenaline. Binding of prenalterol to .beta.-adrenoceptors was also measured. Prenalterol augmented relaxation more than contraction and thus evoked the same qualitative changes of the contraction-relaxation cycle as did isoprenaline. The response developed more slowly than that to isoprenaline, and the effect on relaxation followed a more protracted time course than the effect on contraction. Prenalterol bound non-selectivity to .beta.1- and .beta.2-adrenoceptors in both heart and lung broken cell preparations. pKd [- log Kd] for binding to .beta.-adrenoceptors and pD2 [- log of the concentration giving half maximal effect] values for functional effects in heart were similar, i.e., prenalterol had to occupy half the amount of .beta.-adrenoceptors to evoke half-maximal functional effects. The non-selective .alpha.-blocker, phentolamine, potentiated the effects of prenalterol on contraction, but did not change the equilibrium binding of prenalterol to cardiac .beta.-adrenoceptors. Phentolamine did not change the potency and efficacy of isoprenaline. Although prenalterol qualitatively evoked the same response as isoprenaline, it also exhibited some properties which differed.