Alpha-adrenoceptor-mediated inotropism and beta-adrenoceptor-mediated inotropism in isolated rabbit ventricles: a...

We assessed alpha-adrenoceptor-mediated inotropism in comparison with beta-adrenoceptor-mediated inotropism in whole left ventricle. We used an isolated perfused isovolumic rabbit heart preparation, which enables one to assess contractility and time course of contraction as well as energetic efficiency. Left ventricular pressure (LVP) and volume were measured with an intraventricular fluid-filled balloon. Myocardial oxygen consumption was assessed from the oxygen tension of coronary effluent. Both isoproterenol (n = 7) and phenylephrine (n = 7) increased LV developed pressure (LVDP). Isoproterenol shortened the time to peak pressure (Tmax) from 171 +/- 6 to 142 +/- 7 ms (p less than 0.01) and the time constant of LV pressure decay (TBF) from 44.8 +/- 2.7 to 32.8 +/- 1.9 ms (p less than 0.01). In contrast, phenylephrine slightly but significantly prolonged Tmax from 178 +/- 4 to 184 +/- 4 (p less than 0.05) and did not alter TBF. Both isoproterenol and phenylephrine increased O2 consumption, whereas only phenylephrine increased the force-time integral from 14.0 +/- 1.5 to 17.2 +/- 2.1 g.s (p less than 0.01). These results confirmed in the whole heart preparation that alpha- and beta-adrenoceptor-mediated inotropisms had qualitatively different effects on the time course of contraction and energetic efficiency.