β1- and β2-adrenoceptors in sheep cardiac ventricular muscle

The presence of beta 2-adrenoceptors in the sheep ventricular myocardium was assessed by the radioligand binding technique and functional studies. In membrane preparations, the competition curve between [3H]-dihydroalprenolol and the selective beta 1-antagonist CGP 20712A (0.1 nM-1 mM) was clearly biphasic, and revealed the presence of two different binding sites showing an affinity (pKD) for CGP 20712A of 9.5 +/- 0.9 and 4.5 +/- 0.4, respectively. The relative proportion of beta 1:beta 2 adrenoceptors was about 70:30 in both the right and left ventricle. In ventricular trabeculae driven at 1Hz, isoprenaline (1-300 nM) caused a dose-dependent increase in the force of contraction, the maximum effect being 298 +/- 26 mg, associated with reduction of time to peak tension (t1, clinotropic effect) and relaxation time (t2, 298 +/- 26 mg, associated with reduction of time to peak tension (t1, clinotropic effect) and relaxation time (t2, lusitropic effect). The inotropic dose-response curve for isoprenaline was significantly shifted to the right by pretreatment of the preparations with 0.1 microM CGP 20712A or with the selective beta 2-antagonist ICI 118551 (50 nM). In the presence of CGP 20712A (0.1 microM), isoprenaline, up to a concentration of 10 microM, did not affect either t1 or t2; on the other hand, pretreatment of the preparations with ICI 118551 (50 nM) fully antagonized the clinotropic but not the lusitropic effect of isoprenaline. In the presence of CGP 20712A procaterol (0.01-10 microM), a beta 2-adrenoceptor agonist, induced a positive intropic effect which was not associated with any significant modifications in t1 or t2. This effect was completely abolished by ICI 118551 (50 nM). The positive inotropic action of isoprenaline (1 microM) was associated with a significant decrease in action potential duration measured at -60 mV (220 +/- 8 and 193 +/- 10 ms in the absence and presence of isoprenaline, respectively; P less than 0.05). In the presence of CGP 20712A (0.1 microM) alone, isoprenaline (1 microM) still induced a significant increase in contractility but the action potential profile was only slightly affected. The effects of isoprenaline were fully antagonized by the simultaneous presence of CGP 20712A and ICI 118551 (10 nM). It is concluded that both beta 1- and beta 2-adrenoceptors appear to coexist in sheep ventricular myocardium where their stimulation mediates a positive inotropic effect. However, their functional role on the relaxation phase of the twitch may be different.