Characterization of adrenoceptors mediating positive inotropic responses in the ventricular myocardium of the dog.

1 The pharmacological characteristics of adrenoceptors mediating the positive inotropic action in the dog heart were assessed by the use of blood-perfused papillary muscles and isolated strips of ventricular myocardium.2 On the blood-perfused papillary muscle driven at 2 Hz and in sinus node preparations, phenylephrine induced positive inotropic and chronotropic responses in the same dose range and was much less potent than isoprenaline. The dose-response curve for the chronotropic action of phenylephrine was parallel to that of isoprenaline, whilst the dose-response curve for the inotropic action of phenylephrine was less steep than that of isoprenaline.3 The infusion of pindolol, a beta-adrenoceptor blocking agent, at a rate of 1 mug/min, shifted the isoprenaline dose-response curves to the right, and to the same extent, in both papillary muscle and sinus node preparations. In contrast to isoprenaline, the antagonism of phenylephrine by pindolol was noncompetitive. Phentolamine did not affect the positive inotropic and chronotropic actions of phenylephrine.4 On isolated ventricular strips alpha-adrenoceptor blockade by 10(-6) M phentolamine did not affect dose-response curves to phenylephrine or dopamine. Pindolol shifted the dopamine dose-response curves to the right in a competitive manner and those of phenylephrine in a noncompetitive manner.5 On ventricular strips from reserpine-pretreated dogs phenylephrine and tyramine dose-response curves were shifted markedly to the right and downwards. Desipramine (10(-5) M) which enhanced the action of noradrenaline considerably reduced the myocardial responses of phenylephrine.6 Papaverine (10(-5) M) decreased the threshold concentration of phenylephrine required to stimulate the myocardium and shifted phenylephrine dose-response curves to the left.7 Raising the temperature from 32 degrees C to 37 degrees C shifted phenylephrine dose-response curves to the right; when the temperature was raised from 37 degrees C to 42 degrees C the affinity of the drug was not changed.8 Other alpha-adrenoceptor stimulants, methoxamine and clonidine, decreased the active tension of ventricular strips. The responses to noradrenaline and adrenaline (in the presence of pindolol; 3 x 10(-8) M) were not affected by phentolamine (10(-6) M).9 The results indicate that adrenoceptors mediating positive inotropic responses in the dog ventricle are of the beta-type and that post-synaptic alpha-adrenoceptors are not involved. Phenylephrine acts mainly by releasing noradrenaline from adrenergic nerve endings and partly by a weak direct action on beta-adrenoceptors.