Influences of extracellular calcium ions, verapamil, and calcium antagonistic cations on the positive inotropic effects mediated by alpha- and beta-adrenoceptors in the left atria of rats.

Inotropic responses produced by stimulation of alpha-adrenoceptors were compared with those by isoproterenol in the left atria of rats. The negative frequency-force relationship was abolished or attenuated by isoproterenol, but not counteracted by phenylephrine in the presence of sotalol. The positive inotropic effect of phenylephrine was uniquely modulated by reduction in extracellular Ca2+ concentration. Verapamil at 10(-8) M (at which it produces no negative intropic action) depressed only the effect of phenylephrine. Pretreatment with Ca2+-antagonistic cations in low concentrations depressed the effect of phenylephrine without influencing that of isoproterenol.