Voltage-dependent Vmax blockade in Na+-dependent action potentials after beta 1- and H2-receptor stimulation in...

In isolated papillary muscles of guinea pigs, the influence of isoproterenol, histamine, theophylline, and phenylephrine on the maximal rate of rise (Vmax) of Na+-dependent action potentials and on isometric contractile force was studied under rested state conditions. Isoproterenol (1 x 10(-7) mol/L), histamine (2 x 10(-5) mol/L), and theophylline (2 x 10(-3) mol/L) shifted the voltage dependence of Vmax into the hyperpolarizing direction and, consequently, led to a voltage-dependent Vmax blockade. The alpha-adrenoceptor agonist phenylephrine, on the other hand, proved to be ineffective in depressing Vmax. The beta-receptor blocker pindolol (4 x 10(-6) mol/L) or the H2-receptor blocker cimetidine (4 x 10(-5) mol/L) abolished the inhibitory effects of isoproterenol and histamine, respectively, and caused Vmax to return to the initial control value. A concentration-response relationship analysis at -65 mV revealed that isoproterenol exerted only a weak inhibitory effect on Vmax compared with its positive inotropic action. The IC50 value of the former effect amounted to approximately 5 x 10(-6) mol/L, but the EC50 value of the latter effect was 4 x 10(-8) mol/L. It is, therefore, concluded that, in physiologically relevant concentrations, beta-adrenergic agonists are unlikely to significantly modulate Na+-dependent excitability even in partially depolarized myocardium.