Electrophysiologic mechanisms responsible for inotropic responses to ketamine in guinea pig and rat myocardium.

Inotropic and electrophysiologic effects of ketamine were investigated in cardiac preparations isolated from guinea pigs and rats. Ketamine produced a concentration-dependent negative inotropic effect in electrically driven guinea pig papillary muscles, an effect that was accompanied by a decrease in action potential duration at the 0-mV level (APD0). In contrast, ketamine produced a concentration-dependent positive inotropic effect in rat left atria in the presence of 10(-6) M propranolol. The increase in force of contraction was accompanied by an increase in APD0. Experiments using patch clamp techniques revealed that ketamine reduced the transsarcolemmal Ca2+ current (ICa) as well as the inward rectifier K+ current and delayed outward K+ current in guinea pig single ventricular cells. These results indicate that the shortening of APD0 observed in guinea pig papillary muscles might result from the suppression of ICa. In rat single ventricular cells ketamine reduced the Ca(2+)-insensitive transient outward current (Ito) and did not enhance ICa, suggesting that the ketamine-induced prolongation of APD0 observed in rat left atria is due to a decrease in Ito rather than an increase in ICa. Treatment of rat left atria with the specific Ca(2+)-insensitive Ito inhibitor 4-aminopyridine (2 mM) produced a positive inotropic effect and prolongation of APD0, and these effects were equivalent to those caused by the highest concentration of ketamine. In the presence of 4-aminopyridine, ketamine failed to induce a positive inotropic effect and instead caused a negative inotropic one. In conclusion, the negative and positive inotropic effects of ketamine may result from the suppression of ICa and Ito, respectively. The inhibitory action on these membrane currents may at least in part explain the species and tissue differences in inotropic responses to ketamine.