Effects of thiopental on tension development, action potential, and exchange of calcium and potassium in rabbit ventricular myocardium.

Thiopental in concentrations of 28 and 227 mumoles/liter reduced the tension developed by isolated rabbit ventricular myocardium by 50 and 80%, respectively. The effect was rapid, reversible, and independent of heart rate. Thiopental (227 mumoles/liter) inhibited influx and efflux of potassium to the same extent, so that no net gain or loss occurred. Calcium influx and efflux were also reduced, but calcium was not displaced from the myocardium in association with the decline of tension. The maximum rate of depolarization of the action potential and the time to 50% repolarization were decreased, but resting membrane potential, amplitude, and time to 90% depolarization were unchanged. The negative inotropic effect of thiopental appears to be due to reduced availability of calcium to the myofibrils.