Steroid hormones are thought to have a positive inotropic action, but the precise electrophysiological effects which underlie this positive inotropism remain unknown. The effects of hydrocortisone on the L-type calcium current (I
Ca) of the isolated guinea pig ventricular cells were investigated under the whole-cell voltage-clamp recording. Hydrocortisone (10
-6 to 10
-5 M) increased the amplitude of I
Ca without any changes in the voltage-dependency in I
Ca. This agent retarded the fast inactivation component of I
Ca, but not the slow component. On the other hand, hydrocortisone slightly reduced the slope conductance of inward rectifying potassium current (I
K1) at potentials less than -50 mV. This effect on I
K1 is favorable for the activation of I
Ca. Thus, these results indicate that the positive inotropism of hydrocortisone is mediated, at least in part, by a direct potentiating effect on I
Ca.
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