Relationship between cardiotonic effects and inhibition on cardiac sarcolemmal Na+,K+-ATPase of strophan-thidin at low concentrations.

To evaluate the effects of strophanthidin (Str) on cardiac contractile function and sarcolemmal Na+, K+-ATPase activities in isolated guinea-pig hearts. Isolated guinea-pig hearts were perfused through aorta in a Langendorff mode. Heart rate (HR), left ventricular pressure (LVP), and first derivatives (+/-dp/dt(max)) of LVP were recorded by eight-channel physiological instrument. Cardiac sarcolemmal Na+, K+-ATPase activities were determined with colorimetry. Str 0.1 nmol/L stimulated the Na+, K+-ATPase activities (P<0.05), but had no effect on HR, LVP, and +/-dp/dt(max). Str 1 nmol/L increased +dp/dt(max) (P<0.05) and Na+, K+-ATPase activities (P<0.01). Str 10 and 100 nmol/L significantly increased both LVP (P<0.05) and +dp/dt(max) (P<0.05 or P<0.01), and had no significant effects on Na+, K+-ATPase activities. However, Str 1-100 micromol/L at first enhanced the LVP and +dp/dtmax (P<0.01), then reduced them resulting from irregular contraction, and effects of Str on Na+, K+-ATPase activities revealed a concentration-dependent inhibition (P<0.01). The positive inotropic effects and irregular contraction produced by Str at higher concentrations result from the inhibition of Na+, K+-ATPase activities, and the positive inotropic effects of Str at lower concentrations are not related to the inhibition of the Na+, K+-ATPase activities.