SLOW INACTIVATION OF L-TYPE CALCIUM CURRENT DISTORTS THE MEASUREMENT OF L-TYPE AND T-TYPE CALCIUM CURRENT IN PURKINJE MYOCYTES

We have examined slow inactivation Of L-type calcium current in canine Purkinje myocytes with the whole cell patch clamp technique. Slow ina ctivation is voltage dependent. It is negligible at -50 mV but can ina ctivate more than half of available i(CaL) at -10 mV. There are two ma jor consequences of this slow inactivation. First, standard protocols for the measurement of T-type current can dramatically overestimate it s contribution to total calcium current, and second, the position and steepness of the inactivation versus voltage curve for i(CaL) will dep end on the method of measurement. Given the widespread attempts to ide ntify calcium current components and characterize them biophysically, an important first step should be to determine the extent of slow inac tivation of calcium current in each preparation.