Characteristics of two types of calcium channels in rat pituitary gonadotrophs.

The properties of Ca2+ channels in cultured rat pituitary gonadotrophs were analyzed by the patch-clamp technique. The inward Ca2+ currents, recorded in the presence of 5.2 mM Ca2+ or Ba2+, included a fast, transient component with activation-inactivation kinetics and a delayed component with slower activation. The midpoint of the activation curve lay at -30 mV for the transient component and at -12 mV for the delayed component. At the midpoint, changes in potential of 9.5 and 13 mV induced an e-fold change in the activation of the transient and delayed components, respectively. The rate of inactivation of the first component was strongly voltage dependent. At -43 mV, a 7.4-mV change in potential induced an e-fold change in the fraction of Ca2+ channels available to conduct Ca2+ current. During long-lasting (100-200 ms) low-frequency depolarizing voltage-clamp pulses, the size of the delayed component of the Ca2+ current remained constant. The differential effects of membrane potential on inactivation and the different time constants for activation of the two components of the Ca2+ conductance indicate the presence of two types of Ca2+ channels in the membrane of the gonadotroph: the rapidly inactivating current appears to be attributable to a T-type channel, and the noninactivating current corresponds to the L-type channel described in many other cell types.