Voltage-dependent calcium currents in rat gonadotropes separated by centrifugal elutriation

Centrifugal elutriation of rat pituitary cells yielded two functionally active fractions (6 and 7) that contained 30-40 and 50-60% gonadotropes, respectively. We studied the membrane ionic currents of these cells with the whole cell patch-clamp method. In physiological solution, cells from the enriched fractions displayed a tetrodotoxin-sensitive sodium current. In 20 mM external calcium and zero sodium, the inward current contained two components that were different in threshold, steady-state inactivation, and deactivation. One component was half activated at approximately -25 mV, half inactivated from a holding potential of -61 mV, and deactivated with a time constant of 3 ms. The second component was half activated at 6 mV and deactivated with a time constant of 0.35 ms. These two currents resembled the high-voltage-activated and low-voltage-activated calcium current described in many preparations, including clonal and primary pituitary cells. Bath application of 20 nM gonadotropin-releasing hormone caused a transient and reversible decrease of the calcium current at depolarized voltages and a negative shift of 10 mV in the activation curve. Both effects were observed in a percentage of cells that closely matches the percentage of gonadotropes in the fraction. The shift appeared to affect both components of the current and can partially account for the increased activity of Ca2+ channels at potentials close to the resting value.