Modulation of Ca2+ influx in the ovine somatotroph by growth hormone-releasing factor

Voltage-gated Ca2+ currents were recorded using the nystatin-perforated whole cell recording configuration on the ovine somatotrophs. With the use of Ca(2+)-tetraethylammonium chloride bath solution and Cs+ electrode solution, two types of Ca2+ currents were obtained with a predominant long-lasting (L) current blocked by nifedipine. A transient (T) current was isolated in the presence of nifedipine (3 microM) and was not blocked by omega-conotoxin (5 microM), but diminished to 47 +/- 5% of control by Ni2+ (0.3 mM) or to 52 +/- 10% of control by amiloride (0.5 mM). The nifedipine-blockable L-type current was not affected by omega-conotoxin (5 microM); it was, however, attenuated to 80 +/- 4% of control by Ni2+ (0.3 mM) and to 48 +/- 6% of control by amiloride (0.5 nM). Cd2+ (1 mM) totally prevented both T and L currents. Application of growth hormone-releasing factor (GRF, 10 nM) reversibly increased the amplitude of both Ca2+ currents without modifying their kinetic properties. The effect of GRF was observed approximately 30 s after application, peaked (142 +/- 11% of control, n = 5) rapidly, and lasted > 10 min if GRF treatment was continuous. Intracellular Ca2+ concentration ([Ca2+]i) was increased by GRF (10 nM) within seconds, reaching a peak within 30 s and lasting > 250 s. Blockade of Ca2+ channels (Cd2+, 1 mM) or the use of Ca(2+)-free solution reduced basal [Ca2+]i and significantly (P < 0.05) diminished the effect of GRF on [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)