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 Ca2+-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 mu M) and was not blocked by omega-conotoxin (5 mu M), 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 mu M); it was, however, attenuated to 80 +/- 4% of control by Ni2+ (0.3 mM) and to 48 +/- 6% of control by amiloride (0.5 mM). 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 similar to 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 Ca2+-free solution reduced basal [Ca2+](i) and significantly (P < 0.05) diminished the effect of GRF on [Ca2+](i). The secretion of growth hormone (GH) was increased significantly (P < 0.05) by GRF (10 nM), but the response was totally prevented by the application of Cd2+ (1 mM) or nifedipine (3 mu M). These are the first data to show that GRF acts directly on voltage-gated Ca2+ channels to increase Ca2+ permeability of the ovine somatotroph cell membrane. The subsequent increase in [Ca2+](i) and resultant GH secretion in response to GRF appears to be attributable to this Ca2+ influx.