Modulation of Ca2+ influx in the ovine somatotroph by growth hormone-releasing factor
Chen, C.; Clarke, I.J.
American Journal of Physiology 268(2 Pt 1): E204-E212
1995
ISSN/ISBN: 0002-9513
PMID: 7864095
Accession: 009035047
Voltage-gated Ca-2+ 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 Ca-2+ 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 Ni-2+ (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 Ni-2+ (0.3 mM) and to 48 +- 6% of control by amiloride (0.5 mM). Cd-2+ (1 mM) totally prevented both T and L currents. Application of growth hormone-releasing factor (GRF, 10 nM) reversibly increased the amplitude of both Ca-2+ currents without modifying their kinetic properties. The effect of GRF was observed apprx 30 s after application, peaked (142 +- 11% of control, n = 5) rapidly, and lasted gt 10 min if GRF treatment was continuous. Intracellular Ca-2+ concentration ((Ca-2+)-i) was increased by GRF (10 nM) within seconds, reaching a peak within 30 s and lasting gt 250 s. Blockade of Ca-2+ channels (Cd-2+, 1 mM) or the use of Ca-2+-free solution reduced basal (Ca-2+)-i and significantly (P lt 0.05) diminished the effect of GRF on (Ca-2+)-i. The secretion of growth hormone (GH) was increased significantly (P lt 0.05) by GRF (10 nM), but the response was totally prevented by the application of Cd-2+ (1 mM) or nifedipine (3 mu-M). These are the first data to show that GRF acts directly on voltage-gated Ca-2+ channels to increase Ca-2+ permeability of the ovine somatotroph cell membrane. The subsequent increase in (Ca-2+)-i and resultant GH secretion in response to GRF appears to be attributable to this Ca-2+ influx.