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)
Chen
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Prince Henry's Institute of Medical Research, Clayton, Victoria, Australia.
Clarke
I J
IJ
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002478:Cells, Cultured; D004594:Electrophysiology; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D007425:Intracellular Membranes / Q000378:metabolism; D009994:Osmolar Concentration; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D012756:Sheep
D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D002118:Calcium