EXTRACELLULAR CALCIUM MEDIATES BOTH GRF AND TRH INDUCED GH RELEASE FROM CHICKEN PITUITARY-CELLS

Considerable evidence exists to suggest the necessity of a rise in int racellular calcium following growth hormone releasing factor (GRF) and thyrotropin (TRH) stimulation of GH release in the anterior pituitary gland. In the present study the involvement of this ion in the synerg istic GRF/TRH stimulated release of growth hormone (GH) from the domes tic fowl pituitary gland was investigated. Both GRF and TRH stimulated GH release and Ca-45(2+) influx were significantly (P<0.05) reduced i n the presence of the calcium channel blockers: nifedipine and veraper mil. A simultaneous addition of GRF and TRH produced a significant pot entiation of the GH release equal to 475.7 +/- 11.8% above basal level s (P<0.05). This increase was not associated with an equivalent potent iation of either Ca-45(2+) influx or intracellular calcium levels. Bot h parameters failed to show any synergistic responses. The current stu dy has discounted the involvement of extracellular calcium influx in t he synergistic effect of GRF and TRH and possible regulatory mechanism s are discussed for the observation.