Postsomatostatin hypersecretion of growth hormone from perifused rat anterior pituitary cells is dependent on calcium influx

The role of ionic calcium (Ca2+) in the rebound secretion of growth hormone (GH) following termination of somatostatin (SRIF) administration was investigated in vitro by perifusion of acutely dispersed rat anterior pituitary cells. Treatment with 10 nM SRIF for 40 min significantly reduced the mean GH secretory rate by 3.3 +/- 0.2 ng min-1 representing a 58% decrease from baseline (p < 0.01). Following the withdrawal of SRIF treatment, GH levels surged 3- to 5-fold relative to baseline with the mean secretion rate increasing by 4.5 +/- 0.99 ng min-1 (p < 0.05). GH rebound secretion following SRIF removal from the perifusion medium was completely abolished (p < 0.01) when zero calcium medium (0 Ca2+) or medium containing 2 mM cobalt chloride (Co2+) were administered after SRIF termination. Perifusion with 0 Ca2+ caused the GH release rate to return to above baseline levels. In contrast, Co2+ perifusion caused the GH secretion rate to remain at the level observed during SRIF treatment (-4.52 +/- 0.38 ng min-1 relative to baseline; p < 0.01). Similarly, when cells were exposed to Co2+ alone, a reduction in the rate of GH secretion (-3.96 +/- 0.56 ng min-1; p < 0.01) was evident. After termination of Co2+ treatment, either by itself or following SRIF pretreatment, and upon changing from 0 Ca2+ to normal calcium-containing medium following SRIF pretreatment, a significant overshoot in GH release similar to SRIF withdrawal-induced GH release was observed (p < 0.05 and 0.01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)