Release of Stored, Pre-Labeled Growth Hormone and Prolactin from Perifused Rat Pituitary: Effect of Human Pancreatic Growth Hormone-Releasing Factor-44

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Abstract
Human pancreatic growth hormone-releasing factor-44 (hpGRF-44) differentially stimulates release of stored and newly synthesized rGH without altering rGH synthesis over 3 h in static in vitro incubation; hpGRf-44 also stimulates release of stored, but not newly synthesized, rPRL. To study the time course of pre-labeled, stored hormone release without pharmacologically interrupting synthesis, the current experiments were performed in perifusion. Fifteen minute pulses of 0.1 to 10 nM hpGRf-44 stimulated stored [3H]rGH release (to 890% of base); 1.0 to 10 nM hpGRF-44 stimulated stored [3H]rPRL release (to 440% of base). Pulses of 0.1 to 1.0 mM (Bu)2cAMP also stimulated release of [P3H]rGH (to 570% of base) and [3H]rPRL (to 410% of base). However, peak [3H]rGH and [3H]rPRL responses to hpGRF-44 required 10 min, while peak responses to (Bu)2cAMP required 25 min. Continuous hpGRF-44 stimulated an initial surge of stored [3H]rGH release which was not sustained; the diminishing release was not explained by hpGRF-44 degrdation. Total radioimmunoassayable (RIA) hormone release roughly paralleled release of stored immunoprecipitable (IPn) hormone. Conclusions: in pituitary perifusion, hpGRf-44 stimulates release of both stored rGH and rPRL as shown in static incubation, but (i) the response is biphasic: initial rapid release is followed by a progressively lesser response; and (ii) the response is both more acute and less well sustained than that resulting from exposure to (Bu)2cAMP.