Growth hormone-releasing hormone stimulates cAMP release in superfused rat pituitary cells.

The release of growth hormone (GH) and cAMP was studied in superfused rat pituitary cells by infusing growth hormone-releasing hormone (GHRH) at different doses or a combination of GHRH and somatostatin 14 (SS-14). Three-minute pulses of GHRH caused a dose-dependent GH and cAMP release (effective concentration of 50% of the maximal biological effect is 0.21 nM and 52.5 nM, respectively). The lowest effective doses of GHRH in the superfusion system were 0.03 nM for GH release and 0.3 nM for cAMP discharge when 3-min pulses were applied. The amount of cAMP liberated from the cells was not proportional to GH release: cAMP responses to low doses of GHRH were disproportionally small, and the gradual increase in the release of cAMP after high doses of GHRH was not followed by a parallel rise in GH release. The desensitization induced by repeated pulses or prolonged infusion of GHRH resulted in a greater reduction in GH release than in cAMP liberation. A simultaneous infusion of SS-14 completely blocked GH release stimulated by GHRH but did not inhibit the immediate release of cAMP caused by GHRH. An abrupt decrease in GHRH-stimulated GH release induced by SS-14 was followed by only a minimal reduction in cAMP liberation 9 min later. Our findings indicate that a discharge of cAMP is stimulated after a GHRH pulse, but this effect alone cannot maintain the release of GH. Other steps of the signal transduction mechanisms that are independent of the cAMP route may participate in the process of GH release. The nature of the mechanisms involved in the mediation of GH release may vary with the doses of GHRH used.