THE INTERACTION OF GROWTH HORMONE RELEASING HORMONE WITH OTHER HYPOTHALAMIC HORMONES ON THE RELEASE OF ANTERIOR PITUITARY HORMONES

To determine whether the 29 amino-acid fragment of growth hormone releasing hormone (GHRH) can be combined with other hypothalamic releasing hormones in a single test of anterior pituitary reserve, the responses of anterior pituitary hormones to combinations of an i.v. bolus of GHRH(1-29)NH2 or saline with an i.v. bolus of either LH releasing hormone (LHRH) plus TRH, ovine CRH(oCRH) or saline were studied. Each infusion of GHRH(1-29)NH2 resulted in a rapid increment of the plasma GH value. Infusion of GHRH(1-29)NH2 also caused a small and transient rise in plasma PRL, but no change in the integrated PRL response. The combination of GHRH(1-29)NH2 with LHRH plus TRH caused a larger increment of peak and integrated plasma TSH levels than LHRH plus TRH alone. GHRH(1-29)NH2 did not affect the release of other anterior pituitary hormones after infusion with oCRH or LHRH plus TRH. Because of the finding of potentiation of the TSH-releasing activity of LHRH plus TRH by GHRH(1-29)NH2, the study was extended to the investigation of TSH release after infusion of TRH in combination with either GHRH(1-29)NH2 or GHRH(1-40). In this study the combination of TRH with both GHRH preparations also caused a larger increment of the peak and integrated plasma TSH levels than TRH alone. It is concluded that GHRH(1-29)NH2 possesses moderate PRL-releasing activity apart from GH-releasing activity. In addition, GHRH potentiates the TSH-releasing activity of TRH.(ABSTRACT TRUNCATED AT 250 WORDS)