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

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Abstract
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. Since sequential administration of GHRH(1–29)NH2