EFFECT OF LONG-ACTING ANTAGONISTS OF GROWTH-HORMONE (GH)-RELEASING HORMONE ON GH AND CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE RELEASE IN SUPERFUSED RAT PITUITARY-CELLS

In a recent study, investigating the time course of both GH and cAMP s ecretion induced by GH-releasing hormone (GHRH) in the superfusion sys tem, we found that the amount of cAMP liberated from the cells was not proportional to GH release and that cAMP discharged after a GHRH puls e alone cannot maintain the release of GH. In the present study, two p otent antagonists of GHRH, MZ-4-71 Ibu(0),D-Arg(2),Phe(4-Cl)(6),Abu(15 ),Nle(27)]human GHRH-(1-28)Agm) and MZ-4-243 ([Nac(0),D-Arg(2),Phe(4-C l)(6),Abu(15) Nle(27)]human GHRH-(1-28)Agm) were evaluated for their l ong term effect in the superfusion system and for their ability to inf luence the release of GH and cAMP. Our present findings showed that af ter a 9-min preincubation, antagonist MZ-4-71 and MZ-4-243 at 3 and 1 nM, respectively, caused an inhibition of GH release, stimulated by 1 nM GHRH, similar to that caused by the 100-nM dose of the standard ant agonist ([Ac-Tyr(1),D-Arg(2)]human GHRH-(1-29)NH2). The standard antag onist at 100 nM did not influence the GHRH-induced GH response 30 min later, and the inhibition caused by MZ-4-71 at 30 nM decreased gradual ly to 30% 120 min after the treatment, but the 30-nM dose of MZ-4-243 reduced the GH response by more than 90% even 270 min after its admini stration. During a 2-h incubation with 1 nM GHRH in combination with a 30-min infusion of the standard antagonist, MZ-4-71, MZ-4 -243, or so matostatin, from the 30th to the 60th min, the decrease in GH discharg e preceded the inhibition of cAMP release. After infusion of the antag onists or somatostatin was stopped, GH release resumed sooner than tha t of cAMP. Simultaneous determinations of cAMP and GH in the samples s howed that changes in GH levels were never preceded by a rise or decre ase in cAMP release, in contrast to existing information. The particip ation of more than one signal transduction mechanism in the mediation of the effect of GHRH is very likely, and the balance of these mechani sms may vary with the dose and duration of stimulation.