INVOLVEMENT OF NITRIC-OXIDE IN THE INTERFERON-GAMMA-INDUCED INHIBITION OF GROWTH-HORMONE AND PROLACTIN SECRETION IN ANTERIOR-PITUITARY CELL-CULTURES

In previous work it was shown that the immune cytokine interferon-gamm a (IFN-gamma) inhibits hormone secretion in anterior pituitary (AP) ce ll cultures, an action most likely mediated by folliculostellate (FS) cells. In the present study, we wanted to investigate whether nitric o xide (NO) is involved in this inhibitory action of IFN-gamma. NO synth ase (NOS) inhibitors with affinity for the inducible (iNOS) and the co nstitutive (cNOS) isoform such as N-G-monomethyl-L-arginine (L-NMMA) a nd S-methyl-L-thiocitrulline (SMLT) dose-dependently blocked the inhib itory action of IFN-gamma on GHRH-stimulated GH secretion, and partial ly reversed the inhibitory effect on basal prolactin (PRL) release. In the absence of IFN-gamma these inhibitors significantly augmented bas al PRL release and slightly enhanced GHRH-stimulated GH release. L-N-6 -(1-iminoethyl)lysine (L-NIL), a NOS inhibitor with preferential affin ity for iNOS, abrogated the IFN-gamma effect on GHRH-stimulated GH sec retion and partially reversed IFN-gamma inhibition of PRL release. How ever, L-NIL did not exert a stimulatory effect on basal PRL and GHRH-s timulated GH release by its own. 2,4-diamino-6-hydroxypyrimidine (DAHP ), a NOS inhibitor by interfering with tetrahydrobiopterin (BH4) cofac tor availability, showed the same activity profile as L-NIL. NOS inhib itors blocked or reduced the production of NO as detected by measuring nitrite (NO2-) levels in AP cell cultures and cGMP levels in the NO-r eporter cell line RFL-6. The NOS inhibiting action of L-NMMA was confi rmed by competition experiments with the natural NOS substrate L-argin ine. Thus, in culture medium with lower amounts of L-arginine, L-NMMA blocked the IFN-gamma-induced inhibition of GHRH-stimulated GH release at a lower dose. The inhibition of PRL and GH release by IFN-gamma wa s markedly reduced in L-arginine-depleted medium. The NO donor sodium nitroprusside (SNP) mimicked the inhibitory action of IFN-gamma on GHR H-stimulated GH and basal PRL release. Similarly to IFN-gamma, SNP did not affect basal GH release. As previously reported, inhibition by IF N-gamma occurred only in AP cell populations containing a minimal prop ortion of FS cells. As studied in different cell populations obtained by unit gravity sedimentation in a serum albumin gradient, L-NMMA reve rsed the IFN-gamma effect in the same populations enriched in FS cells . Interestingly, in the absence of IFN-gamma L-NMMA strongly stimulate d basal PRL release in the population most enriched in FS cells. It is concluded that IFN-gamma, through activation of the iNOS pathway prob ably in FS cells enhances the production of NO and that this effect is responsible for the inhibitory action of IFN-gamma on GHRH-stimulated GH release and partially for the IFN-gamma-induced decrease in basal PRL release. On the other hand, NO, likely produced by cNOS, appears t o exert a tonic inhibitory effect on GHRH-stimulated GH and basal PRL release. It seems therefore that low amounts of NO produced constituti vely may take charge of subtle physiological adaptations, and higher l evels of NO produced by iNOS under the influence of IFN-gamma may atte nuate PRL and GH release during emergency conditions of immune and inf lammatory reactions. (C) 1997 Elsevier Science Ireland Ltd.