RESUMO
The anterior pituitary gland is a site of nitric oxide (NO) production and action, suggesting a local regulatory function. We recently reported that NO inhibits in vitro prolactin release. The aim of the present study was to establish the mechanism of action of NO on prolactin release and to determine whether NO is involved in the inhibitory effect of GABA on prolactin release. Since NO exerts its action through cGMP by activating guanylate cyclase in different tissues, we examined the effect of sodium nitroprusside (NP), a NO releaser, on intrapituitary cGMP levels. Incubation of anterior pituitary glands with 0.5 mM NP 4-fold increased intrapituitary cGMP content, but decreased intrapituitary cAMP levels. In addition, we studied the effect of NP on prolactin release in the presence of LY 83583, an inhibitor of guanylate cyclase activity and 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of phosphodiesterase activity. 10 microM LY 83583 and 0.5 mM IBMX blocked the inhibitory effect of NP on prolactin release. (10(-3) M) 8Br-cGMP, an analogue of cGMP, mimicked the effect of NP on prolactin release. On the other hand, NO seems to be involved in the inhibitory effect of GABA on prolactin release since hemoglobin, a scavenger of NO, and Nw-nitro-L-arginine methyl ester, an inhibitor of NO synthase (NOS), blocked the pituitary response to GABA. Moreover, GABA (10(-6) M) stimulated NOS activity by almost 50%. GABA increased intrapituitary cGMP levels and decreased cAMP. Dopamine stimulated NOS activity weakly. These observations suggest that NO, acting through the guanylate cyclase-cGMP pathway, inhibits prolactin secretion. In addition, NO may be involved in the inhibitory effect of GABA and dopamine on prolactin release.
Assuntos
GMP Cíclico/metabolismo , Dopamina/farmacologia , Óxido Nítrico/metabolismo , Prolactina/metabolismo , Ácido gama-Aminobutírico/farmacologia , 1-Metil-3-Isobutilxantina/farmacologia , Aminoquinolinas/farmacologia , Animais , AMP Cíclico/metabolismo , GMP Cíclico/análogos & derivados , GMP Cíclico/farmacologia , Inibidores Enzimáticos/farmacologia , Hemoglobinas/farmacologia , Masculino , NG-Nitroarginina Metil Éster/farmacologia , Neurotransmissores/metabolismo , Inibidores de Fosfodiesterase/farmacologia , Hipófise/efeitos dos fármacos , Hipófise/metabolismo , Prolactina/efeitos dos fármacos , Ratos , Ratos WistarRESUMO
Nitric oxide synthase-containing cells were visualized in the anterior pituitary gland by immunocytochemistry. Consequently, we began an evaluation of the possible role of NO in the control of anterior pituitary function. Prolactin is normally under inhibitory hypothalamic control, and in vitro the gland secretes large quantities of the hormone. When hemipituitaries were incubated for 30 min in the presence of sodium nitroprusside, a releaser of NO, prolactin release was inhibited. This suppression was completely blocked by the scavenger of NO, hemoglobin. Analogs of arginine, such as NG-monomethyl-L-arginine (NMMA, where NG is the terminal guanidino nitrogen) and nitroarginine methyl ester, inhibit NO synthase. Incubation of hemipituitaries with either of these compounds significantly increased prolactin release. Since in other tissues most of the actions of NO are mediated by activation of soluble guanylate cyclase with the formation of cyclic GMP, we evaluated the effects of cyclic GMP on prolactin release. Cyclic GMP (10 mM) produced an approximately 40% reduction in prolactin release. Prolactin release in vivo and in vitro can be stimulated by several peptides, which include vasoactive intestinal polypeptide and substance P. Consequently, we evaluated the possible role of NO in these stimulations by incubating the glands in the presence of either of these peptides alone or in combination with NMMA. In the case of vasoactive intestinal polypeptide, the significant stimulation of prolactin release was augmented by NMMA to give an additive effect. In the case of substance P, there was a smaller but significant release of prolactin that was not significantly augmented by NMMA. We conclude that NO has little effect on the stimulatory action of these two peptides on prolactin release. Dopamine (0.1 microM), an inhibitor of prolactin release, reduced prolactin release, and this inhibitory action was significantly blocked by either hemoglobin (20 micrograms/ml) or NMMA and was completely blocked by 1 mM nitroarginine methyl ester. Atrial natriuretic factor at 1 microM also reduced prolactin release, and its action was completely blocked by NMMA. In contrast to these results with prolactin, luteinizing hormone (LH) was measured in the same medium in which the effect of nitroprusside was tested on prolactin release, there was no effect of nitroprusside, hemoglobin, or the combination of nitroprusside and hemoglobin on luteinizing hormone release. Therefore, in contrast to its inhibitory action on prolactin release NO had no effect on luteinizing hormone release. Immunocytochemical studies by others have shown that NO synthase is present in the folliculostellate cells and also the gonadotrophs of the pituitary gland. We conclude that NO produced by either of these cell types may diffuse to the lactotropes, where it can inhibit prolactin release. NO appears to play little role in the prolactin-releasing action of vasoactive intestinal polypeptide and substance P, but mediates the prolactin-inhibiting activity of dopamine and atrial natriuretic factor.
Assuntos
Arginina/análogos & derivados , Óxido Nítrico/fisiologia , Nitroprussiato/farmacologia , Adeno-Hipófise/metabolismo , Prolactina/metabolismo , Aminoácido Oxirredutases/antagonistas & inibidores , Animais , Arginina/farmacologia , Fator Natriurético Atrial/farmacologia , GMP Cíclico/fisiologia , Dibutiril GMP Cíclico/análogos & derivados , Dibutiril GMP Cíclico/farmacologia , Dopamina/farmacologia , Relação Dose-Resposta a Droga , Guanilato Ciclase/metabolismo , Hemoglobinas/farmacologia , Técnicas In Vitro , Hormônio Luteinizante/análise , Masculino , NG-Nitroarginina Metil Éster , Óxido Nítrico Sintase , Adeno-Hipófise/efeitos dos fármacos , Ratos , Ratos Wistar , Substância P/farmacologia , Peptídeo Intestinal Vasoativo/farmacologia , ômega-N-MetilargininaRESUMO
In order to examine the role of hypothalamic SP in the feedback regulation of prolactin, we studied the effect of prolactin and dopamine on SP concentration and release, and the effect of SP on dopamine release. Hypothalamic fragments from male Wistar rats were incubated in the presence of prolactin, dopamine or SP under basal and K(+)-stimulated conditions. SP (10(-7) M) stimulated dopamine release, while dopamine (10(-7) M) decreased SP content and release. Prolactin (100 ng ml-1) increased SP content and release. An increase in hypothalamic SP content was also found during suckling. In addition, a specific antagonist for SP, Win 62,577, blocked the effect of prolactin and dopamine release. These results show an interaction between SP and dopamine at the hypothalamic level and suggest that SP could mediate the feedback action of prolactin on dopamine release.