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Melanin-concentrating hormone (MCH), a 19-amino acid orexigenic (appetite-stimulating) hypothalamic peptide, is an important regulator of energy homeostasis. It is cleaved from its precursor prepro-MCH (ppMCH) along with several other neuropeptides whose roles are not fully defined. Because pituitary hormones such as growth hormone (GH), ACTH, and thyroid-stimulating hormone affect body weight and composition, appetite, insulin sensitivity, and lipoprotein metabolism, we investigated whether MCH exerts direct effects on the human pituitary to regulate energy balance using dispersed human fetal pituitaries (21–22 wk gestation) and cultured GH-secreting adenomas. We found that MCH receptor-1 (MCH-R1), but not MCH receptor-2, is expressed in both normal (fetal and adult) human pituitary tissues and in GH cell adenomas. MCH (10 nM) stimulated GH release from human fetal pituitary cultures by up to 62% during a 4-h incubation (P < 0.05). Interestingly, neuropeptide EI (10 nM), which is also cleaved from ppMCH, increased human GH secretion by up to 124% in fetal pituitaries. A milder, albeit significant, induction of GH secretion by MCH (20%) was seen in cultured GH-secreting pituitary adenomas. A comparable stimulation of GH secretion was seen when cultured mouse pituitary cells were treated with MCH. Treatment of cultured GH adenoma cells with MCH (100 nM) induced extracellular signal-regulated kinases 1 and 2 phosphorylation, suggesting activation of MCH-R1. In aggregate, these data suggest that MCH may regulate pituitary GH secretion and imply a potential cross-talk mechanism between appetite-regulating neuropeptides and pituitary hormones.

Intact or oophorectomized (OVX) female rats were given moderate doses of testosterone for 12 wk. Insulin-stimulated glucose transport with submaximal insulin concentrations was studied with the euglycemic clamp technique. Glycogen synthesis and 2-deoxy-D-glucose uptake were measured during the clamp in the extensor digitorum longus, white and red portions of the gastrocnemius, and in the soleus muscles by tracer technique. Testosterone treatment resulted in elevations of circulating testosterone, increased plasma insulin concentrations, and a marked decrease in insulin-stimulated glucose transport. In control animals, glycogen synthesis and 2-deoxy-D-glucose transport increased with increasing concentrations of type 1 fibers. Testosterone inhibited glycogen synthesis and 2-deoxy-D-glucose transport to approximately 50% in all muscles except 2-deoxy-D-glucose transport in intact rats. Glycogen synthesis in the liver was not affected. Testosterone administration also resulted in changes in muscle morphology. The relative number of type 1 fibers decreased, whereas type 2 fibers increased. This was most pronounced in red muscles. There was also a decrease in capillary density after testosterone treatment. It was concluded that testosterone administered to female rats is followed by marked insulin resistance. This is correlated to alterations in muscle morphology with fewer type 1 fibers and a lower degree of capillarization, which are both known to be characteristics of insulin-insensitive muscles.

Ta Gueule.