Analysis of autofeedback mechanisms in the secretion of pro-opiomelanocortin-derived peptides by melanotrope cells of <i>Xenopus laevis</i>

The secretion of most pituitary hormones is under the control of feedback mechanisms. The feedback control of α-melanophore-stimulating hormone (α-MSH) from melanotrope cells is controversial. The possible existence of an autofeedback exerted by α-MSH or other POMC-derived peptides on melanotrope cells of the amphibian Xenopus laevis has been investigated. α-MSH or its potent agonist 4-norleucine, 7-d-phenylalanine-α-MSH has no effect on the release of radiolabeled POMC-derived peptides or immunoreactive β-endorphin from superfused neurointermediate pituitary lobes. Melanin concentrating hormone, previously reported to have an α-MSH-like effect on melanophores, did not affect α-MSH secretion. Neurointermediate lobe superfusate, which contains a mixture of POMC-derived peptides, failed to affect the secretory activity of melanotropes. It is concluded that in X. laevis the secretory activity of melanotropes is not under the control of short-term auto-feedback mechanisms involving α-MSH or other POMC-derived peptides