Ca2+ oscillations in melanotropes of Xenopus laevis: their generation, propagation, and function

The melanotrope cell of the amphibian Xenopus laevis is a neuroendocrine transducer that converts neuronal input concerning the color of background into an endocrine output, the release of alpha-melanophore-stimulating hormone (alpha-MSH). The cell displays intracellular Ca(2+) oscillations that are thought to be the driving force for secretion as well as for the expression of genes important to the process of background adaptation. Here we review the functioning of the Xenopus melanotrope cell, with emphasis on the role of Ca(2+) oscillations in signal transduction in this cell. We start by giving a general overview of the evolution of Ca(2+) as an intracellular messenger molecule. This is followed by an examination of the melanotrope as a neuroendocrine integrator cell. Then, the evidence that Ca(2+) oscillations drive the secretion of alpha-MSH is reviewed, followed by a similar analysis of the evidence that the same oscillations regulate the expression of proopiomelanocortin (POMC), the precursor protein for alpha-MSH. Finally, the possible importance of the pattern of Ca(2+) signaling to melanotrope cell function is considered.