The extracellular calcium-sensing receptor increases the number of calcium steps and action currents in pituitary melanotrope cells

Secretion of α-melanophore-stimulating hormone (α-MSH) from the neuroendocrine melanotrope cells in the intermediate lobe of the pituitary gland of the clawed frog Xenopus laevis is regulated by various inhibitory, stimulatory and autocrine factors. The neuropeptide sauvagine stimulates α-MSH secretion by changing the pattern of intracellular Ca2+ oscillations and the electrical properties of the cell membrane. In the present study we investigated whether another secreto-stimulator, the extracellular Ca2+-sensing receptor (CaR), also affects the Ca2+ oscillatory pattern and electrical membrane properties. Using high-speed dynamic video-imaging we show that activation of the CaR with the specific agonist l-phenylalanine (l-Phe) changes the Ca2+ oscillatory pattern by increasing the number of Ca2+ steps, which are the “building blocks” of the oscillations. Moreover, using patch-clamp electrophysiology it is demonstrated that l-Phe affects membrane properties by increasing frequency and duration of action currents. Compared to sauvagine, the CaR has different effects on the action current parameters, suggesting that multiple mechanisms regulate the electrical properties of the melanotrope cell membrane and, thereby, the Ca2+ oscillation-dependent level of α-MSH secretion.