Melanotrope Cells of Xenopus laevis Express Multiple Types of High-Voltage-Activated Ca2+ Channels

Pituitary melanotrope cells are neuroendocrine signal transducing cells that translate physiological stimuli into adaptive hormonal responses. In this translation process, Ca2+ channels play essential roles. We have characterised which types of Ca2+ current are present in melanotropes of the amphibian Xenopus laevis, using whole‐cell, voltage‐clamp, patch‐clamp experiments and specific blockers of the various current types. Running an activation current–voltage relationship protocol from a holding potential (HP) of −80 mV/or −110 mV, shows that Xenopus melanotropes possess only high‐voltage activated (HVA) Ca2+ currents. Steady‐state inactivation protocols reveal that no inactivation occurs at −80 mV, whereas 30% of the current is inactivated at −30 mV. We determined the contribution of individual channel types to the total HVA Ca2+ current, examining the effect of each channel blocker at an HP of −80 mV and −30 mV. At −80 mV, ω‐conotoxin GVIA, ω‐agatoxin IVA, nifedipine and SNX‐482 inhibit Ca2+ currents by 21.8 ± 4.1%, 26.1 ± 3.1%, 24.2 ± 2.4% and 17.9 ± 4.7%, respectively. At −30 mV, ω‐conotoxin GVIA, nifedipine and ω‐agatoxin IVA inhibit Ca2+ currents by 33.8 ± 3.0, 24.2 ± 2.6 and 16.0 ± 2.8%, respectively, demonstrating that these blockers substantially inhibit part of the Ca2+ current, independently from the HP. We have previously demonstrated that ω‐conotoxin GVIA can block Ca2+ oscillations and steps. We now show that nifedipine and ω‐agatoxin IVA do not affect the intracellular Ca2+ dynamics, whereas SNX‐482 reduces the Ca2+ step amplitude. We conclude that Xenopus melanotrope cells express all four major types of HVA Ca2+ channel, as well as the resulting currents, but no low‐voltage activated channels. The results provide the basis for future studies on the complex regulation of channel‐mediated Ca2+ influxes into this neuroendocrine cell type as a function of its role in the animal's adaptation to external challenges.