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

Abstract

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, omega-conotoxin GVIA, omega-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, omega-conotoxin GVIA, nifedipine and omega-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 omega-conotoxin GVIA can block Ca2+ oscillations and steps. We now show that nifedipine and omega-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.

Dopamine D₂-Receptor Activation Differentially Inhibits N- and R-Type Ca²⁺ Channels in <i>Xenopus</i> Melanotrope Cells

“…Since we have previously shown [10]that the ω-conotoxin inhibition of the total current depends on the state of channel inactivation, we also tested the apomorphin-induced inhibition under ω-conotoxin at an HP of –30 mV, i.e., when part of the current is inhibited [10]. Compared to an HP of –80 mV, at –30 mV ω-conotoxin inhibited a larger part of the total I Ca , namely 33.7 ± 3.0% (p < 0.05), whereas for the peak and sustained currents the inhibitions were 31.7 ± 3.0 and 35.9 ± 3.4%, respectively (fig.…”

“…In a previous study we showed that Xenopus melanotropes possess all classes of HVA Ca 2+ currents. Of those only the R-type current has fast activation and inactivation kinetics [10]. Therefore, we hypothesize that at least R-type Ca 2+ currents are inhibited by D 2 -receptor activation.…”

“…This secretory cell is a suitable cell model for such a study, as it expresses L-type, P/Q-type, N-type and R-type Ca 2+ currents channels [10]and, moreover, possesses a wide variety of G-protein-coupled receptors [11]. Xenopus melanotropes release α-melanophore- stimulating hormone (α-MSH), which causes darkening of the skin [12].…”

About a snail, a toad, and rodents: animal models for adaptation research

“…Calcium channels play a crucial role in determining the secretory activity of Xenopus melanotropes (Zhang et al, 2005). Melanotropes from black background-adapted toads have a 2 times larger diameter, a 2.5-fold larger membrane capacitance, a 2-fold higher high-voltage-activated (HVA) current density, a 10% smaller Ca 2+ -dependent inactivation, and L-type channels with 5 times slower activation and inactivation kinetics.…”

“…Such Ca 2+ oscillations are generated at the plasma membrane through opening of N-type and R-type Ca 2+ channels (Lieste et al, 1998a; Zhang et al, 2005) and may drive the secretion of αMSH. By mobilizing intracellular calcium stores (“calcium-induced calcium release”; Koopman et al, 1999) the Ca 2+ influx also initiates a self-propagating Ca 2+ wave that enters the nucleus (Scheenen et al, 1996; Koopman et al, 2001) and possibly induces gene expression (Scheenen et al, 1994, 1996; Lieste et al, 1998b; Jenks et al, 2003; Van den Hurk et al, 2005b; Kuribara et al, 2010a).…”