An extracellular sulfhydryl group modulates background Na+ conductance and cytosolic Ca2+ in pituitary cells

Abstract

Treatment of GH3 pituitary cells with p-chloromercurybenzenesulfonate (PCMBS) increased the cytosolic Ca2+ concentration ([Ca2+]i). This effect was reversed by dithiothreitol and blocked by L-type Ca2+ channel antagonists or Na+ removal. PCMBS increased membrane conductance and depolarized the plasma membrane. Apart from minor effects on K+ and Ca2+ channels, PCMBS increased (6 times at -80 mV) an inward Na+ current whose properties were similar to those of a background Na+ conductance (BNC) described previously, necessary for generation of spontaneous electrical activity. In rat lactotropes and somatotropes in primary culture, PCMBS also produced a Na+-dependent [Ca2+]i increase, whereas little or no effect was observed in thyrotropes, corticotropes, and gonadotropes. The Na+ conductance elicited by PCMBS in somatotropes seemed to be the same as that stimulated by the hypothalamic growth hormone (GH)-releasing hormone, which regulates membrane excitability and GH secretion. The BNC studied here could play a physiological role, regulating excitability and spontaneous activity, and explains satisfactorily the [Ca2+]i-increasing actions of the mercurials reported previously in several excitable tissues.