Action currents generate stepwise intracellular Ca2+ patterns in a neuroendocrine cell

It is believed that specific patterns of changes in the cytosolic-free calcium concentration ([Ca2+](i)) are used to control cellular processes such as gene transcription, cell proliferation, differentiation, and secretion. me recently showed that the Ca2+ oscillations in the neuroendocrine melanotrope cells of Xenopus laevis are built up by a number of discrete Ca2+ rises, the Ca2+ steps. The origin of the Ca2+ steps and their role in the generation of long-lasting Ca2+ patterns were unclear. By simultaneous, noninvasive measuring of melanotrope plasma membrane electrical activity and the [Ca2+](i), we show that numbers, amplitude, and frequency of Ca2+ steps are variable among individual oscillations and are determined by the firing pattern and shape of the action currents. The general Na+ channel blocker tetrodotoxin had no effect on either action currents or the [Ca2+](i). Under Na+-free conditions, a depolarizing pulse of 20 mM K+ induced repetitive action currents and stepwise increases in the [Ca2+](i). The Ca2+ channelblocker CoCl2 eliminated action currents and stepwise increases in the [Ca2+](i) in both the absence and presence of high K+. We furthermore demonstrate that the speed of Ca2+ removal from the cytoplasm depends on the [Ca2+](i), also between Ca2+ steps during the rising phase of an oscillation, It is concluded that Ca2+ channels, and not Na+ channels, are essential for the generation of specific step patterns and, furthermore, that the frequency and shape of Ca2+ action currents in combination with the Ca2+ removal rate determine the oscillatory pattern.