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. We 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⁺ channel blocker 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 Na²⁺ 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.