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 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.
Lieste
J R
JR
Department of Cellular Animal Physiology, Institute of Cellular Signaling and Nijmegen Institute for Neurosciences, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands.
Koopman
W J
WJ
Reynen
V C
VC
Scheenen
W J
WJ
Jenks
B G
BG
Roubos
E W
EW
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002478:Cells, Cultured; D004594:Electrophysiology; D005260:Female; D016257:Fura-2 / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008856:Microscopy, Fluorescence; D018408:Patch-Clamp Techniques; D010902:Pituitary Gland / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D014982:Xenopus laevis
D002121:Calcium Channel Blockers; D013779:Tetrodotoxin; D012964:Sodium; D011188:Potassium; D002118:Calcium; D016257:Fura-2