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All classes of calcium channel couple with equal efficiency to exocytosis in rat melanotropes, inducing linear stimulus‐secretion coupling
Author(s) -
Mansvelder Huibert D.,
Kits Karel S.
Publication year - 2000
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2000.t01-1-00327.x
Subject(s) - exocytosis , chemistry , biophysics , voltage dependent calcium channel , channel blocker , membrane potential , calcium , endocrinology , medicine , secretion , biology , biochemistry , organic chemistry
1 The contribution of low voltage‐activated (LVA) T‐type Ca 2+ channels and four different types of high voltage‐activated (HVA) Ca 2+ channel to exocytosis, and the relationship between calcium influx and exocytosis during action potentials (APs) were studied in pituitary melanotropes. 2 Selective HVA Ca 2+ channel blockers reduced exocytosis, monitored by membrane capacitance measurements, proportional to the reduction in Ca 2+ influx. The efficacy of Ca 2+ in stimulating exocytosis did not change in the presence of the Ca 2+ channel blockers, indicating that all HVA Ca 2+ channels act together in stimulating exocytosis. 3 The relationship between Ca 2+ influx and exocytosis during the AP was examined using APs recorded from spontaneously active melanotropes as command templates under voltage clamp. Under voltage clamp, multiphasic Ca 2+ currents were activated over the entire duration of the APs, i.e. during the rising phase as well as the plateau phase. The maximum amplitude of the Ca 2+ current coincided with the peak of the AP. 4 The relationship between Ca 2+ entry and exocytosis was linear for the different phases of the AP. Also, the influx of Ca 2+ through LVA T‐type channels stimulated exocytosis with the same efficacy as through the HVA channels. 5 APs of increasing duration (∼50 to ∼300 ms) evoked increasing amounts of exocytosis. The number of entering Ca 2+ ions and the capacitance change were linearly related to AP duration, resulting in a fixed relationship between Ca 2+ entry and exocytosis. 6 The results show that Ca 2+ ions, entering a melanotrope, couple with equal strength to exocytosis regardless of the channel type involved. We suggest that the linear relationship between Ca 2+ entry and secretion observed under physiological conditions (during APs), results from the equal strength with which LVA and HVA channels in melanotropes couple to exocytosis. This guarantees that secretion takes place over the entire duration of the AP.