Type III tastecells form classical chemical synapses and release neurotransmitter is initiated by employing exocytosis that is triggered by Ca2+ ions entering through voltage-gated (VG) Са2+-channels. The density of cell packing in the taste bud is such that the intercellular space is two orders of magnitude smaller than the intracellular one and, therefore, the concentration of extracellular ions, including Ca2+ ions, can vary considerably when the electrical activity of the taste buds changes due to the redistribution of ions between the cytoplasm and external medium. If this is the case, then the reliability of the synapse in type III cells requires the existence of a mechanism that would ensure the independence of the release of the neurotransmitter from the concentration of extracellular Ca2+ in the physiologically relevant range, that is, an increase in the intracellular Ca2+ concentration should be invariant with respect to extracellular Ca2+. To examine this idea, we analyzed intracellular Ca2+ signals generated by type III taste cells in response to stimulation at different concentrations of extracellular Ca2+. As was found, both VG Ca2+ currents in type III cells and intracellular Ca2+ signals, which subsequently trigger the release of the neurotransmitter, and its secretion proper, remain invariant when the extracellular Ca2+ varies in the physiological range. It is shown that the Ca2+ current is invariant with respect to the concentration of extracellular Ca2+ in the 1-2 mM range of the bath Ca2+, and for the curve of depolarization- induced increase in the concentration of intracellular Ca2+ from the concentration of the bath Ca2+ plateau is in the range 0.5-5 mM Ca2+. These data confirm the existence in the taste cells of type III is not clear at the moment the mechanism that can ensure the invariance of the secretion of neurotransmitters by ensuring the stability of intracellular Ca2+ signals at different concentrations of Ca2+ ions in the extracellular environment.
2+-channels
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