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Neurons are polarized into compartments such as the soma, dendrites, and axon terminals, each of which has highly specialized functions. To test whether Ca2+ is differently handled in different compartments of a neuron, we investigated Ca2+ clearance mechanisms in somata of supraoptic magnocellular neurosecretory cells (MNCs) and in their axon terminals located in neurohypophyses. Using patch-clamp and microfluorometry techniques, Ca2+ transients were evoked by depolarizing pulses. Endogenous Ca2+ binding ratios (κS) and Ca2+ clearance rates were calculated from the decay phases of Ca2+ transients according to the single compartment model. Mean values of κS were 79 ± 2.6 in somata of MNCs and 187 ± 19 in axon terminals. Ca2+ clearance rate in axon terminals, which were calculated from time derivative of Ca2+ decay and the κS values, were approximately threefold higher than in somata. In response to external Na+ reduction, Ca2+ clearance rates were reduced by 65% in axon terminals, but did not change in somata. Immunohistochemical assays confirmed that K+-dependent Na+/Ca2+ exchanger (NCKX2) was specifically localized to neurohypophysial axon terminals and was not found in somata. In somata, inhibition of sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) pumps, mitochondrial Ca2+-uniporter, and plasma membrane Ca2+-ATPase (PMCA) pumps decreased Ca2+ clearance rate by 48, 27, and 21%, respectively. These results suggest that neurohypophysial axon terminals have greater Ca2+ clearance power than somata because of the specific localization of NCKX2, and that Ca2+ clearance in somata of MNCs is mediated by SERCA pumps, mitochondrial uniporter, and PMCA pumps.

the journal of neuroscience/the journal of neuroscience

Distribution of K+-Dependent Na+/Ca2+Exchangers in the Rat Supraoptic Magnocellular Neuron Is Polarized to Axon Terminals