The Journal of Neurophysiology Vol. 83 No. 1 January 2000, pp. 81-89
Copyright ©2000 by the American Physiological Society

Two Mechanisms That Raise Free Intracellular Calcium in Rat Hippocampal Neurons During Hypoosmotic and Low NaCl Treatment

 ¹Institute for Neurobiology, University of Amsterdam, 1098 SM Amsterdam, The Netherlands; and  ²Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710

Borgdorff, Aren J., George G. Somjen, and Wytse J. Wadman. Two Mechanisms That Raise Free Intracellular Calcium in Rat Hippocampal Neurons During Hypoosmotic and Low NaCl Treatment. J. Neurophysiol. 83: 81-89, 2000. Previous studies have shown that exposing hippocampal slices to low osmolarity (_o) or to low extracellular NaCl concentration ([NaCl]_o) enhances synaptic transmission and also causes interstitial calcium ([Ca²⁺]_o) to decrease. Reduction of [Ca²⁺]_o suggests cellular uptake and could explain the potentiation of synaptic transmission. We measured intracellular calcium activity ([Ca²⁺]_i) using fluorescent indicator dyes. In CA1 hippocampal pyramidal neurons in tissue slices, lowering _o by ~70 mOsm caused "resting" [Ca²⁺]_i as well as synaptically or directly stimulated transient increases of calcium activity ([Ca²⁺]_i) to transiently decrease and then to increase. In dissociated cells, lowering _o by ~70 mOsm caused [Ca²⁺]_i to almost double on average from 83 to 155 nM. The increase of [Ca²⁺]_i was not significantly correlated with hypotonic cell swelling. Isoosmotic (mannitol- or sucrose-substituted) lowering of [NaCl]_o, which did not cause cell swelling, also raised [Ca²⁺]_i. Substituting NaCl with choline-Cl or Na-methyl-sulfate did not affect [Ca²⁺]_i. In neurons bathed in calcium-free medium, lowering _o caused a milder increase of [Ca²⁺]_i, which was correlated with cell swelling, but in the absence of external Ca²⁺, isotonic lowering of [NaCl]_o triggered only a brief, transient response. We conclude that decrease of extracellular ionic strength (i.e., in both low _o and low [NaCl]_o) causes a net influx of Ca²⁺ from the extracellular medium whereas cell swelling, or the increase in membrane tension, is a signal for the release of Ca²⁺ from intracellular stores.