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Volume 271, Number 46, Issue of November 15, 1996 pp. 29067-29072
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Na⁺-dependent Release of Mg²⁺ from an Intracellular Pool in Rat Sublingual Mucous Acini

(Received for publication, April 3, 1996, and in revised form, September 9, 1996)

Guo H. Zhang and James E. Melvin ^¶

From the Departments of  Dental Research and ^¶ Neurobiology and Anatomy, University of Rochester, Rochester, New York 14642

Muscarinic stimulation induces release of Mg²⁺ from an intracellular pool in rat sublingual mucous acini (Zhang, G. H., and Melvin, J. E. (1992) J. Biol. Chem. 267, 20721-20727). In the present study we examined the interdependence of Mg²⁺ mobilization on intracellular Na⁺ and Ca²⁺ by monitoring the intracellular free concentrations of Na⁺ ([Na⁺]_i), Mg²⁺ ([Mg²⁺]_i), and Ca²⁺ ([Ca²⁺]_i) using ion-sensitive fluorescent indicators. Gramicidin increased the intracellular concentrations of all three ions. Comparable to agonist-stimulated mobilization of Mg²⁺, the gramicidin-induced [Mg²⁺]_i increase was independent of extracellular Mg²⁺ indicating release of Mg²⁺ from an intracellular pool. Clamping the [Ca²⁺]_i near 30 nM with the Ca²⁺-selective chelator BAPTA failed to alter the [Na⁺]_i or [Mg²⁺]_i increases generated by gramicidin. In contrast, depletion of intracellular Na⁺ markedly suppressed the muscarinic-stimulated [Mg²⁺]_i increase, whereas the [Ca²⁺]_i increase was similar to that seen in physiological extracellular Na⁺. These results revealed that intracellular Mg²⁺ mobilization did not directly relate to the [Ca²⁺]_i, but required an increase in [Na⁺]_i. Consistent with this hypothesis, increasing [Na⁺]_i by activating Na⁺ influx via the Na⁺/H⁺ exchanger also increased the [Mg²⁺]_i. The Na⁺/Mg²⁺ exchange inhibitor quinidine suppressed both the gramicidin- and muscarinic-induced discharge of internal Mg²⁺. These results suggest that release of Mg²⁺ from an intracellular pool is mediated by a Na⁺-dependent Mg²⁺ transport mechanism in salivary acinar cells.

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