Intracellular Mg2+ movement during muscarinic stimulation. Mg2+ uptake by the intracellular Ca2+ store in rat sublingual mucous acini.

The muscarinic agonist carbachol produces a sustained elevation in the cytosolic free Mg2+ concentration ([Mg2+]i) by mobilizing an intracellular Mg2+ 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 investigated the relationship between the agonist-induced mobilization of the intracellular Ca2+ store and Mg2+ movement in acini loaded with the Ca(2+)-selective chelator bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) by monitoring [Mg2+]i using dual wavelength microfluorometry of the Mg(2+)-sensitive fluorescent indicator mag-fura-2. The resting [Mg2+]i in BAPTA-loaded acini was comparable to unloaded acini (0.36 +/- 0.01 mM (n = 29) versus 0.35 +/- 0.01 mM (n = 119)). In contrast to the > 40% sustained increase in [Mg2+]i in unloaded acini, carbachol stimulation of BAPTA-loaded acini induced a rapid transient [Mg2+]i decrease (approximately 30%), followed by a slower recovery to the prestimulated [Mg2+]i in 3-4 min. Furthermore, in Ca(2+)-free medium or when Ca2+ influx was blocked with La3+ or Ni2+, carbachol induced a sustained decrease in [Mg2+]i (approximately 40%). Reintroducing extracellular Ca2+ resulted in recovery of [Mg2+]i, even in the absence of extracellular Mg2+. 8-(Di-ethylamino)octyl 3,4,5-trimethoxybenzoate, an inhibitor of the inositol 1,4,5-trisphosphate-sensitive intracellular Ca2+ release pathway, completely blocked the carbachol-induced decrease in [Mg2+]i, whereas thapsigargin, a Ca(2+)-ATPase inhibitor that empties the inositol 1,4,5-trisphosphate-sensitive intracellular Ca2+ store, stimulated a decrease in [Mg2+]i. The thapsigargin-induced Mg2+ uptake was into the same intracellular pool as that activated by carbachol because stimulation with thapsigargin after carbachol did not induce a further decrease in [Mg2+]i. Taken together, these results strongly suggest that depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ store activates Mg2+ uptake, most likely to maintain charge balance of the intracellular Ca2+ pool.