INTRACELLULAR MG2- MG2+ UPTAKE BY THE INTRACELLULAR CA2+ STORE IN RATSUBLINGUAL MUCOUS ACINI( MOVEMENT DURING MUSCARINIC STIMULATION )

The muscarinic agonist carbachol produces a sustained elevation in the cytosolic free Mg2+ concentration ([Mg2+](i)) by mobilizing an intrac ellular Mg2+ pool in rat sublingual mucous acini (Zhang, G. H., and Me lvin, J. E. (1992) J. Biol. Chem. 267, 20721-20727). In the present st udy, we investigated the relationship between the agonist-induced mobi lization of the intracellular Ca2+ store and Mg2+ movement in acini lo aded with the Ca2+-selective chelator bis(o-aminophenoxy)ethane-N,N,N' ,N'-tetraacetic acid (BAPTA) by monitoring [Mg2+](i) using dual wavele ngth microfluorometry of the Mg2+-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) Dersus 0.35 +/- 0.01 mM (n = 119)). In contrast to the >40% sustained increase in [Mg2+](i) in unl oaded acini, carbachol stimulation of BAPTA-loaded acini induced a rap id transient [Mg2+](i) decrease (similar to 30%), followed by a slower recovery to the prestimulated [Mg2+](i). in 3-4 min. Furthermore, in Ca2+-free medium or when Ca2+ influx was blocked with La3+ or Ni2+, ca rbachol induced a sustained decrease in [Mg2+](i) (similar to 40%). Re introducing extracellular Ca2+ resulted in recovery of [Mg2+](i), even in the absence of extracellular Mg2+. 8-(Diethylamino)octyl 3,4,5-tri methoxybenzoate, an inhibitor of the inositol 1,4,5-trisphosphate-sens itive intracellular Ca2+ release pathway, completely blocked the carba chol-induced decrease in [Mg2+](i), whereas thapsigargin, a Ca2+-ATPas e inhibitor that empties the inositol 1,4,5-trisphosphate-sensitive in tracellular Ca2+ store, stimulated a decrease in [Mg2+](i). The thapsi gargin-induced Mg2+ uptake was into the same intracellular pool as tha t activated by carbachol because stimulation with thapsigargin after c arbachol did not induce a further decrease in [Mg2+](i). Taken togethe r, 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.