Secretagogue-induced mobilization of an intracellular Mg2+ pool in rat sublingual mucous acini.
The regulation of the intracellular free Mg2+ concentration ([Mg2+]i) was monitored in rat sublingual mucous acini using dual wavelength microfluorometry of the Mg(2+)-sensitive dye mag-fura-2. Acini attached to coverslips and superfused continuously with a Mg(2+)-containing medium (0.8 mM) have a steady-state [Mg2+]i of 0.35 +/- 0.01 mM. Adjusting the extracellular Mg2+ concentration to 0 and 10 mM or removing extracellular Na+ did not alter the resting [Mg2+]i. Stimulation with the Ca(2+)-mobilizing, muscarinic agonist, carbachol, induced a sustained increase in [Mg2+]i (approximately 50%; t1/2 < 20 s; Kd approximately 1.5 microM), the magnitude and the duration of which were unchanged in Mg(2+)-depleted medium indicating that the rise in [Mg2+]i was generated by Mg2+ release from an intracellular Mg2+ pool. Forskolin, which increases the intracellular cAMP content, produced a small, transient increase in the [Mg2+]i (< 10%). Muscarinic stimulation in a Ca(2+)-free medium blunted the initial increase in [Mg2+]i by approximately 50%, whereas the sustained increase in [Mg2+]i was lost. When the muscarinic-induced increase in [Ca2+]i was blocked by 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate, an inhibitor of the agonist-sensitive intracellular Ca2+ release pathway, both the initial and the sustained phases of the increase in [Mg2+]i were virtually eliminated. Thapsigargin and 2,5-di-(terbutyl)-1,4-benzohydroquinone, which increase [Ca2+]i by inhibiting microsomal Ca(2+)-ATPase, caused a dramatic increase in [Mg2+]i. Stimulation in a Na(+)-free medium or in the presence of bumetanide, an inhibitor of Na+/K+/2Cl- cotransport, blunted the agonist-induced rise in [Mg2+]i (approximately 50%), whereas ouabain, a Na+,K(+)-ATPase inhibitor, had no significant effect. FCCP (carbonyl cyanide p-trifluoromethoxyphenylhydrazone), a mitochondrial uncoupler, mobilized an intracellular Mg2+ pool as well. The carbachol-induced increase in [Mg2+]i was markedly inhibited by FCCP (approximately 80%), suggesting that the same pool(s) of Mg2+ were primarily involved. The above results provide strong evidence that Ca(2+)-mobilizing agonists increase cytoplasmic free [Mg2+] by releasing an intracellular pool of Mg2+ that is associated with a rise in the [Na+]i.
Zhang
G H
GH
Department of Dental Research, University of Rochester, New York 14642.
Melvin
J E
JE
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology*; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D005456:Fluorescent Dyes; D005707:Gallic Acid / Q000031:analogs & derivatives; D006873:Hydroquinones / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology; D008297:Male; D008985:Monensin / Q000494:pharmacology; D009061:Mouth Mucosa / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013050:Spectrometry, Fluorescence; D013373:Substance P / Q000494:pharmacology
D002121:Calcium Channel Blockers; D005456:Fluorescent Dyes; D006873:Hydroquinones; C019359:2,5-di-tert-butylhydroquinone; D013373:Substance P; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; C006014:8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate; D010042:Ouabain; D005707:Gallic Acid; D002217:Carbachol; D008985:Monensin; D008274:Magnesium