Na+ gradient-dependent Mg2+ transport in smooth muscle cells of guinea pig tenia cecum.
Thin strips of guinea pig tenia cecum were loaded with the Mg2+ indicator furaptra, and the indicator fluorescence signals measured in Ca2+-free condition were converted to cytoplasmic-free Mg2+ concentration ([Mg2+]i). Lowering the extracellular Na+ concentration ([Na+]o) caused a reversible increase in [Mg2+]i, consistent with the inhibition of Na+ gradient-dependent extrusion of cellular Mg2+ (Na+-Mg2+ exchange). Curve-fitting analysis indicated that the relation between [Na+]o and the rate of rise in [Mg2+], had a Hill coefficient of approximately 3, a [Na+]o at the half-maximal rate of rise of approximately 30 mM, and a maximal rate of 0.16 +/- 0.01 microM/s (mean +/- SE, n = 6). Depolarization with 56 mM K+ shifted the curve slightly toward higher [Na+]o without significantly changing the maximal rate, suggesting that the Na+-Mg2+ exchange was inhibited by depolarization. The maximal rate would correspond to a flux of 0.15-0.4 pmol/cm2/s, if cytoplasmic Mg2+ buffering power (defined as the ratio of the changes in total Mg2+ and free Mg2+ concentrations) is assumed to be 2-5. Ouabain (1-5 microM) increased the intracellular Na+ concentration, as assessed with fluorescence of SBFI (sodium-binding benzofuran isophthalate, a Na+ indicator), and elevated [Mg2+]i. In ouabain-treated preparations, removal of extracellular Na+ rapidly increased [Mg2+]i, with an initial rate of rise roughly proportional to the degree of the Mg2+ load, and, probably, to the Na+ load caused by ouabain. The enhanced rate of rise in [Mg2+]i (up to approximately 1 microM/s) could be attributed to the Mg2+ influx as a result of the reversed Na+-Mg2+ exchange. Our results support the presence of a reversible and possibly electrogenic Na+-Mg2+ exchange in the smooth muscle cells of tenia cecum.
Tashiro
M
M
Department of Physiology, The Jikei University School of Medicine, Tokyo, Japan. tashiro@jikei.ac.jp
Konishi
M
M
D000818:Animals; D001572:Benzofurans; D055592:Biophysical Phenomena; D001703:Biophysics; D002432:Cecum / Q000378:metabolism*; D004988:Ethers, Cyclic; D005110:Extracellular Space / Q000378:metabolism; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D017136:Ion Transport; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials; D009130:Muscle, Smooth / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
D001572:Benzofurans; D004988:Ethers, Cyclic; D005456:Fluorescent Dyes; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; C061665:sodium-binding benzofuran isophthalate; D012964:Sodium; D008274:Magnesium; D011188:Potassium; D016257:Fura-2