Regulation of intracellular free magnesium concentration in the taenia of guinea‐pig caecum.

1. In the taenia isolated from the guinea‐pig caecum, changes in free intracellular magnesium concentration ([Mg2+]i) were measured based on the separation of the chemical shift of the alpha‐ and beta‐peaks of ATP obtained with nuclear magnetic resonance (NMR). 2. When external Mg2+ was increased from 1.2 to 12 and 40 mM, [Mg2+]i increased from 0.34 +/‐ 0.05 to 0.39 +/‐ 0.07 and 0.61 +/‐ 0.18 mM (n = 3), respectively, in 125‐150 min. In the absence of Ca2+, the increase was greater, i.e. [Mg2+]i reached 0.92 +/‐ 0.05 (n = 3) and 3.37 mM (n = 1), respectively. 3. In Ca(2+)‐ and Mg(2+)‐free solution, [Mg2+]i decreased to about 10 microM in 100 min. This decrease was not affected by substitution of Na+ with K+, but the recovery of [Mg2+]i on Mg2+ readmission was slower in the absence of Na+. This recovery was prevented by Ca2+ (2.4 mM), but Na+ readmission produced full recovery of [Mg2+]i in the presence of Ca2+. 4. When Na+ was substituted with K+ in the absence of Ca2+, [Mg2+]i increased from 0.38 +/‐ 0.01 mM to 0.56 +/‐ 0.04 mM (n = 4) in 100 min. Substitution of Na+ with N‐methyl‐D‐glucamine produced a much greater increase (to 2.1 mM in 100 min). Removal of the external K+ in the absence of Ca2+ also increased [Mg2+]i to 0.66 +/‐ 0.11 mM (n = 4) in 150 min, but no clear change was observed in the presence of Ca2+. Full recovery of [Mg2+]i could be produced in normal solution even when the intracellular phosphocreatine and ATP concentrations were 70‐80% of the control. 5. It is concluded that in the smooth muscle of the guinea‐pig taenia caeci an Na(+)‐Mg2+ exchange process and possibly also an ATP‐driven Mg2+ pump are playing important roles in maintaining free [Mg2+]i at about 0.3 mM under the physiological condition. In addition to this, Ca2+ inhibits both Mg2+ efflux and influx, probably by reducing the membrane permeability to Mg2+ and by competing with Mg2+ in the Na(+)‐Mg2+ exchange of the plasma membrane.