Free magnesium in sheep, ferret and frog striated muscle at rest measured with ion‐selective micro‐electrodes

Abstract

Neutral carrier-based liquid membrane microelectrodes were constructed which are suitable for continuous measurements of [Mg2+]i in cardiac and skeletal muscle preparations. The electrodes show a Nernstian behavior in pure MgCl2 solutions. In the presence of a constant ionic background chosen to simulate the cytoplasmic composition, the calibration function flattens progressively with lower [Mg2+], due to the interference of K+ and Na+. The response to changes in [Mg2+] is less than 0.5 s. In quiescent preparations at room temperature (23.degree. C), the following basal [Mg2+]i were determined: 3.5 mM (sheep Purkinje fibers), 3.1 mM (sheep ventricular muscle), 3.0 mM (ferret ventricular muscle) and 3.3 mM (frog skeletal muscle). In cardiac tissue, electrical stimulation does not measurably affect the basal [Mg2+]i. In the presence of 0.5 mM-Mg2+o, the calculated Mg2+ equilibrium potentials. EMg, are in the range of -23 to -25 mV, suggesting that Mg2+ is not passively distributed across the sarcolemma in striated muscle. Further studies were performed on sheep Purkinje fibers to investigate the effect of various experimental interventions on [Mg2+]i. Elevating [Mg2+]o from 0.5 to 10 mM resulted in a reversible increase of [Mg2+]i. The initial rate of increase corresponds to a Mg2+ influx of 0.42 p-mol/cm2.s, or a Mg permeability, PMg, of 1.6 .times. 10-8cm/s. Increasing PCO2 [CO2 partial pressure] from nominally 0 to 100 mmHg (Tris-buffered vs. bicarbonate-buffered Tyrode solution) produced a reversible decrease in [Mg2+]i by roughly 0.45 mM, probably due to Mg2+ binding the newly formed intracellular HCO3-. The effect of metabolic poisoning on [Mg2+]i was assessed by exposure to cyanide, iodoacetic acid and 2-4-dinitrophenol. No significant increase in [Mg2+]i indicative of a liberation of Mg2+ from ATP was observed.