THE REGULATION OF INTRACELLULAR MG-2+ IN GUINEA-PIG HEART, STUDIED WITH MG-2+-SELECTIVE MICROELECTRODES AND FLUOROCHROMES

Because of the reported presence of a Na+-Mg2+ exchanger in guinea-pig but not in ferret myocardium, the Mg2+ extrusion mechanism in guinea-pig myocardium has been reinvestigated using Mg2+- and Na+- selective microelectrodes and the fluorochromes mag-fura-2 and -5. The mean [Mg2+]i measured with microelectrodes in trabeculae or papillary muscles was 0.72 mmol/l (n = 22, thirteen experiments; range 0.42-1.23 mmol/l). Increasing [Mg2+]o from 0.5 mmol/l to either 10.5 or 20 mmol/l caused small increases in [Mg2+]i. Decreasing [Na+]o by 50% had no effect on the [Mg2+]i and there was no change in [Na+]i on increasing [Mg2+]o from 0.5 to 10-5 mmol/l. Varying pH(o) or changing pH(i) with NH4Cl did not influence the [Mg2+]i. In vitro calibration of mag-fura-2 and -5 using the ratio method gave values for K(d)' (experimentally determined dissociation constant) of 22-2 +/- 2.7 (mean +/- S.D., n = 7) and 25.7 +/- 1.3 (n = 4) mmol/l respectively. Mag-fura-2 reacted to physiological concentrations of Ca2+ and mag-fura-5 to changes in pH. In isolated myocytes, Na+ removal gave an apparent increase of [Mg2+]i with mag-fura-2 but not with mag-fura-5. However, when the pH(i) was altered with NH4Cl mag-fura-5 showed an apparent decrease in [Mg2+]i on application and an apparent increase on removal, with a time course similar to the pH(i) changes. It is concluded that Mg2+ extrusion in guinea-pig myocardium is not via a Na+-Mg2+ exchanger. The use of mag-fura-2 and -5 are limited in their application because of Ca2+ and H+ sensitivity respectively.