The regulation of intracellular Mg2+ 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 Mg(2+)‐ 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 pHo or changing pHi 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 pHi 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 pHi 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.