Myocardial magnesium transport: effect of gramicidin S and epinephrine

Initial magnesium transport in low magnesium Ringer in the frog myocardium consists of at least two systems, a fast system with high capacity with a time constant of 30 seconds and a slow transport system that operates with a time constant of 165 seconds. Both transport systems appear to be electroneutral and can operate either against an electrochemical gradient or with an electrochemical gradient. The fast transport system can transport Mg2+ in an outward direction at 1.84 p mol cm-2 sec-1; the slow system causes Mg2+ to be transported outward at 0.05 p mol cm-2 sec-1. Gramicidin S (5 microM) decreases the slow outward transport system to 0.01 p mol cm-2 sec-1 and at concentrations greater than 1 microM inhibits not only slow magnesium outflux in low calcium Ringer but also inhibits magnesium influx during recovery of Mg2+ in Ringer. Gramicidin S at 5 microM decreases Mg2+ influx from .04 p mol cm-2 sec-1 to 0.01 p mol cm2 sec-1 indicating that influx and efflux may take place on the same transport system. In the presence of 10 mM Mg2+ Gramicidin S increases magnesium content. Epinephrine increases magnesium efflux and overcomes the inhibition of Mg2+ efflux by Gramicidin S.