Magnesium exchange in rat ventricle.

1. The exchange of cellular Mg with external (28)Mg in the rat left ventricle was measured in vivo and, under conditions approximating a steady state, in an isolated, working rat heart perfused and contracting at 36-38 degrees C.2. About 98% of cellular Mg exchanged at a single rate.3. The rate of exchange in vivo was the same as that observed in independent in vitro measurements of the influx and efflux at the physiological external Mg(2+) concentration of 0.56 mM. The rate was 0.15 +/- 0.02 m-mole/(kg dry ventricle.min) or 0.21 +/- 0.02 p-mole/(cm(2).sec).4. In the perfused heart the dependence of the influx on the external Mg concentration was hyperbolic with an apparent V(max) of 0.31 +/- 0.04 m-moles/(kg dry weight.min) and an apparent K(m) of 0.57 +/- 0.08 mM.5. The Mg efflux into a solution containing 2.8 mM-Mg was markedly faster than that into a Mg-free solution.6. These results are interpreted as consistent with a carrier-mediated transport of Mg across the plasma membrane.