EFFECTS OF MG2+ ON BASAL AND BETA-ADRENERGIC-STIMULATED DELAYED RECTIFIER POTASSIUM CURRENT IN FROG ATRIAL MYOCYTES

1. The effects of internal Mg2+ ions on the delayed rectifier potassium current (I(K)) of bull-frog atrial myocytes were studied using the whole-cell configuration of the patch-clamp technique with a perfusable patch electrode. 2. Initial variations in I(K) amplitude were dependent on [Mg2+]i. With [Mg2+] > 1 mM, the amplitude of I(K) usually decreased after initiating the whole-cell recording configuration (run-down); with [Mg2+]i < 1 mM, I(K) usually increased (run-up). Mg2+ blocked I(K) with an apparent half-maximal effect of 0.6 mM [Mg2+]i. 3. The basal free [Mg2+]i, indicated by the amplitude of I(K) before run-up or run-down, was estimated from the relationship between [Mg2+]i and I(K) to be 0.8 mM. 4. The amplitude of both the activation curve and the instantaneous voltage-current relationship was decreased by increasing [Mg2+]i. Under these conditions, the voltage dependence of I(K) was not affected. 5. The rate of activation of the current at + 40 mV was slowed by increasing [Mg2+]i with little effect on the rate of deactivation at - 50 mV. This is in contrast to the effects of isoprenaline, which speeded activation and slowed deactivation. 6. Isoprenaline increased I(K) on average by about 2.5 pA/pF, whether I(K) had previously run down or not, and regardless of [Mg2+]i. The reversibility of isoprenaline was partially inhibited at [Mg2+]i < 1 mM. 7. It is concluded that Mg2+ affects I(K) via several mechanisms that might include a Mg2+-dependent phosphatase.