Influence of magnesium and extracellular calcium reduction on ouabain-treated sinoatrial node cells in rabbit heart.

The influence of the extracellular Mg on the slow regenerative potential induced by Ca removal was investigated by using rabbit SA node tissue treated with ouabain. When the SA node tissue was superfused with ouabain (10(-5) M), the preparation became quiescent. Further superfusion with Mg-free and Ca-free solution produced a small regenerative potential reaching 21 +/- 3 mV (n = 5). Superfusion with high Mg, Ca-free solution containing 5.19 mM Mg inhibited the regenerative potential. Superfusion of the ouabain-treated SA node tissue with Ca-free (2 mM EGTA) solution containing normal Mg or Mg-free, Ca-free (2 mM EGTA) solution produced a slow regenerative potential reaching 45 +/- 2 mV (n = 5) or 44 +/- 3 mV (n = 5). Increasing the external Mg to 3.6 mM or 6.18 mM in the Ca-free, EGTA solution caused a significant inhibition of slow regenerative potential. The addition of diltiazem (0.08 mM) blocked both normal action potential and slow regenerative potential induced by superfusion with Mg-free, Ca-free (2 mM EGTA) solution. These results suggest that Mg inhibits the development of the slow regenerative potential resulting from inward currents through the Ca channel.