Contracture and hyperpolarization of the rabbit sinoatrial node cells in Na-depleted solution

A study was made on both hyperpolarization and contracture which developed in the rabbit sinoatrial node cell during perfusion with Na-depleted solution. The membrane was hyperpolarized in 19 out of 27 specimens (-69.5 +/- 6.0 mV), while in the remaining 8 specimens the value of the membrane potential remained low (-39.4 +/- 2.7 mV). Slope conductance increased as the membrane was hyperpolarized in Na-depleted solution, but not when the value of the membrane potential remained at low. The amplitude of hyperpolarization increased with decrease in Na concentration, increase in Ca concentration and increase in perfusate temperature. A marked increase in diastolic tension was observed in 23 out of 29 specimens but in the 6 remaining specimens the increase in tension was small. Tension increased with decrease in Na concentration, increase in Ca concentration, and increase in perfusate temperature. Pretreatment with ouabain caused a consistent increase in contracture tension, while its effect on the membrane hyperpolarization was variable among different specimens. These results may be explained by assuming that intracellular Ca concentration increased with Na-Ca exchange in Na-depleted solution. In addition to contracture, increase in intracellular Ca concentration might have caused an increase of membrane K conductance and produced marked hyperpolarization.