OUABAIN POTENTIATION OF RAPID COOLING CONTRACTURE OF CAFFEINIZED CARDIAC MUSCLES IN CALCIUM DEPRIVED MEDIUM

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To obtain further evidence concerning the intracellular action of ouabain, we observed the effect of this compound on contractures evoked by rapid cooling in the presence of caffeine (rapid cooling caffeine contraeture; RCCC), in cardiac muscles, under conditions of a Ca deprived medium. The experiments were carried out at 20 C using cat papillary muscles and frog ventricle strips, with the exception of cooling (2°C). The muscles were first immersed in normal solution, electrically driven, then immersed in 1 mM EDTA solution for 6 min. The strips were then immersed for 10 min in a caffeine solution prepared by eluting a no-Ca solution containing 50 mM caffeine through Dowex A-I, Ca-chelating resin. Finally, the caffeine solution was exchanged with fresh caffeine solution of 2°C to evoke the RCCC. Ouabain (1×10^-7-1×10^-5 M) markedly potentiated RCCC. An ouabain-induced increase of RCCC did not appear in the resting strips unless the tissues were electrically stimulated. ³H-ouabain occupied a considerable cellular space (0.55) at the appearance of the ouabain potentiation of RCCC (¹⁴C-inulin space; 0.20). The appearance of the ouabain potentiation of RCCC was independent of changes in ²²Na, Ca²⁺ and ATP contents in the strips. A possible mechanism of the potentiating effect of ouabain on contraction is discussed.