Shortening of the first action potential plateau phase and potentiation of isometric contraction are simultaneously found in rabbit papillary muscle after 1 min of rest. The ionic nature of these events was investigated. Shortening of the plateau phase is not markedly prevented by activators of the slow inward current (5'-guanyl imidodiphosphate, 10(-5) M plus isoproterenol, 10(-7) M) but is completely eliminated by potassium current blockade with 4-aminopyridine (2 X 10(-4) M). The latter suggests the key role of the outward current increase in plateau phase shortening. It is further potentiated by high external K+ and Ca2+ as well as by ouabain 5 X 10(-7) M. It is concluded that simultaneous alteration of mechanical and electrical activity after rest might be related to accumulation of Ca2+ inside the cardiac cell and K+ outside it as the result of sodium pump inhibition in quiescent rabbit papillary muscle.
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