Calcium ion effects on the rising phases of action potentials obtained from guinea-pig papillary muscles at different potassium concentrations

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In guinea-pig papillary muscles, action potentials, rates of rise ( v ̇ max and conduction velocities were measured at various potassium concentrations. An increase in potassium reduced v ̇ max in an S-shaped relation to the membrane potential but conduction velocity was influenced differently. At 18 to 20 m m K 0 +, the rising phase often included a remainder of the fast depolarization and v ̇ max showed two peaks; the first peak could be abolished by TTX (10 −6 m), the second peak by D600 (1 μg/ml, 1 Hz). The v ̇ max (second peak) of the rising phase of the slow responses as well as overshoot and amplitude was clearly correlated ( r = 0.99) to the log of Ca 0 2+. The increase in Ca 0 2+ concentration caused an elevation of both overshoot and amplitude of 34.22 mV per decade. In contrast, the v ̇ max of the first peak was correlated to the linear Ca 0 2+ concentration. At K 0 + 20 m m increase of Ca 0 2+ led to a marked increase of the first and Na +-dependent peak of v ̇ . This v ̇ max was increased 4.23 times by a Ca 0 2+ increase from 2.5 to 10 m m. Accordingly, impaired conduction due to 20 m m K 0 + was almost normalized by 10 m m Ca 0 2+. A Ca 0 2+-increase (2.5 to 10 m m) shifted the curve of Na +-inactivation by 4.26 mV in the depolarization direction and reduced v ̇ max at normal membrane potential. The results confirm the dependence of the rising phase and the overshoot of slow responses to the log Ca 0 2+ but indicate strong and simultaneous Ca 0 2+-effects on recovery of the Na +-system which may add to or even dominate the overall changes in v ̇ max of slow responses.