We have investigated the electrical and mechanical effects of reducing the bathing K concentration, K0, over the range from 4-0 mM in guinea-pig papillary muscle and in sheep Purkinje fibres. 2. In papillary muscle, reducing K0 to zero produces a negative shift in the resting potential and an initial increase in action potential duration. An increase of twitch tension ensues, followed by a reduction in actin potential duration and, eventually, an increase in tonic tension. This increase in tonic tension is often accompanied by a decrease of twitch tension. Finally, transient depolarizations and after contractions are produced. 3. In voltage clamped Purkinje fibres, K0 reduction decreases the slope conductance at the more negative potentials and reduces the pace-maker current, iK2. Twitch tension increases rapidly and voltage dependent tonic tension develops. After even very short exposures to very low K0 (1 mM and below), an oscillatory transient inward current and accompanying aftercontraction can be seen. The oscillatory transient inward current and aftercontraction are similar to those described for cardiotonic steroid intoxication by Kass, Lederer, Tsien & Weingart (1978). 4. Prolonged exposure to 0 K0 leads to the development of a slow current 'creep'. This current is activated by depolarization and has a reversal potential of -6.7 +/- 3.5 mV. The development of this creep current is accompanied by an increasing 'creep' in tonic tension with the same time course. On repolarization both the current creep and creep in tension recover with time courses still similar to each other. 5. Fluctuations appear in both the tension and current records during exposure to low K0. The tension and current fluctuations have similar principal frequencies (about 1 Hz). 6. Ca0 removal, substituting Ba0 for Ca0 or adding Mn0 (2 mM) can each remove the transient inward current, aftercontraction, fluctuations of current and tension, and creep current as well as the increase of twitch and tonic tension. 7. Replacing Ca0 by Sr0 leads to an increased inotropic effect of low K0 with altered kinetics and appears to abolish the transient inward current, aftercontraction and fluctuations of current and tension. 8. It is concluded that Ca1 plays a central role in the inotropic and arrhythmogenic effects of low K0. Possible mechanisms of Ca1 control are discussed in light of the results that have been presented.
