The interactions of protons, calcium and potassium ions on cardiac Purkinje fibres.

1. In sheep Purkinje fibres, acidosis shifts the steady‐state current‐voltage relation in an inward direction over a wide range of negative potentials. The reversal potential for iK2 is shifted in a positive direction. These changes are consistent with K+ accumulation in the extracellular spaces of the Purkinje fibre. 2. These effects of acidosis are completely prevented by increasing [K]0 from the normal range (2.7‐‐5.4 mM) to 10.8 mM. 3. Increasing [Ca]0 from 2 to 6 mM also produces an inward shift in the steady‐state current‐voltage relation and a positive shift of the iK2 reversal potential. 4. In 10.8 mM‐[K]0 these effects are absent even when [Ca]0 is increased to 20 mM. 5. One possible explanation is that K+ ions may protect the Na pump from inhibition by protons and Ca2+ ions. Alternative explanations are also discussed.