Effect of intracellular and extracellular pH on contraction in isolated, mammalian cardiac muscle.

1. Intracellular pH (pHi) and Na+ activity were recorded (ion‐selective microelectrodes) in guinea‐pig papillary muscle and the sheep cardiac Purkinje fibre while simultaneously recording twitch tension. The effects of intracellular acidosis and alkalosis upon contraction were investigated. 2. A fall of pHi produced by reducing pHo was associated with a fall of twitch tension. Similarly, a rise of pHi produced by raising pHo produced a rise of twitch tension. The time course of the changes in tension correlated with the time course of changes of pHi rather than pHo. These results are consistent with previous work showing that acidosis inhibits contraction and that the inhibition depends upon a fall of pHi. 3. Changes of pHi were produced while maintaining pHo constant at 7.4. Removal of NH4Cl or addition of sodium acetate (pHo 7.4) reduced pHi but this gave either an increase of tension (papillary muscle) or an initial fall followed by a subsequent recovery of tension (Purkinje fibre). The increase or recovery of tension occurred despite the fact that there was an intracellular acid load. Thus, reducing pHi at constant pHo can increase tension whereas reducing pHi at low pHo (6.4, see paragraph 2) inhibits tension. 4. The increase of recovery of tension during intracellular acidosis produced at a constant pHo (7.4) was associated with a rise of intracellular sodium activity (aiNa). Amiloride (1.5 mmol/l), an inhibitor of Na(+)‐H+ exchange, prevented the rise of aiNa during intracellular acidosis and also prevented the recovery of tension. It is concluded that the increase or recovery of tension at low pHi is secondary to a rise of aiNa caused by stimulation of Na(+)‐H+ exchange. A rise of aiNa will elevate Ca2+ via sarcolemmal Na(+)‐Ca2+ exchange and thus will elevate tension. 5. An intracellular acidosis produced by reducing pHo (6.4) does not elevate aiNa in the Purkinje fibre. In papillary muscle, aiNa rises but this occurs slowly and the rise is 50% smaller than that seen when the same intracellular acidosis is induced at normal pHo (7.4). The net depression of tension under these conditions thus correlates with the lack of a large rise of aiNa. 6. Knowing the quantitative dependence of tension upon both aiNa and pHi in the two tissues it is possible to predict the recovery of twitch tension during intracellular acidosis at constant pHo (7.4), using the changes of pHi and aiNa measured under these conditions.(ABSTRACT TRUNCATED AT 400 WORDS)