Influence of sodium‐hydrogen exchange on intracellular pH, sodium and tension in sheep cardiac Purkinje fibres.

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Abstract
1. The influence of sarcolemmal Na+-H+ exchange upon intracellular Na+ activity (aNai), intracellular pH (pHi) extracellular surface pH (pHs) and tonic tension was investigated in sheep cardiac Purkinje fibres. Intracellular ion activities were measured with liquid sensor ion-selective micro-electrodes. A two-micro-electrode voltage-clamp was also used to control membrane potential while simultaneously recording tonic tension. 2. Inhibition of the sarcolemmal Na+-K+ pump by strophanthidin (10 .mu.mol/l) produced a rise in aNai an increase in [Ca2+]i as evidenced by a rise in tonic tension, and a fall in pHi of 0.1-0.3 units. The intracellular acidosis has been shown previously to be linked to the rise in [Ca2+]i (Vaughan-Jones, Lederer and Eisner, 1983). 3. Amiloride (1-2 mmol/l), an inhibitor of Na+-H+ exchange, produced a small reversible decrease in pHi and aNai. Both effects became more pronounced in strophanthidin-exposed fibres. In addition, pHi decreased during application of strophanthidin and this decrease was reversibly inhibited by amiloride. It is concluded that sarcolemmal Na+-H+ exchange in stimulated following inhibition of the Na+-K+ pump. 4. In strophanthidin-exposed fibres, a rise in [Ca2+]i resulted in an intracellular acidosis which could still be observed in the presence of amiloride (1 mmol/l). This suggests that the fall in pHi was not caused by a modulatory effect of [Ca2+]i on sarcolemmal Na+-H+ exchange. 5. Tetrodotoxin (TTX) produced a small fall in aNai (ca. 0.5 mmol/l) which was not augmented in the presence of strophanthidin. Furthermore, the effects on aNai of TTX and amiloride were additive. Thus, the influence of amiloride on aNai does not involve blockade of voltage-gated Na+ channels. 6. The stoicheiometry of Na+-H+ exchange, estimated from the rates of change of pHi and aNai in amiloride, appeared to be electroneutral (1:1). The stoicheiometry was unaffected by changes in pHi. 7. In strophanthidin-exposed fibres (i.e. aNai is elevated), the recovery of pHi from an intracellular acidosis (brought about by brief exposure to NH4Cl) was slowed greatly by amiloride (1-2 mmol/l). The rise in aNai that occurred during pHi recovery was also reduced by amiloride. It is concluded that Na+-H+ exchange can be stimulated by a fall in pHi under conditions where aNai is elevated. However, at a given pHi, its rate of recovery was slower in the presence than in the absence of strophanthidin. This slowing was at least partly due to the increase in the rate of production of intracellular acid known to take place during Na+-K+ pump inhibition. 8. Repeated application of amiloride in the presence of strophanthidin indicated that Na+-H+ exchange is increasingly activated by pHi in the range 7.2-6.7, showing little saturation at pHi, 6.7. The small effect of amiloride on aNai and pHi, at resting pHi (7.2), indicates that Na+-H+ exchange also participates in the regulation of steady-state pHi. 9. Amiloride inhibited the tonic contracture which developed in strophanthidin. However, amiloride did not inhibit the fall in aNai which occurred following elevation of [Ca2+]o (strophanthidin-exposed fibres). This latter observation suggests that amiloride does not directly inhibit sarcolemmal Na+-Ca+ exchange. The drug''s inhibitory effect on tonic tension is therefore likely to be mediated indirectly via its effects on aNai and pHi. 10. The increased Na+ influx on Na+-H+ exchange contributes to the rise of aNai in strophanthidin, and therefore, via Na+-Ca+ exchange, contributes indirectly to the rise in [Ca2+]i. Na+-H+ exchange therefore augments the intracellular ''Ca2+ overload'' which is characteristic of preparations exposed to cardioactive steroids.