Intracellular acidosis can increase evoked Ca(2+i)-transient amplitude (e.g. Choi et al. 2000). We have examined the role played by sarcolemmal Na⁺-H⁺ exchange (NHE) and Na⁺-HCO₃^– cotransport (NBC).

Ventricular myocytes, isolated from guinea-pig and rat (humanely killed by cervical dislocation), were AM-loaded with Fluo-3, and field stimulated at 0.3 Hz. Acetate (80 mM) was superfused to reduce pH_i. In parallel experiments, pH_i was monitored (AM-loaded carboxy SNARF-1). All experiments were conducted at 37 °C. Results are expressed as means ± S.E.M. and P values were calculated with Student’s paired t test.

With Hepes-buffered superfusates, reducing pH_i by 0.46 ± 0.02 pH units (n = 17) activated NHE. In guinea-pig myocytes, there was an initial 20 % decrease in Fluo-3 fluorescence-transient amplitude and then, within 90 s, a secondary increase to 121 ± 9.4 % (n = 9) of control, despite continuing acidosis. In the presence of the selective NHE-1 inhibitor, cariporide (30 µM), the secondary increase (now from 77% to 80 ± 3.7 % of control; n = 9) was significantly inhibited (P < 0.05). Similar results were obtained with rat myocytes (n = 10, P < 0.05). Thus, NHE activity is involved in the acid-induced increase in Ca(2+i)-transient amplitude.

With 22 mM HCO₃^–-5 % CO₂-buffered superfusates, reducing pH_i by 0.28 ± 0.01 pH units (n = 25) activated both NHE and NBC. In guinea-pig myocytes, there was an initial 15 % decrease followed by a secondary increase that was maximal 2-3 min after adding acetate (amplitude 116 ± 0.03 % of control, n = 9), despite the continuing acidosis. In 30 µM cariporide, after the initial decrease, the transient amplitude displayed some recovery (to 94 ± 0.04% of control, n = 8), but significantly less than in acetate alone (P < 0.05). In the presence of both cariporide and 10 µM S0859, a selective NBC inhibitor (Chen & Vaughan-Jones, 2001) there was a further significant reduction in amplitude at 2-3 min (now to 79 ± 0.03% of control, n = 11, P < 0.05), such that there was virtually no recovery from the initial decrease. In the rat, cariporide similarly attenuated the secondary recovery (n = 6, P < 0.05), but there appeared to be no additional inhibitory effect of adding S0859 (n = 7). Therefore during acidosis in the guinea-pig but not the rat, NBC activity is also involved in the acid-induced rise of systolic Ca(2+i).

The increase of systolic Ca(2+i) during acidosis is not due to recovery of pH_i, which remained acidic over the time period. It is most likely driven by a rise of Na⁺₁ through Na⁺ influx on NHE and NBC. By acting on sarcolemmal Na⁺-Ca²⁺ exchange, this can indirectly increase SR Ca²⁺ loading and release. The greater part of the Ca(2+i)-transient increase appears to depend on NHE activity although, in guinea-pig myocytes, a smaller involvement of NBC is apparent. The stimulatory effect of NHE and NBC on Ca(2+i) will help to counteract the inhibitory effect of intracellular acidosis on contraction.

This work was funded by the BHF and Wellcome Trust. We thank Dr H. Kleeman of Aventis, for kindly providing cariporide and S0859