Threshold [Ca2+]m required to prevent net NADH oxidation. Up to a threshold concentration of [Ca2+]m (dotted lines in A–B), the extent of NADH oxidation at the end of the 100-second stimulation period was linearly correlated with [Ca2+]m (A, diastolic rhod-2 F/F0; B, systolic rhod-2 F/F0). Beyond the threshold, further increases [Ca2+]m did not alter the NADH balance. Filled and open symbols: 5 and 15 mmol/L Na+ groups, respectively.