Mechanism of early contractile failure during hypoxia in intact ferret heart: Evidence for modulation of maximal Ca²⁺-activated force by inorganic phosphate

We tested the hypothesis that accumulation of H⁺ or inorganic phosphate (Pi) is responsible for the early contractile failure of hypoxia by measuring maximal Ca²⁺-activated pressure and ³¹P nuclear magnetic resonance spectra in Langendorff-perfused ferret hearts at 30°C. Maximal Ca²⁺-activated pressure was identified by the saturation of pressure with respect to [Ca²⁺](o) observed during tetani as [Ca²⁺](o) was increased to 15 mM in HEPES-buffered, 100% O₂-bubbled perfusate and during hypoxia induced by bubbling with room air or with 100% N₂. Tetani were produced by pacing at 8-12 Hz following exposure to ryanodine (1-5 μM), an inhibitor of Ca²⁺ release from the sarcoplasmic reticulum, and were elicited once a minute to measure maximal Ca²⁺-activated pressure during acquisition of nuclear magnetic resonance spectra. An inverse correlation was observed between [Pi] and maximal Ca²⁺-activated pressure (r = 0.87 mean, n = 12), with an average decline of 8.6% in pressure per 1 μmol/g wet wt. increase in [Pi]. Intracellular pH (pH(i)) showed no significant correlation with maximal Ca²⁺-activated pressure (r = 0.49 mean, n = 12). Two other protocols, 1) pacing at variable rates and 2) gated measurements at two different times during the tetanus, were also used to correlate [Pi], pH(i), and maximal Ca²⁺-activated pressure. These protocols confirmed the highly significant correlation between [Pi] and maximal Ca²⁺-activated pressure, as well as the lack of correlation with pH(i). Acidosis induced by NH₄Cl (20 mM) or by bubbling with 95% O₂/5% CO₂ was associated with < 20% depression of maximal Ca²⁺-activated pressure in the pH(i) range down to 6.8, but much greater depression at lower pH(i). The data are consistent with depression of maximal Ca²⁺-activated force during the early phase of hypoxia by Pi but not by H⁺.