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 degrees 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 microM), 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 mumol/g wet wt. increase in [Pi]. Intracellular pH (pHi) showed no significant correlation with maximal Ca2+-activated pressure (r = 0.49 mean, n = 12). Two other protocols, pacing at variable rates and gated measurements at two different times during the tetanus, were also used to correlate [Pi], pHi, 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 pHi. Acidosis induced by NH4Cl (20 mM) or by bubbling with 95% O2/5% CO2 was associated with less than 20% depression of maximal Ca2+-activated pressure in the pHi range down to 6.8, but much greater depression at lower pHi. The data are consistent with depression of maximal Ca2+-activated force during the early phase of hypoxia by Pi but not by H+.