and the slow inward Ca current 8 during hypoxia show little change over the period in which early contractile failure develops, suggesting that Ca 2+ entry remains unimpeded. Direct measurements of intracellular free Ca 2+ concentration ([Ca 2+ ],) using aequorin 9 show no change in Ca 2+ transients during the early phase of hypoxia. Therefore, experimental evidence does not implicate a decline in the Ca 2+ transient as the mechanism of early contractile failure but, instead, points to a

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... e in the responsiveness of the contractile proteins to Ca 2+ . Such a decrease might be caused by changes in the levels of intramyocardial metabolites during hypoxia -for example, decline of adenosine triphosphate (ATP), 10 accumulation of H + ," or increase of inorganic phosphate (Pi). 12 The concentration of phosphocreatine (PCr), ATP, Pi, and H + during ischemia or hypoxia have been measured analytically (e.g., Kammermeier et al 13 or Doorey and Barry 14 ) and by 3I P nuclear magnetic resonance (NMR)."~1 9 A decline in [PCr], an accumulation of Pi, and acidosis are the major changes during early hypoxia. [ATP] decreases little during the period in which developed tension declines by more than 50%. A small decline in [ATP] has little effect on the relation between [Ca 2+ ], and developed tension in skinned cardiac muscle preparations (the Ca 2+ -tension relation), and any effect would most likely be in the direction of sensitizing the contractile machinery to Ca 2 " 1 ". 20 " 22 by guest on July 23, 2018 http://circres.ahajournals.org/ Downloaded from 100%O2 79%N2/21%O2 100%N, 100%-0% J 5 sec 1 min 5 sec FIGURE 2. Estimation of maximal Ca 2 + -activated pressure during hypoxia. The gas bubbling the perfusate was changed from 100% O2 to 79% N 2 I21% O2 and finally to 100% N 2 as indicated in the bar above the pressure record. Tetani were elicited once a minute, and NMR spectra were collected sequentially using 64-pulse acquisition (2.13 minutes). The developed pressure was normalized by the pressure of the tetanus in the control period, i.e., the first tetanus in this figure.