Correlation between myocardial contractile force and cytosolic inorganic phosphate during early ischemia

To test the role of inorganic phosphate (P-i) in down-regulation of myocardial contractile force at the onset of ischemia, P-i of rat hearts was determined with P-31 nuclear magnetic resonance spectroscopy. Forty cycles of brief hypoperfusion (30% of baseline flow for 33 s) were used to achieve a time resolution of 0.512 s for comparing dynamic changes in P-i and contractile force. Initial control values of left ventricular developed pressure (LVP), heart rate, and oxygen consumption were 136 +/- 11 mmHg, 236 +/- 4 beats/min, and 95 +/- 3 mu l O-2 . min(-1). g(-1); these values were unchanged at the end of the experiment. During the first 10 s of hypoperfusion, P-i increased at a rate (percentage of the total observed change) faster than the decrease in LVP;P-i and LVP then changed at the same rate during the remainder of the hypoperfusion. ADP did not change in advance of LVP. Intracellular pH did not change. The results indicate that P-i plays an important role in initiating the downregulation of myocardial contractile force at the onset of ischemia. Perfusion pressure also declined faster than LVP at the onset of ischemia, indicating potential importance of vascular collapse in contractile down-regulation during early ischemia.