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 my ocardial contractile force at the onset of ischemia, P-i of rat hearts was determined with P-31 nuclear magnetic resonance spectroscopy. For ty 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 cha nges in P-i and contractile force. Initial control values of left vent ricular developed pressure (LVP), heart rate, and oxygen consumption w ere 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 (perce ntage 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 hy poperfusion. ADP did not change in advance of LVP. Intracellular pH di d 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 co llapse in contractile down-regulation during early ischemia.