P-31-NMR STUDY OF NORMOXIC AND ANOXIC PERFUSED TURTLE HEART DURING GRADED CO2 AND LACTIC-ACIDOSIS

We studied the effects of graded acidosis (both CO2 and lactic acid) and anoxia on intracellular pH (pH(i)) regulation, high-energy phosphates, and mechanical function of isolated perfused hearts of the turtle (Chrysemys picta bellii) at 20-degrees-C using P-31-nuclear magnetic resonance (NMR) spectroscopy. During CO2 acidosis, anoxia had no effect on apparent nonbicarbonate buffer value (d[HCO3-]/dpH(i) = 71 and 89 mM/pH in normoxia and anoxia, respectively) or on pH(i) regulation (dpH(i)/dpH(e) = 0.52 and 0.43 in normoxia and anoxia, respectively, where pH(e) is extracellular pH). During normoxic lactic acidosis, dpH(i)/dpH(e) was similar to the values observed in CO2 acidosis and averaged 0.55 overall. During anoxic lactic acidosis, however, similar regulation occurred over only a narrow range of pH(e), and then dpH(i)/dpH(e) increased to > 1.0 at pH(e) < 7.1. Creatine phosphate (CP), calculated as the area of the NMR peak, fell more in response to normoxic CO2 acidosis than to normoxic lactic acidosis; in anoxia, the fall in CP was further increased but to similar extreme levels (10-20% of control) in both acid perfusions. Cardiac output and maximum rate of pressure development each fell during acidosis in similar fashion in all protocols, and the responses were similar in normoxic and anoxic hearts. Heart rate, in contrast, decreased during acidosis, but this effect was more pronounced when hearts were anoxic. We conclude that the effect of acidosis on cardiac function can depend on the type of acidosis imposed. Based on the heart's insensitivity to anoxia alone, we suggest that anoxia may normally depress function indirectly via its effect on intracellular acid-base state.