NMR study of rat diaphragm exposed to metabolic and compensated metabolic acidosis

When exposed to hypercapnia, several muscles deteriorate with respect to their mechanical performance. Exposure to metabolic acidosis and, perhaps surprisingly, to compensated metabolic acidosis has the same effect on the diaphragm. The mechanisms involved in these effects remain unclear. If the diaphragmatic intracellular pH (pH(i)) is assumed to decrease with hypercapnia, to remain unchanged during metabolic acidosis, and to increase during compensated metabolic acidosis, it would appear that different mechanisms must be responsible for the depreciation in the diaphragm's mechanical performance. The present experiments using ³¹P nuclear magnetic resonance (³¹P-NMR) spectroscopy were undertaken to determine the effect of metabolic acidosis and compensated metabolic acidosis on pH(i) and on high-energy phosphate metabolites in the resting rat diaphragm. A whole diaphragm was slightly stretched while being stitched onto a fiberglass mesh. The area approximated that at functional residual capacity. It was superfused in the NMR sample tube with a phosphate-free Krebs-Ringer bicarbonate solution ([HCO₃^-] = 6 meq/l) equilibrated with either 95% O₂-5% CO₂ or 98.75% O₂-1.25% CO₂). Spectra were acquired during 15-min intervals for control (30 min of normal Krebs-Ringer bicarbonate superfusate, equilibrated with 95% O₂-5% CO₂), for 120 min of exposure to either form of acidosis and for 60 min of recovery with normal superfusate. The pH(i) decreased rapidly during metabolic acidosis but did not change significantly during compensated metabolic acidosis. In both forms of acidosis, phosphocreatine declined gradually but not significantly, whereas ATP and inorganic phosphate did not change at all. The results suggest that HCO₃^- passes freely through the diaphragmatic sarcolemma, very much like the cardiac sarcolemma. Overall, it seems unlikely that the previously observed decrease in mechanical force during acidosis is due to any loss of high-energy phosphate stores.