Figure 5. Magnetic resonance spectroscopic data in the diaphragm from 24 animals during saline (S) or aminophylline administration. (A) Inorganic phosphate/phosphocreatine ratio (Pi/PCr) with and without aminophylline administration during rest, pacing, and recovery. Pi/PCr approaches 1 immediately after the onset of pacing in saline-treated animals. Conversely, A40 treatment prevents the increase in Pi/PCr. A10 and A20 treatments produce intermediate increases in Pi/PCr. (B) Adenosine triphosphate (ATP) concentration does not change as a function of pacing or aminophylline dose. (C) intracellular pH (pHi) does not vary during pacing as a function of aminophylline dose. S = saline; A10 = aminophylline 10 mg/kg followed by 1 mg *symbol* kg sup -1 *symbol* h sup -1; A20 = 20 mg/kg followed by 2 mg *symbol* kg sup -1 *symbol* h sup -1; and A40 = 40 mg/kg followed by 4 mg *symbol* kg sup -1 *symbol* h sup -1.