Control of respiration by products of ATP hydrolysis was examined in the in situ rat heart using a purpose-built nuclear magnetic resonance (NMR) coil. The in situ ratio of phosphocreatine to ATP concentrations ([PCr]/[ATP]) was 2.30 +/- 0.05, free Mg2+ concentration ([Mg2+]) was 0.57 mM, and cytosolic pH was 7.35 +/- 0.03 (n = 7). Basal inorganic phosphate concentration ([P-i]) was below NMR detection but was estimated to be 0.83 mM. The [ATP]/[ADP] [P-i] ratio, free ADP concentration ([ADP]), and free energy of ATP hydrolyses (Delta G(ATP)) were calculated to be 700,000 +/- 78,000 M(-1), 18 +/- 3 mu M, and -63.93 +/- 0.33 kJ/mol in situ, respectively (n = 7). In contrast, in the Langendorff perfused rat heart [ATP]/[ADP] [P-i] was only 76,140 +/- 12,830 M(-1), [ADP] was 65 +/- 9 mu M, and Delta G(ATP) was -59.92 +/- 0.48 kJ/mol (n = 7), all indicative of a lower energy state in vitro. Epinephrine infusion in situ (0.9 mu g . min(-1) . kg(-1)) increased the rate-pressure product 2.05-fold. During stimulation [ATP] was stable at 97 +/- 3% signal intensity, [PCr] declined by 25%, and [P-i] increased to 1.83 mM. Cytosolic pH was 7.27 +/- 0.01 and [Mg2+] was 0.64 +/- 0.05 mM. [PCr]/[ATP] declined to 1.83 +/- 0.13, and [ATP]/[ADP] [P-i] fell to 108,000 +/- 15,000 M(-1). Delta G(ATP) only fell marginally to -59.56 +/- 0.49 kJ/mol. Free [ADP] increased threefold to 55 +/- 10 mu M. Infusion of 2.8 +/- 0.5 mu g . min(-1) . kg(-1) epinephrine increased the rate-pressure product 2.7-fold, further reduced [ATP]/[ADP] [P-i] (5% of basal), and elevated [ADP] more than fourfold without changing [ATP]. We conclude that the in situ heart is highly energetic compared with isolated perfused hearts and operates at a different metabolic ''set-point.'' Because free [ADP] and [P-i] in situ approximate apparent Michaelis constants for mitochondrial respiration in vitro and increase with increased cardiac work, we conclude that each fulfills the criteria for the kinetic control of O-2 consumption in the in situ rat myocardium.
