Regulation of the Oxidative Phosphorylation Rate in the Intact Cell

The mechanisms that underlie the balance between the consumption and oxidative generation of ATP in the intact cell are not well-defined. Cytosolic inorganic phosphate (P_i) and ADP levels, the cytosolic ATP/ADP ratio, and the cytosolic phosphorylation potential (PP) have all been proposed as major regulatory variables, the latter as a component of a “near-equilibrium” thermodynamic regulatory scheme. Therefore, the potential regulatory roles of these variables in the intact cell were evaluated with ³¹P NMR and Langendorff perfused rat hearts; in this preparation, the tissue oxygen consumption rate (MV0₂) can be varied over a wide range. When the exogenous carbon source was varied, none of the proposed regulatory parameters, i.e., the ATP/ADP ratio, PP, or cytosolic ADP level, were found to be uniquely related to MV0₂. Rather, ADP levels at a given MV0₂ decreased progressively for the exogenous carbon sources in the following order: glucose, glucose + insulin, palmitate + glucose, lactate, pyruvate + glucose, and octanoate + glucose. In the octanoate and pyruvate groups, MV0₂^_1 was linearly dependent upon [ADP]⁻¹ with apparent K_m values being in the range previously observed in isolated mitochondria. A similar trend was observed in the MV0₂-[P_i] relationship. The present findings suggest that exogenous carbon sources which effectuate deregulation of intramitochondrial NADH generation lower cytosolic ADP and P_i to levels which are limiting to the rate of oxidative phosphorylation. For other carbon sources, the processes controlling the rate of NADH generation also participate in determining the rate of oxidative ATP synthesis. However, this control must be exerted kinetically rather than through a near-equilibrium thermodynamic mechanism as indicated by the present data and prior kinetic studies of the ATP synthetic process in both isolated mitochondria and intact myocardium [La Noue, K. F., et al. (1986) Biochemistry 25, 7667–7675; Kingsley-Hickman, P., et al. (1987) Biochemistry 26, 7501–7510].