THE DYNAMIC REGULATION OF MYOCARDIAL OXIDATIVE-PHOSPHORYLATION - ANALYSIS OF THE RESPONSE-TIME OF OXYGEN-CONSUMPTION

Although usually steady-state fluxes and metabolite levels are assesse d for the study of metabolic regulation, much can be learned from stud ying the transient response during quick changes of an input to the sy stem. To this end we study the transient response of O-2 consumption i n the heart during steps in heart rate. The time course is characteriz ed by the mean response time of O-2 consumption which is the first sta tistical moment of the impulse response function of the system (for mo no-exponential responses equal to the time constant). The time course of O-2 uptake during quick changes is measured with O-2 electrodes in the arterial perfusate and venous effluent of the heart, but the venou s signal is delayed with respect to O-2 consumption in the mitochondri a due to O-2 diffusion and vascular transport. We correct for this tra nsport delay by using the mass balance of O-2,O- with all terms (e.g. O-2 consumption and vascular O-2 transport) taken as function of time. Integration of this mass balance over the duration of the response yi elds a relation between the mean transit time for O-2 and changes in c ardiac O-2 content. Experimental data on the response times of venous [O-2] during step changes in arterial [O-2] or in perfusion flow are u sed to calculate the transport time between mitochondria and the venou s O-2 electrode. By subtracting the transport time from the response t ime measured in the venous outflow the mean response time of mitochond rial O-2 consumption (t(mito)) to the step in heart rate is obtained. In isolated rabbit heart we found that t(mito) to heart rate steps is 4-12 s at 37 degrees C. This means that oxidative phosphorylation resp onds to changing ATP hydrolysis with some delay, so that the phosphocr eatine levels in the heart must be decreased, at least in the early st ages after an increase in cardiac ATP hydrolysis. Changes in ADP and i norganic phosphate (P-i) thus play a role in regulating the dynamic ad aptation of oxidative phosphorylation, although most steady state NMR measurements in the heart had suggested that ADP and P-i do not change . Indeed, we found with P-31-NMR spectroscopy that phosphocreatine (PC r) and P-i change in the first seconds after a quick change in ATP hyd rolysis, but remarkably they do this significantly faster (time consta nt similar to 2.5 s) than mitochondrial O-2 consumption (time constant 12 s). Although it is quite likely that other factors besides ADP and P-i regulate cardiac oxidative phosphorylation, a fascinating alterna tive explanation is that the first changes in PCr measured with NMR sp ectroscopy took exclusively place in or near the myofibrils, and that a metabolic wave must then travel with some delay to the mitochondria to stimulate oxidative phosphorylation. The t(mito) Slows with falling temperature, intracellular acidosis, and sometimes also during reperf usion following ischemia and with decreased mitochondrial aerobic capa city. In conclusion, the study of the dynamic adaptation of cardiac ox idative phosphorylation to demand using the mean response time of card iac mitochondrial O-2 consumption is a very valuable tool to investiga te the regulation of cardiac mitochondrial energy metabolism in health and disease.