Oxygen-dependence of metabolic rate in the muscles of craniates
We present evidence that oxygen consumption $$ (V_{{{\text{O}}_{2} }} ) $$ is oxygen partial pressure $$ (P_{{{\text{O}}_{2} }} ) $$ dependent in striated muscles and $$ P_{{{\text{O}}_{2} }} $$-independent in the vasculature in representatives of three craniate taxa: two teleost fish, a hagfish and a rat. Blood vessel $$ V_{{{\text{O}}_{2} }} $$ displayed varying degrees of independence in a $$ P_{{{\text{O}}_{2} }} $$ range of 15–95 mmHg, while $$ V_{{{\text{O}}_{2} }} $$ by striated muscle tissue slices from all species related linearly to $$ P_{{{\text{O}}_{2} }} $$ between 0 and 125 mmHg, despite $$ V_{{{\text{O}}_{2} }} $$ rates varying greatly between species and muscle type. In salmon red muscle, lactate concentrations fell in slices incubated at a $$ P_{{{\text{O}}_{2} }} $$ of either 30 or 100 mmHg, suggesting aerobic rather than anaerobic metabolism. Consistent with this finding, potential energy, a proxy of ATP turnover, was $$ P_{{{\text{O}}_{2} }} $$-dependent. Our data suggest that the reduction in $$ V_{{{\text{O}}_{2} }} $$ with falling $$ P_{{{\text{O}}_{2} }} $$ results in a decrease in ATP demand, suggesting that the hypoxic signal is sensed and cellular changes effected. Viability and diffusion limitation of the preparations were investigated using salmon cardiac and skeletal muscles. Following the initial $$ P_{{{\text{O}}_{2} }} $$ depletion, reoxygenation of the Ringer bathing salmon cardiac muscle resulted in $$ V_{{{\text{O}}_{2} }} {\text{s}} $$ that was unchanged from the first run. $$ V_{{{\text{O}}_{2} }} $$ increased in all muscles uncoupled with p-trifluoromethoxylphenyl-hydrazone (FCCP) and 2,4-dinitrophenol (DNP). Mitochondrial succinate dehydrogenase activity, quantified by reduction of 3-(4,5-dimethylthiazol)-2,5-diphenyl-2H-tetrazolium bromide (MTT) to formazan, was constant over the course of the experiment. These three findings indicate that the tissues remained viable over time and ruled out diffusion-limitation as a constraint on $$ V_{{{\text{O}}_{2} }} $$.