Respiration and oxidative phosphorylation by mitochondria of red and white skeletal muscle

Abstract

Mitochondria from red and white skeletal muscle of the rabbit were compared polarographically with pyruvate–malate, α-glycerophosphate, and NADH as substrates. With pyruvate–malate, the organelles from the two muscle types did not differ in O2 uptake rate ([Formula: see text] mitochondrial protein per minute at 28°) or in ADP/O ratios [Formula: see text]. However, the respiratory control ratios (r.c.r.) were significantly higher in white than in red muscle mitochondria: 5.5 ± 0.3 versus 4.4 ± 0.2 (mean ± s.e.), respectively. The respiration of the white muscle mitochondria with α-glycerophosphate was similar to that with pyruvate–malate and exhibited ADP/O and r.c.r. values of 1.5 ± 0.1 and 3.0 ± 0.1, respectively. The corresponding values for red muscle mitochondria were all lower with this substrate. The organelles from both tissues were uncoupled with NADH as substrate and yielded O2 rates of less than 5% of those with pyruvate–malate.Assuming that the in vivo permeability characteristics of the organelles resemble those described above, it can be calculated that direct penetration of glycolytically generated NADH into the mitochondria would probably be too slow for its reoxidation to occur in either muscle type at a rate compatible with aerobic carbohydrate utilization. However, the intramitochondrial portion of the α-glycerophosphate shuttle has the capacity to transport reducing equivalents to the respiratory chain at the required rate. It is as yet uncertain whether it shares this role with a malate–oxaloacetate shuttle.