The heterogeneity of muscle mitochondria demonstrated by discontinuous density-gradient centrifugation

Abstract

When ribosomes were isolated from the same muscles 6h after intraperitoneal injection of the steroids it was shown that the minimum effective doses capable of causing a significant (P<0.05) depression in their amino acid-incorporating ability in vitro were: corticosterone, 20mg/kg body wt.; prednisolone, 20mg/kg body wt.; triamcinolone, 0.05mg/kg body wt.; betamethasone, 1mg/kg body wt.; dexamethasone, 0.05mg/kg body wt.; triamcinolone acetonide, 0.05mg/kg body wt. Although it is true that the low catabolic potencies of corticosterone and prednisolone were reflected in the high doses required to affect ribosomal activity in vitro, the other steroids, on this criterion, were in the wrong order, since betamethasone would be expected to be less catabolic than triamcinolone, which itself would be expected to have a catabolic potential similar to that of its acetonide. A much more satisfactory correlation between catabolic potential and diminished ribosomal activity was observed on the basis of duration of steroid action. For example, 12 h after a single dose (20mg/kg body wt.) of corticosterone muscle ribosomes had regained normal activity, whereas those from animals receiving a similar dose of prednisolone still retained diminished activity. Likewise it was possible to separate, as pairs, triamcinolone and betamethasone from dexamethasone and triamcinolone acetonide. Triamcinolone, the least catabolic of these four, ceased to be effective 18h after injection, whereas the acetonide was still effective 48h after administration. Cardiac-muscle ribosomes were not affected as a result of steroid treatment, in agreement with the absence of any effect on heart weights. These results serve to emphasize again the close correlation between changes in ribosomal activity and cellular determinants of catabolic decline in skeletal muscle. Nevertheless the mechanism of the decrease in incorporating ability of isolated ribosomes, which will be discussed, still remains obscure.