Skeletal muscle from normal cows and from cows affected with clinical Johne's disease was analysed for nitrogen, protein, hydroxyproline, myoglobin, RNA, DNA, acid soluble ribonucleotides and ATP contents. Creatine phosphokinase (CPK) activity was also assayed in aqueous muscle extracts.

In wasted muscle from cows with Johne's disease the marked reduction in mean muscle-cell size seen histologically was apparently responsible for the increase in DNA content from 0·34 mg per g to 0·87 mg per g fresh muscle. This was confirmed by an associated reduction in non-collagen protein from about 155 mg per g to 125 mg per g and an increase in the proportion of connective tissue (about 35 per cent. measured by its hydroxyproline content).

The mean apparent cellular concentrations of the various muscle constituents expressed in amounts per g DNA fell to between 30 and 63 per cent. of the corresponding normal values and the mean cell size had decreased from 2·96 to 1·16 mg fresh weight per g DNA.

ATP content was statistically correlated with muscle CPK activity for both healthy and wasted muscle samples; the reduced cellular concentration of ATP in wasted muscle was therefore probably related to its decreased CPK activity.

Muscular atrophy in Johne's disease may, therefore, be partly associated with a failure of protein synthesis due to a limited availability of ATP via the CPK reaction. Possible precipitating factors are also discussed.


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