EFFECT OF CORTICOSTERONE ON PROTEIN-DEGRADATION IN ISOLATED RAT SOLEUS AND EXTENSOR DIGITORUM LONGUS MUSCLES

The net catabolic effect of glucocorticoids on protein metabolism is w ell documented but the acute and chronic effect of glucocorticoids on protein breakdown remains controversial. In the present studies protei n breakdown was measured by the release of tyrosine fi-om the isolated soleus and extensor digitorum longus (EDL) muscles of control rats an d rats treated with corticosterone (10 mg/ 100 g body weight/day) for 5 days. The effect of corticosterone in arresting growth was confirmed since corticosterone-treated rats weighed significantly less than con trol rats after 2, 3, 4 and 5 days of treatment (P<0.001). Furthermore , the weights of soleus and EDL muscles from corticosterone-treated ra ts were significantly reduced (P<0.001, at least P<0.05 respectively) compared with muscles from control rats on days 3-5. In the EDL muscle tyrosine release was significantly elevated after corticosterone trea tment for 2 days (257 +/- 21 nmol/g tissue/h, P<0.05), 3 days (205 +/- 9 nmol/g tissue/h, P<0.01), 4 days (255+/-20 nmol/g tissue/h, P<0.005 ) and 5 days (218 +/- 8 nmol/g tissue/h, P<0.05) compared with EDL fro m control rats (192 +/- 13, 171 +/- 7, 187 +/- 7, 180 +/- 12 nmol/g ti ssue/h respectively). In the soleus muscle, tyrosine release was signi ficantly elevated after corticosterone treatment for 2 days (226 +/- 1 4 nmol/g tissue/h, P<0.001), 3 days (223 +/- 16 nmol/g tissue/h, P<0.0 01) and 4 days (199 +/- 10 nmol/g tissue/h, P<0.001) compared with con trol rats (158 +/- 7, 132 +/- 6 and 153 +/- 7 nmol/g tissue/h respecti vely). After 5 days there was no significant difference in tyrosine re lease from soleus muscle between corticosterone-treated (176 +/- 15 nm ol/g tissue/h) and control rats (157 +/- 6 nmol/g tissue/h). Plasma gl ucose concentrations were not significantly different in rats heated w ith corticosterone and control rats whilst insulin levels were signifi cantly raised in the corticosterone-treated rats on all days compared with control rats (P<0.05 on day 1; P<0.001 on days 2, 3, 4 and 5). It is suggested that insulin may have prevented hyperglycaemia developin g in the corticosterone-treated rats. Results fi-om these studies indi cate that the acute effect of glucocorticoids is to increase muscle pr oteolysis but this is not maintained with longer-term treatment.