Effects of the cyclo-oxygenase inhibitor, fenbufen, on clenbuterol-induced hypertrophy of cardiac and skeletal muscle of rats.

1. When rats were fed with clenbuterol for 7 days skeletal muscle mass increased by 21% in the tonic soleus and phasic plantaris muscles and a 16% hypertrophy of the heart was also induced. Fenbufen, fed to rats for the same period, blocked the hypertrophy of the heart but not that of the skeletal muscles. 2. When feeding of fenbufen commenced 3 days before the administration of clenbuterol, plasma prosta-glandin F2 alpha (PGF2 alpha) was reduced by 79%; there was again no effect of fenbufen on clenbuterol-induced increases in the RNA or protein content of plantaris, nor in the increased area of fast or slow twitch fibres in the soleus. In the heart the clenbuterol-induced increases in the RNA (+21%) and protein content (+20%) were totally inhibited. 3. The effects of clenbuterol on heart muscle appear to be mediated by a cyclo-oxygenase metabolite of arachidonic acid whilst the effects on skeletal muscle are not.