Studies on protein metabolism during isoproterenol-induced cardiac hypertrophy

Chronic treatment with isoproterenol (ISO) (30 mug/100 g body wt/day for 21 days) causes a hypertrophy of the heart (approximately 40%). Protein determination made on homogenates of the heart show similar percentages of protein in the control and treated animals. Thus, ISO presumably stimulates protein synthesis in the heart, as its total protein content is increased by chronic treatment. Studies of the stimulation of protein synthesis during a 9-day treatment with ISO have been carried out by following the incorporation of 14C-labeled amino acids in vivo into heart protein. This stimulation attained a maximum after 5 days of treatment and then declined. In other experiments, groups of rats received a single injection of ISO and were killed at various times (from 0 to 48 hr) after the injection. Stimulation of protein synthesis was maximal 3 hr after the injection, and it slowly decreased until the 36th hr. After that time the rate of protein synthesis was equal to that of control animals. Other groups of rats received daily injections of ISO for 9 days and were killed at various times after the ninth injection. Protein synthesis was still stimulated by ISO; however, the stimulation was observed only during the first 12 hr following the injection of the 9th day. There was an apparent inhibition of protein synthesis which lasted from the 30th to the 48th hr following the ninth injection. The rate of protein degradation did not seem to be affected by a chronic treatment with ISO. Thus, the increased rate of protein synthesis seems to be the sole factor responsible for the increase of the total protein content of the heart, even if the stimulation of the incorporation of labeled amino acids decreased after a few days of treatment.