The relation between reduced protein degradation and elevated adenosine 3',5'-monophosphate in isolated rat atria.

We studied the effect on protein degradation of agents that alter intracellular adenosine 3′,5′-monophophate (cyclic AMP) in isolated rat right atria. Protein degradation rate was determined by measurement of the rate of release of L-tyroslne from atria cultured in the presence of cycloheximide. Stimulation of adenylate cyclase activity by either L-isoproterenol, glucagon, prostaglandin Ei, prostaglandin E2 or cholera toxin produced a 23–43% decrease in tyrosine release rate. L-Isoproterenol (1 fiM) lowered the protein degradation rate both in the presence (27 ± 3%) or absence (25 ± 2%) of insulin. In constrast, the a-adrenergic agonist methoxamine (10 JIM) increased the rate of tyrosine efflux. The phosphodiesterase inhibitors papaverine (0.1 mM) and theophylline (1 mM) reduced tyrosine release rate by 62 ± 2% and 25 ± 2%, respectively. The cyclic nucleotide analog dibutyryl cyclic AMP produced a 23 ± 4% reduction in tyrosine efflux rate, an effect that may be in part due to butyrate released by hydrolysis. Both carbamylcholine and nitroprusside, agents which activate guanylate cyclase, had no apparent effect on the rate of overall protein degradation. In experiments with atria labeled in vitro with radioactive tyrosine, the degradation rate of soluble cell proteins fractionated by gel filtration was uniformly decreased by L-isoproterenol over a wide molecular weight range. Comparison of degradation rate to protein kinase activation, high energy phosphate levels and beating rate show that suppression of degradation is more closely correlated with mechanical activity or energy depletion than with cyclic AMP content. Therefore, these data do not establish whether cyclic AMP modifies protein degradation directly, or by effects secondary to increased contractile activity.