Epinephrine-induced automaticity of canine cardiac Purkinje fibers and its relationship to the adenylate cyclase-adenosine 3',5'-monophosphate system.

We studied the relationship between epinephrine-induced increases in automaticity and in the adenylate cyclase-adenosine 3',5'-monophosphate (cAMP) system in canine cardiac Purkinje fibers. In intact Purkinje fiber bundles superfused with Tyrode's solution, epinephrine induced a concentration-dependent increase in automaticity and in cAMP content. Both of these effects were reduced by propranolol. Decreasing the temperature of the superfusate from 37 degrees to 25 degrees C blocked the epinephrine-induced increase in automaticity, but not the increase in cAMP content. Addition of the metabolic blocking agent, iodoacetate, to the superfusate did not block the effect on Purkinje fiber automaticity, but the cAMP content did not increase. In Purkinje fiber bundle whole homogenates adenylate cyclase was stimulated more by isoproterenol than by epinephrine. Low concentrations of phenylephrine decreased adenylate cyclase activity; higher concentrations induced an increase toward control values. Iodoacetate did not significantly alter cyclase activity and did not affect its response to epinephrine. These students have shown that although intact Purkinje fiber bundles respond to epinephrine with increases in automaticity and cAMP content, these two events can be dissociated under appropriate conditions.