Functional and Structural Assessment of Intercellular Communication: Increased Conduction Velocity and Enhanced Connexin Expression in Dibutyryl cAMP-Treated Cultured Cardiac Myocytes

Remodeling of conduction pathways in the hypertrophic response to myocardial injury is a potential mechanism leading to the development of anatomic substrates of lethal arrhythmias. To delineate the responsible mechanisms and to directly relate changes in intercellular coupling at gap junctions with electrophysiological alterations, we studied the effects of cAMP, a mediator of cardiac hypertrophy, on action potential conduction velocity and connexin expression in neonatal rat ventricular myocyte cultures. Conduction velocity was measured with an optical activation mapping technique in cells loaded with the voltage-sensitive dye RH-237. Action potentials were conducted 24% to 29% more rapidly (P2-fold after a 4-hour exposure) but no change in the Cx43 synthesis rate. Northern blot analysis demonstrated a time-dependent increase in the amount of Cx43 mRNA, with a maximum 3.3-fold increase after 4 hours of exposure to 1 mmol/L db-cAMP; cycloheximide did not block this effect. In contrast, Cx45 mRNA levels were not altered significantly after db-cAMP treatment. Thus, cAMP causes a significant increase in conduction velocity that appears to be attributable largely to enhanced expression of proteins responsible for intercellular communication. Cx43 and Cx45 levels appear to be upregulated by cAMP by disparate molecular mechanisms.