We examined the involvement of the Na(+)/Ca(2+) exchanger in the automaticity of 
the pulmonary vein myocardium with a specific inhibitor, SEA0400. Action 
potentials were recorded from the myocardial layer of isolated guinea-pig 
pulmonary vein preparations, and Ca(2+) transients were recorded from the 
cardiomyocytes. Spontaneous electrical activity was observed in 17.7% of the 
preparations, which was inhibited by either SEA0400 or ryanodine. In quiescent 
preparations, ouabain induced electrical activity and spontaneous Ca(2+) 
transients, which were inhibited by SEA0400, as well as ryanodine. These results 
provide pharmacological evidence that the Na(+)/Ca(2+) exchanger underlies the 
automaticity of the pulmonary vein myocardium.