RESUMEN
To reinvestigate the possible regulating activity of the Na-Ca exchange mechanism on the rest potentiation phenomenon post rest contraction force development was studied in isometrically contracting rat papillary muscles, stimulated at a rate of 30/min and submitted to pauses varying from 5 to 60 seconds. The Na-Ca exchange activity was analysed using the relationship between the amplitude of the first post rest contractions and the steady state control contractions (relative potentiation) and by the changes of internal Na that are known to affect the Na-Ca exchange activity. Maneuvers that increased the internal Na concentration as high rate of stimulation and high external Na produced an increase of the relative potentiation. This was seen when stimulation rate was changed from 30/min to 60/min, even in low extracellular. Na concentration. On the other hand, the blockade of the fast Na channel by propafenone and the reduction of the external Na concentration, that are known to reduce internal Na, diminished the relative potentiation. These results suggest that the activity of the Na/Ca exchange mechanism might control the rest potentiation effects in cardiac muscle (AU)