Effects of dopamine on porcine myocardial action potentials and contractions at 37 degrees C and 32 degrees C

Background: Little information exists on the effects of drugs with cardiova scular action in hypothermia, and some findings have indicated paradoxic ef fects of dopamine in this setting. As we have not found any data on the ele ctrophysiologic and contractile effects of dopamine on the heart in hypothe rmia, we decided to study this in pig myocardium, since pigs have a cardiov ascular system more similar to that of humans than other animals. Methods: Excised muscle strips from pig ventricular septum were mounted in an organ bath. After 45 min of equilibration at 37 degreesC or 32 degreesC, resting and action potentials, time to peak contraction and contractile fo rce were recorded during pacing with a frequency of 60/min. Dopamine at 4 m uM or 8 muM was added and new recordings were made after 15 min. Results: Cooling to 32 degreesC caused a prolongation of contraction by 48% and the contractile force increased by 39%. The mem- brane action potentia l duration at 50% and 90% repolarization levels increased at 32 degreesC by 28% and 16% respectively. Dopamine significantly (P<0.05) increased the co ntractile force and membrane action potential duration at 50% and 90% repol arization levels both in normothermia and in hypothermia, whereas the durat ion of the contraction was not significantly changed. Conclusion: Cooling to 32<degrees>C significantly prolongs the myocardial a ction potential and the contraction duration. Dopamine increases the contra ctile force and prolongs the action potential both at 37 degreesC and at 32 degreesC.