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

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Abstract
Little information exists on the effects of drugs with cardiovascular action in hypothermia, and some findings have indicated paradoxic effects of dopamine in this setting. As we have not found any data on the electrophysiologic and contractile effects of dopamine on the heart in hypothermia, we decided to study this in pig myocardium, since pigs have a cardiovascular system more similar to that of humans than other animals. Excised muscle strips from pig ventricular septum were mounted in an organ bath. After 45 min of equilibration at 37 degrees C or 32 degrees C, resting and action potentials, time to peak contraction and contractile force were recorded during pacing with a frequency of 60/min. Dopamine at 4 microM or 8 microM was added and new recordings were made after 15 min. Cooling to 32 degrees C caused a prolongation of contraction by 48% and the contractile force increased by 39%. The membrane action potential duration at 50% and 90% repolarization levels increased at 32 degrees C by 28% and 16% respectively. Dopamine significantly (P<0.05) increased the contractile force and membrane action potential duration at 50% and 90% repolarization levels both in normothermia and in hypothermia, whereas the duration of the contraction was not significantly changed. Cooling to 32 degrees C significantly prolongs the myocardial action potential and the contraction duration. Dopamine increases the contractile force and prolongs the action potential both at 37 degrees C and at 32 degrees C.