Effect of dopamine on the transmembrane potentials of guinea-pig heart preparations

The effects of dopamine on the transmembrane action potentials of electrically driven left auricle and right papillary muscle of guinea pig were studied in Tyrode solution and in 25 mM K+-Tyrode solution. Under physiological condition dopamine at concentration less than 10(-6) M had no effect, at concentrations between 10(-6) and 10(-4) M it slightly increased the resting potential (RP), caused a marked increase of the overshoot (OS), but did not change the maximum rate of rise (Vmax) of the action potential (AP). The duration of AP was significantly enhanced by 10(-6) and 10(-5) M but was shortened by 10(-4) M dopamine. The effects of dopamine on AP were antagonized by pindolol (4 X 10(-7) M) but not by phentolamine (5 X 10(-6) M). In the atrium in which the fast sodium channels were blocked by partial depolarization (25 mM K+-Tyrode) and the preparation was inexcitable, dopamine in high concentrations (10(-4)--2.5 X 10(-4) M) restored the electrical and mechanical activity. Dopamine induced APs having very slow rate of rise and low RP, characteristics of the given extracellular K+ concentration. The slow electrical responses were unaffected by haloperidol (10(-5) M), but were abolished by pindolol. The effects of dopamine and adrenaline on the depressed fast Na+ system (using 4 mM to 20 mM K+-Tyrode solution) were compared in the papillary muscle. The steady state inactivation of Vmax was shifted by 6 mV to more negative potentials by adrenaline (5 X 10(-6) M) and by 3 mV to more negative potentials by dopamine (2.5 X 10(-4) M).(ABSTRACT TRUNCATED AT 250 WORDS)