The effects of histamine on the transmembrane electrical activity of cells of small preparations (0.5 X 0.5 mm) of guinea-pig and rabbit sinoatrial- and atrioventricular-nodes were studied. Histamine at concentrations above 10(-7) mol/l increased the firing rate, the rate of diastolic depolarization, the maximum diastolic potential, the amplitude and the maximum rate of depolarization of the action potential of pacemaker cells of rabbit and guinea-pig sinoatrial cells and rabbit atrioventricular cells. These effects were antagonized by the H2-receptor blocker cimetidine (2.5 X 10(-6)mol/l) but they were not modified by the H1-receptor blocker chlorphenamine (2.5 and 5 X 10(-6)mol/l). Small preparations of guinea-pig atrioventricular node did not exhibit spontaneous activity, but it was induced by histamine and blocked by cimetidine. Histamine increased the maximum upstroke velocity of propagated action potential of cells of the central part of complete atrioventricular node in both species studied. These effects were blocked by cimetidine, but not by chlorphenamine. It is concluded that the increase in automaticity induced by histamine in guinea-pig and rabbit sinoatrial and atrioventricular nodes was due to stimulation of H2-receptors. Histamine did not depress electrical activity of atrioventricular node cells, but rather increased it. This effect was due to H2-receptor stimulation.
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