Differential effects of histamine on left and righ guinea-pig atria

In electrically stimulated strip from right guinea-pig atria histamine produced a positive inotropic effect and it increased the amplitude and duration of calcium-dependent slow action potentials, which suggests an increase in the slow inward current. These effects were apparently mediated by H2-receptors. On strips from left atria histamine produced an H1-receptor mediated positive inotropic effect, without alteration of slow action potentials. The mechanism of this positive inotropic effect remains unknown.