Effects of Histamine on Intracellular Na⁺ Activity and Twitch Tension in Guinea Pig Papillary Muscles

Effects of histamine on the twitch tension, intracellular Na⁺ activity, and action potential were simultaneously and continuously measured in the isolated cardiac papillary muscle of guinea pigs. In the driven fiber at 60 beats/ min, histamine induced a concentration-dependent positive inotropic effect, and apparently increased the amplitude of the action potential and shortened the duration at 3μM, and significantly decreased by 6.7% from the control value of 4.5 ±0.1 mM (n = 6, p < 0.05) at 0.3μM. Cimetidine at 10μM did not produce any significant effect but partially antagonized the action of histamine. The respective concentrations of 1 and 3μM histamine significantly decreased by 5.4 and 14.3% from the control 5.6±0.1mM (n = 6, p<0.05) in the presence of 10μM cimetidine. Chlorpheniramine at 10μM significantly reduced the twitch tension and, and prolonged the action potential duration. Histamine at 1μM increased the twitch tension and decreased from 4.5+0.1 to 4.1±0.1mM (n = 5, p<0.05) in the presence of 10 μM chlorpheniramine. The combined application of 10μM cimetidine and 10μM chlorpheniramine reduced the twitch tension and abolished most effects of histamine. In high potassium solution (16.2mM), 1μM histamine significantly decreased by 12.9% from the control of 3.1+0.2 mM (n = 5, p<0.05). The treatment of 0.5 or 1βM ouabain increased the tension development and, and abolished the decrease in produced by the effect of histamine. The results suggest that histamine can directly stimulate Na-K pump in sarcolemma through means of the activation of both H₁- and H₂-receptors.