[The effects of histamine on the electrical and mechanical properties of the heart].

The effects of histamine on the electrical and mechanical properties of the right and left atria and papillary muscle of the guinea-pig's heart were studied. The experiments were performed using conventional techniques of intracellular recording of the electrical activity and isometric tension. It was found that the distribution of the histaminergic receptors is different in the several tissues studied. The H2 receptors predominate in the right atrium, whereas the H1 in the left one. There are both kinds of receptors in the ventricular tissues. The main effects found were: a) In the right atrium: a positive chronotropic effect, a shortening of the action potential and an increase in excitability. b) In the left atrium only a shortening of the action potential was seen. The other electrical and mechanical parameters were not affected by histamine. c) In ventricular muscle a biophasic response in the action potential duration was observed, low concentrations of histamine induced a shortening of the action potential, whereas larger concentrations provoked a lengthening. Also low doses of histamine induced a negative inotropic effect, secondary to the activation of H1 receptors, while concentrations larger than 10(-8) M provoked a positive inotropic effect due to the H2 receptor activation. Also a decrease in excitability was seen mainly due to the activation of the H1 receptors stimulation. In every tissue studied the effects of histamine were seen with concentrations as low as 1 X 10(-10) M, being the H1 receptors who shown a greater affinity, since the activation of the H2 receptors were obtained only with doses larger than 1 X 10(-8) M.(ABSTRACT TRUNCATED AT 250 WORDS)