Background: Histamine is one of the biologically active substances that activate adenyl cyclase enzymatic system through H2 receptor. The aim of the study is demonstration of the presence of membranous histamine receptors in cardiomyocytes and determination of their role in function of the cardiac muscle cell.
Material/Methods: The experiments were carried out on 20 rabbits divided into two experimental groups. Electric and mechanical activity of cardiac muscle trabecula was registered by glass microelectrodes' method in group I and by saccharose slit method in group II.
Results: In group I after administration of 2.5 µMil-1 of histamine mean rhythm rate increased to 61&plusm;2.7 stimulationsimin-1, contractile tone increased by 55.7&plusm;4.9% in relation to the control values, relaxation time shortened to 139.2&plusm;1.8 ms, while time to pike decreased to 229.6&plusm;2.5 ms. The following effects were observed after administration of 5 µMil-1 of histamine: mean rhythm rate increased to 76.4&plusm;4.5 i min-1, amplitude of the potentials was unchanged, while their duration shortened to 227.7&plusm;3.2 ms, contractile tone increased by 122.4&plusm;12.9%, average. In group II contractile tone increased by mean 110%, time to pike decreased to 103&plusm;1.5 ms, while relaxation time - to 210.2&plusm;4.2 ms. Frequency of spontaneous stimulations as well as amplitude and duration of the action potential remained unchanged in all of the experiments.
Conclusions: Positive chronotropic and inotropic action of histamine added to the extracellular fluid point to the presence of histaminergic receptor in rabbit's cardiac muscle. H2 histaminergic receptors are situated not only on the external surface of cellular membrane of rabbit's right heart atrium trabeculae, but also inside the cells. Experiments with histamine administration by the "cut end" method suggest that the role of intracellular H2 histaminergic receptors is associated with controlling the contractile processes of the cardiac muscle.

Keywords: Histamine, H2 receptor, cardiomyocytes