Role of histamine in cardiac muscle function--an attempt at demonstrating the presence of intracellular histaminergic H2-type receptors.

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 AND 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 microM x l-1 of histamine mean rhythm rate increased to 61 +/- 2.7 stimulations x min-1, contractile tone increased by 55.7 +/- 4.9% in relation to the control values, relaxation time shortened to 139.2 +/- 1.8 ms, while time to pike decreased to 229.6 +/- 2.5 ms. The following effects were observed after administration of 5 microM x l-1 of histamine: mean rhythm rate increased to 76.4 +/- 4.5 x min-1, amplitude of the potentials was unchanged, while their duration shortened to 227.7 +/- 3.2 ms, contractile tone increased by 122.4 +/- 12.9%, average. In group II contractile tone increased by mean 110%, time to pike decreased to 103 +/- 1.5 ms, while relaxation time--to 210.2 +/- 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.

Respiratory potentials recorded from the human hindbrain.

Using a nasopharyngeal and a neck electrode slow potentials synchronous with respiration were registered. Similar potentials were recorded in a patient with apnoea due to widespread paralysis of the Landry type. The potentials are thought to be of medullary origin.

Effects of calmidazolium and dibutyryl cyclic AMP on the longitudinal internal resistance in sinus node strips.

The effects of calmidazolium (10(-7) mol/l) and db cAMP (10(-3) mol/l) on the longitudinal internal resistance (ri) in the rabbit sinus node strips were studied by means of microelectrode and extracellular recording techniques. Calmidazolium (CDZ) decreased ri by 27%. An addition of db cAMP to the CDZ-containing Tyrode potentiated this effect by further 14%. db cAMP initially decreased ri by 17%, then it enhanced this resistance by 7% above the pretreatment control value. An addition of CDZ to the db cAMP-containing Tyrode reduced ri by 41% below the pretreatment control. Possible mechanisms underlying these effects were discussed in terms of cAMP, Cai, and calmodulin action on the cell coupling.

Effects of forskolin on the longitudinal internal resistance (ri) in rabbit sinus node strips.

The effect of several concentrations of 7-0-hemisuccinyl, 7-deacetyl forskolin on ri was studied by means of microelectrode and single sucrose gap techniques. Twenty mumol.l-1 of forskolin added to the sucrose gap lowered ri by 15% below the control value. The drug applied at 50 mumol.l-1 initially decreased ri by 31%, and then it increased the resistance by 43% above the control value. The possible mechanisms underlying these effects are discussed in terms of cAMP and Ca action on the cell coupling.

Effects of cyclic nucleotides on the longitudinal internal resistance in the rabbit sinus node.

The effect of dibutyryl cyclic AMP as well as dibutyryl cyclic GMP on the longitudinal internal resistance was studied in the rabbit sinus node strips by means of microelectrode and sucrose gap techniques. Dibutyryl cyclic AMP added to the sucrose gap decreased the resistance and increased the spontaneous rate whereas dibutyryl cyclic GMP evoked the opposite effects. These results suggest that cyclic nucleotides influence the cell coupling in the sinus node.

A method of measurement of longitudinal resistances in an isolated cardiac and smooth muscle preparation.

The longitudinal internal as well as external resistance was measured by means of simultaneous intracellular and sucrose gap voltage recordings. The longitudinal internal resistance could be calculated from a formula comprising the shunt resistance, membrane action potential and transgap action potential before and after shunting. The longitudinal external resistance was calculated from another formula comprising the same quantities.