Arrhythmogenic effect of sympathetic histamine in mouse hearts subjected to acute ischemia

Abstract

The role of histamine as a newly recognized sympathetic neurotransmitter has been presented previously, and its postsynaptic effects greatly depended on the activities of sympathetic nerves. Cardiac sympathetic nerves become overactivated under acute myocardial ischemic conditions and release neurotransmitters in large amounts, inducing ventricular arrhythmia. Therefore, it is proposed that cardiac sympathetic histamine, in addition to norepinephrine, may have a significant arrhythmogenic effect. To test this hypothesis, we observed the release of cardiac sympathetic histamine and associated ventricular arrhythmogenesis that was induced by acute ischemia in isolated mouse hearts. Mast cell-deficient mice (MCDM) and histidine decarboxylase knockout (HDC(-/-)) mice were used to exclude the potential involvement of mast cells. Electrical field stimulation and acute ischemia-reperfusion evoked chemical sympathectomy-sensitive histamine release from the hearts of both MCDM and wild-type (WT) mice but not from HDC(-/-) mice. The release of histamine from the hearts of MCDM and WT mice was associated with the development of acute ischemia-induced ventricular tachycardia and ventricular fibrillation. The incidence and duration of induced ventricular arrhythmias were found to decrease in the presence of the selective histamine H(2) receptor antagonist famotidine. Additionally, the released histamine facilitated the arrhythmogenic effect of simultaneously released norepinephrine. We conclude that, under acute ischemic conditions, cardiac sympathetic histamine released by overactive sympathetic nerve terminals plays a certain arrhythmogenic role via H(2) receptors. These findings provided novel insight into the pathophysiological roles of sympathetic histamine, which may be a new therapeutic target for acute ischemia-induced arrhythmias.

Figure 1

Morphological evaluation of hearts and SCG neurons from WT (A), MCDM (B) and HDC^−/− mice (C). Heart and SCGs were obtained from the same mouse. Black arrowheads identify mast cells. Magnified images of the boxed areas are shown in the bottom left of the corresponding panels (A₁, B₁ and C₁). No positive staining of tryptase was found in MCDM sections (B₁). SCG sections were double stained with antibody to DβH (green) and HA (red). All mice strains were positive for DβH (A₂, B₂ and C₂). Majority of WT mice and MCDM SCG neurons were simultaneously positive for HA (A_3–4 and B_3–4), whereas HDC^−/− SCG sections were negative for HA (C₃). Experiments were repeated in three mice from each strain, and five sections of heart or SCG from each one mouse were observed. Scale bar, 50 μm.

Figure 2

Coronary HA and NE overflows from perfused mice isolated hearts. (A) Effect of EFS on coronary HA (A₁) and NE overflows (A₂) in WT, MCDM and HDC^−/− mouse hearts. Each point represents the mean ± SD value of HA or NE concentration in the first 5 min of coronary effluent after EFS (n = 5). (B) Effect of global stop-flow ischemia followed with reperfusion on coronary HA (B₁) and NE overflows (B₂) in WT, MCDM and HDC ^−/− mouse hearts. Ischemia was applied for 10 min after an initial stabilization period of 30 min. Each point represents the mean ± SD value of HA concentration in the coronary effluent after reperfusion (n = 12–15).

Figure 3

Effect of different durations of global stop-flow ischemia on coronary HA (A) and NE overflows (B) from WT and MCDM hearts after reperfusion. Ischemia was applied for 10, 20 or 30 min after an initial stabilization period of 30 min. Each point represents the mean ± SD value of HA concentration in the coronary effluent in the first 5 min of reperfusion. *P < 0.05 between hearts of different phenotypes subjected to the same ischemia time by ANOVA followed by a Student-Newman-Keuls t test (n = 5).

Figure 4

Effect of 6-OHDA on EFS (A) and global stop-flow ischemia (B) induced coronary HA and NE overflows in WT mice and MCDM. (A) The frequency of EFS applied was 8 Hz for 60 s with pulse duration of 1 ms. Bars represent the mean ± SD values of HA (A₁) and NE (A₂) concentrations in the first 5 min of coronary effluent after EFS. (B) Ischemia was applied for 10 min after an initial stabilization period of 30 min. Bars represent the mean ± SD values of HA (B₁) and NE (B₂) concentrations in the coronary effluent in the first 5 min of reperfusion. **P < 0.01 versus the 6-OHDA untreated group of corresponding phenotype by Student t test (n = 5).

Figure 5

Effect of global stop-flow ischemia on ventricular arrhythmias in mice isolated hearts during a 30-min reperfusion. Ischemia was applied for 10 min after an initial stabilization period of 30 min. (A) ECG tracings from one heart of WT, MCDM and HDC^−/−mouse, respectively, showing the first 60 s and the last 10 s of the 30-min reperfusion. (B) The incidence of VT and VF is expressed as percentages of the total number of hearts used in each group. Bars represent the percentage values; *P < 0.05 versus WT group and ⁺P < 0.05 versus the MCDM group by χ² test. (C) Duration of VT and VF represents the cumulative duration of arrhythmia during the 30-min reperfusion. Bars represent the mean ± SD values of duration time. *P < 0.05 versus WT group, ⁺P < 0.05 versus MCDM group by ANOVA followed by Student-Newman-Keuls t test (n = 12–15).

Figure 6

Effect of atenolol and famotidine, alone and in combination, on ventricular arrhythmias induced by global stop-flow ischemia in isolated hearts of WT mice and MCDM mice during a 30-min reperfusion. Ischemia was applied for 10 min after an initial stabilization period of 30 min. (A) The incidence of VT and VF is expressed as percentages of the total number of hearts used in each group. Bars represent the percentage values. *P < 0.05 versus untreated group; ⁺P < 0.05 versus either atenolol alone or famotidine alone group by χ² test. (B) The duration of VT and VF represents the cumulative duration of arrhythmia during the 30-min reperfusion. Bars represent the mean ± SD values of duration time. *P < 0.05 versus untreated group; ⁺P < 0.05 versus either atenolol alone or famotidine alone group by ANOVA followed by Student-Newman-Keuls t test. (C) Effect of atenolol and famotidine, alone or in combination, on coronary HA overflow obtained from the same isolated hearts as in (A) and (B). Bars represent the mean ± SD values of HA concentrations in the coronary effluent in the first 5 min of reperfusion (n = 10–12).

Figure 7

Effect of HA and NE, alone and in combination, on ventricular arrhythmogenesis in isolated hearts of WT mice (A) and MCDM mice (B). Exogenous reagents were applied for 5 min after an initial stabilization period of 30 min. Surface ECG recording began when exogenous reagents were applied and lasted for 30 min. Bars represent the mean ± SD values of duration time. *P < 0.05 versus respective HA or NE treatment group by ANOVA followed by Student-Newman-Keuls t test (n = 5).

Figure 8

Myocardial cAMP levels in isolated hearts of WT, MCDM and HDC^−/−mice. The basal groups of myocardial cAMP levels were obtained from hearts without being subjected to global stop-flow ischemia. The ischemia-induced myocardial cAMP levels were obtained from hearts subjected to a 10-min global stop-flow ischemia followed by 30-min reperfusion with or without famotidine treatment. Bars represent the mean ± SD values of cAMP levels. *P < 0.05 versus corresponding basal group; ⁺P < 0.05 versus corresponding famotidine untreated group; ^§P < 0.05 versus famotidine untreated group of HDC^−/− mice by ANOVA followed by Student-Newman-Keuls t test (n = 8–12).