Antiarrhythmic effects of beta3-adrenergic receptor stimulation in a canine model of ventricular tachycardia

Abstract

Background: Beta3-adrenergic receptor (beta3-AR) stimulation inhibits cardiac contractility.

Objective: This study sought to test the hypothesis that beta3-AR stimulation is antiarrhythmic.

Methods: We implanted a radio transmitter for continuous electrocardiogram monitoring in 18 dogs with a tendency for high incidence of spontaneous ventricular tachycardia (VT). Ten of 18 had subcutaneous continuous BRL37344 (beta3-AR agonist) infusion (experimental group) for 1 month. The other dogs were controls. Western blotting studies were performed on tissues sampled from the noninfarcted left ventricular free wall of all dogs that survived the 60-day follow-up period.

Results: Phase 2 VT appeared significantly later in the experimental group than in the control group (P <.05). The number of VT episodes in the experimental group was significantly lower than in the control group during both the first month (0.5 +/- 0.95 episodes/day vs. 2.6 +/- 2.3 episodes/day) and the second month (0.2 +/- 0.2 episode/day vs. 1.2 +/- 1.1 episodes/day, P <.05 for both). The experimental group had shorter QTc than control (P <.002). The experimental group had decreased protein levels for sodium calcium exchanger and dihydropyridine receptor, increased beta3-AR expression, without changes in beta1-AR, beta2-AR. The average heart weight and the left ventricular free wall thickness in the experimental group (226 +/- 17 g and 15.1 +/- 1.2 mm, respectively) was significantly lower than in the control group (265 +/- 21 g and 17.4 +/- 2.5 mm, respectively, P <.05 for both). There was no difference in the incidences of sudden cardiac death in these 2 groups of dogs.

Conclusion: Beta3-AR stimulation significantly reduces the occurrence of ventricular tachycardia.

Figure 1

The thickness of left ventricle free wall and myocardial infarct size in two groups. *, p< 0.05; versus control group.

Figure 2

Number of spontaneous phase-2 VT episodes after surgery. VT episodes more than 20/day were clipped. BRL (-) indicates control group; BRL (+) indicates experimental group. Red bar indicates BRL37344 infusion interval. Note that dogs 2 and 6 of the experimental group had no VT for an entire 60-day period.

Figure 3

Spontaneous VF and SCD. These episodes were from both the control group (A-B) and the experimental group (C-F). All VF episodes were initiated by a premature ventricular contraction that occurred before the end of the T wave (R on T).

Figure 4

QTc intervals at different time points. The vertical bar indicates the standard deviation. *, p< 0.05; **, p<0.01 versus control group

Figure 5

Protein expression of beta-ARs. The immunoblots are representative of the experiments summarized in the graph. Each bar represents the mean ± SD. Data are expressed as the ratio of control group. *, p< 0.05; versus control group.

Figure 6

Examples of Western blots of sodium-calcium exchanger (NaCaX), dihydropyridine receptor (DHPR), sarcoplasmic reticulum Ca²⁺-ATPase (SERCA2a), type-2 ryanodine receptor (RyR2), hERG, KvLQT1, MinK and GAPDH in left ventricular free wall homogenates of 3 controls dogs and 3 experimental group dogs. The band signals of DHPR and NaCax were significantly decreased in the experimental group. There was no significant difference in SERCA2a, RyR2, hERG, KvLQT1 and MinK expression between the control group and the experimental group.

Figure 7

Protein expression of nerve group factor (NGF), growth associated protein 43 (GAP43), and tyrosine hydroxylase (TH) in left ventricle free wall. Representative Western blotting bands of NGF, GAP 43, TH, and GAPDH in three control dogs and three experimental group dogs are shown. Each bar represents the mean ± SD. Data are expressed as the ratio of control group. There was no significant difference in NGF, TH and GAP43 expression between the control and experimental groups.

Figure 8

Examples of immunostaining of growth associated protein 43 (GAP43, A-B) and tyrosine hydroxylase (TH, C-D) in the control (A, C) and experimental group (B, D). A and B are longitude section, while C and D are transverse section. Both TH and GAP43 immunoreactivity (brown, arrow) was abundant in both control and experimental groups. However, there was no significant difference in either TH or GAP43 immunoreactivity between the two groups. Objective: 20×, Scale bar, 100 μm.