Direct effects of chronic beta-adrenergic receptor blockade on left ventricular and myocyte function in a model of tachycardia-induced congestive heart failure.
BACKGROUND
Chronic beta-receptor blockade (beta-blockade) has been reported to improve symptoms and increase survival in patients with congestive heart failure (CHF); however, whether the mechanisms for the effects of beta-blockade in CHF are due to modulating chronotropy, inotropy, or both remains unknown. To address this issue, left ventricular function and isolated myocyte function were examined with chronic beta-blockade in a rapid pacing model of CHF, thereby eliminating potential chronotropic effects of beta-blockade.


RESULTS
Pigs were randomly assigned to three groups of six pigs each: supraventricular tachycardia (SVT): 3 weeks of atrial pacing at 240 beats/min; SVT/beta-blockade: 3 weeks of rapid pacing and beta-blockade (25 mg atenolol twice daily on days 14-21 of pacing); control group, sham control animals. This dosage schedule for beta-blockade was chosen because catecholamines are persistently elevated by day 14 in this model of CHF. Left ventricular fractional shortening and end-diastolic dimension were measured by echocardiography in the conscious state with a resting ambient heart rate. Isolated left ventricular myocyte function was examined using high-speed videomicroscopy. Supraventricular tachycardia caused left ventricular dilation (5.4 +/- 0.1 vs 3.5 +/- 0.1 cm) and reduced fractional shortening (12 +/- 1% vs 35 +/- 1%) compared with control animals (P < .05). The SVT/beta-blockade group showed no significant effects on left ventricular size or function compared with the SVT group, but their ambient resting heart rate was reduced by 20% relative to the SVT group (P < .05). Myocyte shortening was reduced in the SVT group (2.2 +/- 0.1% vs 4.5 +/- 0.1%, P < .05) compared with the control group and increased from SVT-only values with beta-blockade (2.7 +/- 0.1%, P < .05). Similarly, myocyte shortening velocity was similarly reduced in the SVT and SVT/beta-blockade groups (31 +/- 1 and 32 +/- 1 microns/s) compared with the control group (51 +/- 1 microns/s, P < .05). With SVT/beta-blockade myocyte contraction duration was prolonged (525 +/- 5 ms) compared with SVT-only or control values (469 +/- 9 and 473 +/- 4 ms, P < .05). Thus, institution of beta-1-selective blockade during the development of SVT-induced CHF altered the temporal characteristics of the myocyte contraction process, which resulted in improved myocyte shortening.


CONCLUSIONS
In a model of CHF due to the maintenance of a chronically elevated heart rate, institution of beta-1-selective blockade during the progression of the CHF process minimally affected left ventricular size and function. At the level of the myocyte, chronic beta-1-receptor blockade prolonged the contraction interval and thereby increased myocyte shortening. These unique results suggest that a contributory mechanism for the effects of beta-blockade in the setting of CHF is chronotropic modulation.
Spinale
F G
FG
Department of Surgery and Anesthesiology, Medical University of South Carolina, Charleston 29425, USA.
Johnson
W S
WS
Wang
Y
Y
Wang
Z
Z
Mukherjee
R
R
Hebbar
L
L
Jones
B U
BU
D000319:Adrenergic beta-Antagonists / Q000008:administration & dosage / Q000627:therapeutic use*; D000704:Analysis of Variance; D000818:Animals; D001262:Atenolol / Q000008:administration & dosage / Q000627:therapeutic use*; D002304:Cardiac Pacing, Artificial; D002395:Catecholamines / Q000097:blood; D018929:Cell Culture Techniques; D004195:Disease Models, Animal; D006333:Heart Failure / Q000188:drug therapy* / Q000209:etiology / Q000503:physiopathology; D006439:Hemodynamics / Q000187:drug effects / Q000502:physiology; D009200:Myocardial Contraction / Q000187:drug effects* / Q000502:physiology; D009206:Myocardium / Q000166:cytology; D011897:Random Allocation; D012016:Reference Values; D013552:Swine; D013617:Tachycardia, Supraventricular / Q000150:complications*; D016277:Ventricular Function, Left / Q000187:drug effects*
D000319:Adrenergic beta-Antagonists; D002395:Catecholamines; D001262:Atenolol