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Beta-adrenoceptor mediated signal transduction in congestive heart failure in cardiomyopathic (UM-X7.1) hamsters
1Division of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, Winnipeg, Manitoba, Canada.
Insights
Severe congestive heart failure in hamsters shows reduced beta 1-adrenoceptors and impaired Gs-protein function. Increased Gi-protein activity and expression contribute to adrenergic signal loss in failing hearts.
Area of Science:
- Cardiovascular Physiology
- Molecular Cardiology
- Signal Transduction
Background:
- Congestive heart failure (CHF) involves complex alterations in cardiac signaling pathways.
- The role of beta-adrenoceptor (β-AR) mediated signal transduction in severe CHF remains incompletely understood.
Purpose of the Study:
- To investigate the status of β-ARs, G-proteins, and adenylyl cyclase (AC) activity in severe CHF.
- To elucidate the molecular mechanisms underlying adrenergic dysfunction in failing hearts.
Main Methods:
- Studied cardiomyopathic hamster hearts at severe stages of failure (220-275 days old).
- Assessed β-AR density and affinity (Kd) using radioligand binding.
- Measured AC activity, G-protein functional activity (cholera and pertussis toxin stimulation), protein content (immunoblotting), and mRNA levels (Northern blot).
Main Results:
- Marked decrease in cardiac β1-AR density, with no change in β2-AR density or receptor affinity.
- Attenuated isoproterenol-stimulated AC activity and significantly depressed forskolin, NaF, and Gpp(NH)p-stimulated AC activities.
- Depressed functional activity and increased content/bioactivity of inhibitory G-proteins (Gi).
- Increased mRNA levels for both stimulatory (Gs) and inhibitory (Gi) G-proteins.
- Evidence of Gs-protein uncoupling from the catalytic AC site.
Conclusions:
- Adrenergic support loss in severe CHF involves reduced β1-AR numbers and impaired Gs-protein function.
- Increased Gi-protein content and bioactivity contribute to the observed signaling deficits.
- These alterations collectively disrupt cardiac adrenergic signaling in advanced heart failure.
Abstract:
In view of the lack of information regarding the status of beta-adrenoceptor mediated signal transduction mechanisms at severe stages of congestive heart failure, the status of beta-adrenoceptors, G-proteins and adenylyl cyclase activities was examined in 220-275 day old cardiomyopathic hamster hearts. Although no changes in the Kd values for beta 1- and beta 2-adrenoceptors were seen, the number of beta 1-adrenoceptors, unlike that of beta 2-adrenoceptors, was markedly decreased in cardiac membranes from failing hearts. The activation of adenylyl cyclase in the failing hearts by different concentrations of isoproterenol was also attenuated in comparison to the control preparations. The basal adenylyl cyclase activity in cardiac membranes from the failing hearts was not altered; however, the stimulated enzyme activities, when measured in the presence of forskolin, NaF or Gpp(NH)p were depressed significantly. The functional activity of Gs-proteins (measured by cholera toxin stimulation of adenylyl cyclase) was depressed whereas that of Gi-proteins (measured by pertussis toxin stimulation of adenylyl cyclase) was increased in the failing hearts. Not only were the Gs- and Gi-protein contents (measured by immunoblotting) increased, the bioactivities of these proteins as determined by ADP-ribosylations in the presence of cholera toxin and pertussis toxin, respectively, were also higher in failing hearts in comparison to the control values. Northern blot analysis revealed that the signals for Gs- and Gi-protein mRNAs were augmented at this stage of heart failure. These results indicate that the loss of adrenergic support at severe stages of congestive heart failure in cardiomyopathic hamsters may involve a reduction in the number of beta 1-adrenoceptors, and an increase in Gi-protein contents as well as bioactivities in addition to an uncoupling of Gs-proteins from the catalytic site of adenylyl cyclase in cardiac membrane.