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Inactivation of (Gialpha) proteins increases arrhythmogenic effects of beta-adrenergic stimulation in the heart
1Pharmakologisches Kerninstitut, Universitäts-Krankenhaus Eppendorf, Hamburg, Germany.
Abstract:
Chronic treatment of rats with carbachol downregulates M-cholinoceptors and inhibitory, pertussis toxin (PTX)-sensitive G proteinalpha-subunits (Gialpha) and sensitizes the heart to arrhythmogenic effects of isoprenaline (ISO), suggesting a causal relationship. To test this hypothesis by a more direct and quantitative approach, nine groups of rats were treated for 24 h with increasing doses of PTX (1.25-200 microg/kg i.v.). Inactivation of cardiac Gialpha was determined biochemically by 32P-ADP-ribosylationin vitro and functionally by measuring contractile effects of carbachol. Effects of ISO were studied in spontaneously beating right atria (RA) and isolated papillary muscles (PM; paced at 1 Hz). PTX increased heart rate in conscious animals (ECG) with a bell-shaped dose-dependency (maximal increase 120 beats/min at 7.5 microg/kg). PTX dose-dependently inactivated 25-85% of total cardiac Gialpha, which linearly correlated with a loss of the direct negative chronotropic effect of carbachol in atria, but not with a loss of its indirect negative inotropic effect in PM. The latter was resistant up to PTX 20 microg/kg (=70% inactivation). The decrease in Gialpha closely correlated with an increased efficacy of ISO to induce spontaneous contractile activity (automaticity) in PM. At 3 micromol/l ISO, all PM from PTX 200 microg/kg beat spontaneously compared to 10% in control. In contrast, pretreatment with PTX only modestly and not clearly dose-dependently increased the inotropic potency of ISO (PTX 100 microg/kg: EC50 28v 81 nmol/l in control) and did not affect the chronotropic effect of ISO. The disparity of the functional consequences of PTX treatment suggest that under physiological conditions, Gialpha serve mainly to suppress arrhythmogenic, but not or to a minor extent, positive chronotropic or inotropic effects of beta-adrenoceptor activation.
Insights
Pertussis toxin (PTX) inactivates cardiac G-alpha-inhibiting subunits (Gialpha), increasing heart rate and sensitizing the heart to isoprenaline-induced arrhythmias. Gialpha primarily suppresses arrhythmogenic effects, not positive chronotropic or inotropic effects.
Area of Science:
- Cardiovascular Pharmacology
- G protein signaling
- Cardiac electrophysiology
Background:
- Chronic carbachol treatment downregulates M-cholinoceptors and inhibitory G-alpha-inhibiting subunits (Gialpha).
- This downregulation sensitizes the heart to isoprenaline (ISO)-induced arrhythmogenic effects, suggesting a causal link.
- A direct, quantitative approach is needed to confirm this hypothesis.
Purpose of the Study:
- To quantitatively assess the relationship between G-alpha-inhibiting subunit (Gialpha) inactivation and cardiac sensitivity to isoprenaline (ISO).
- To investigate the role of G-alpha-inhibiting subunits (Gialpha) in suppressing isoprenaline-induced arrhythmogenic effects.
Main Methods:
- Rats were treated with varying doses of pertussis toxin (PTX) to inactivate cardiac G-alpha-inhibiting subunits (Gialpha).
- G-alpha-inhibiting subunit (Gialpha) inactivation was measured biochemically (32P-ADP-ribosylation) and functionally (carbachol effects).
- Effects of isoprenaline (ISO) on heart rate (ECG) and contractility (papillary muscles, atria) were assessed.
Main Results:
- Pertussis toxin (PTX) increased heart rate in a bell-shaped dose-dependent manner.
- PTX dose-dependently inactivated cardiac G-alpha-inhibiting subunits (Gialpha), correlating with reduced carbachol effects.
- G-alpha-inhibiting subunit (Gialpha) inactivation correlated with increased isoprenaline (ISO) efficacy in inducing spontaneous contractile activity (automaticity) in papillary muscles.
- PTX pretreatment only modestly increased isoprenaline (ISO) inotropic potency and did not affect its chronotropic effect.
Conclusions:
- G-alpha-inhibiting subunits (Gialpha) primarily suppress the arrhythmogenic effects of beta-adrenoceptor activation.
- G-alpha-inhibiting subunits (Gialpha) play a minor role in modulating the positive chronotropic or inotropic effects of beta-adrenoceptor activation.
- These findings clarify the specific role of G-alpha-inhibiting subunits (Gialpha) in cardiac regulation and arrhythmogenesis.