© 1999 by European Society of Cardiology
Copyright © 1999, European Society of Cardiology
Modulation of the pacemaker current If by β-adrenoceptor subtypes in ventricular myocytes isolated from hypertensive and normotensive rats
aDepartment of Preclinical and Clinical Pharmacology, University of Firenze, Firenze, Italy
bInstitute of Pharmacology and Toxicology, Universitad Complutense de Madrid, Madrid, Spain
* Corresponding author. Tel.: +39-055-4271-264; fax: +39-055-4271-285; e-mail: mugelli@server1.pharm.unifi.it
Objective: Both β1- and β2-adrenoceptors (β1-AR and β2-AR) are functionally present in human and rat ventricular myocytes. The two receptor subtypes are differently regulated during the development of myocardial hypertrophy and failure. If is expressed in human and rat ventricular myocytes. In hypertrophied myocytes isolated from old spontaneously hypertensive rats (SHR) the density is much larger than in age-matched normotensive Wistar Kyoto (WKY). Due to the possible relevance of If as an arrhythmogenic mechanism in the rat and human ventricle, we studied and compared the effects of β1-AR and β2-AR stimulation on If in both hypertrophied and normal left ventricular myocytes of 18-month old SHR and WKY. Methods: The whole-cell configuration of the patch-clamp technique was employed. Noradrenaline (NA, 1 µM) was used to stimulate β1-AR and isoprenaline (ISO, 1 µM) in the presence of the β1-AR antagonist CGP 20712A (0.1 µM) to stimulate β2-AR. Results: In SHR, NA increased If by causing a 10.8±0.9 mV (n=10) positive shift in the voltage of maximal activation (V1/2); this effect was completely reversed by CGP 20712A. β2-AR stimulation was effective in seven out of 13 cells tested, where it caused a small positive shift in V1/2 (4.0±1.7 mV). Cyclopentyladenosine (CPA), a selective A1-receptor agonist, reversed the effect of NA; the antiadrenergic action of CPA was abolished in cells pre-incubated with pertussis toxin (PTX) to block inhibitory G proteins (Gi). In PTX-treated cells the shift in V1/2 caused by both β2-AR (9.6±1.7 mV, n=6, p<0.05) and β1-AR (17.6±1.9 mV, n=7, p<0.05) was significantly greater than in control cells. Both β-AR subtypes modulated If activation also in WKY: β1-AR shifted V1/2 by 16.0±1.4 mV (n=15) and β2-AR by 4.2±1.1 mV (n=7). However, in PTX-treated WKY cells only the β2-AR effect was potentiated (shift in V1/2: 11.4±1.4 mV, n=9, p<0.01), while the β1-AR response was unchanged (18.9±4.2 mV, n=5, n.s.). Conclusions: If expressed in SHR hypertrophied ventricular myocytes is modulated by catecholamines mainly through the stimulation of the β1-AR subtype. The β1-AR response is, however, significantly lower than that observed in myocytes from normotensive rats, probably as a consequence of the presence of an increased inhibitory activity of Gi proteins. This post-receptorial control may be seen as a mechanism to limit the arrhythmogenicity of β-AR stimulation in myocardial hypertrophy and failure.
KEYWORDS β-AR, β-adrenoceptor; β1-AR, β1-adrenoceptor; β2-AR, β2-adrenoceptor; CPA, Cyclopentyladenosine; Gi, Inhibitory GTP-binding protein; Gs, Stimulatory GTP-binding protein; gf,max, Maximal specific conductance of If; ISO, Isoprenaline; NA, Noradrenaline; PTX, Pertussis toxin; SHR, Spontaneously hypertensive rats; V1/2, Voltage of half maximal activation of If; WKY, Wistar Kyoto rats