A study on the effect of β-adrenergic stimulation on the electrophysiology of the isolated heart

Background: A coordinated heart beat relies on the propagation of a rapid depolarising event throughout the atria and ventricles and the subsequent coupling of this electrical signal to a transient contraction in every atrial and ventricular cardiomyocyte. The rate of propagation, known as conduction velocity (CV) is mainly determined by cellular expression of Na channels and gap junctional proteins (connexins), however there is emerging evidence that both proteins may be functionally regulated by intrinsic signaling processes. Previous studies indicate that stimulation of the β-adrenergic pathway increases CV, but little consistent data exists on the magnitude, associated adrenoreceptor pharmacology or time course of the effect. This study investigates the effect of β-AR stimulation – using either the β-agonist isoproterenol (ISO) or by directly raising cAMP via addition of Forskolin (Fsk) and/or 3-Isobutyl-1-methylxanthine (IBMX) - on ventricular CV in the intact rat heart. The aim was to measure the response of CV to β-AR stimulation and investigate the mechanisms behind this response. Action potential (AP) and intracellular Ca2+ measurements were also made to determine the effect of β-AR stimulation on cellular electrophysiology over the same time-course as the CV response to β-AR stimulation. Methods: Adult male Wistar rats (250-350g) were euthanized by cervical dislocation and excised hearts retrogradely perfused with modified Tyrode's solution. CV measurements were taken using a custom-built probe, consisting of bipolar stimulating and recording electrode pairs placed flat against the epicardium of the left ventricle (LV). The CV probe also incorporated a fibre-optic light guide, allowing ratiometric measurements of voltage and intracellular Ca2+ from the LV epicardium. Results and Conclusions: β-AR stimulation increased LV longitudinal CV by approximately 10%. This increase in CV was found to be cAMP mediated. This effect was not due to changes in Ca2+ handling alone and although an increase in AP amplitude (APA) suggested that INa was increased, the magnitude was thought insufficient to explain the change in CV. This suggested a potential role for gap junction conductance (GJC) in mediating CV changes. This view was supported by preliminary data indicating the magnitude of the response was larger when measuring transverse CV: transverse conduction involves proportionally more GJC than longitudinal conduction. β-AR stimulation was confirmed to increase CV, a response mediated via β1AR subtype, and which required an increase in cAMP: cAMP was increased by activation of adenylyl cyclase (AC) with forskolin (Fsk) or through inhibition of phosphodiesterases (PDEs) by IBMX. The increase in CV was shown to be mediated through the cAMP sensitive kinase, PKA; another cAMP target, Epac, appeared not have a role in this pathway. Understanding the regulation of CV by β-AR stimulation is crucial to understanding sympathetic regulation of the heart and may lead to further understanding of the interplay between downregulated β-AR signaling and arrhythmia generation in the diseased heart.