Extracellular Diadenosine Tetraphosphate Suppresses Ectopic Proarrhythmicity in the Myocardial Tissue of the Pulmonary Veins in Adult but not in Neonatal Rats

Diadenosine tetraphosphate (Ap4A) belongs to a group of endogenous purine compounds that have recently been considered as new neurotransmitters or cotransmitters in the autonomic nervous system. It has been shown that Ap4A affects electrophysiology of a pacemaker and working myocardium and modulates adrenergic control of the heart in adult mammals. Nevertheless, the physiological role of Ap4A in the regulation of bioelectric properties in the pulmonary vein (PV) myocardium has not yet been investigated. It is well known that myocardial tissue in the wall of the PV acts as source of the ectopic proarrhythmic activity that underlies supraventricular arrhythmias like atrial fibrillation. The aim of the present study was to elucidate the effects Ap4A on bioelectrical properties and proarrhythmic ectopy in PVs in adult rats and at early postnatal ontogenesis. Thus, the resting potentials and the electrically evoked and spontaneous action potentials were recorded with the use of the standard microelectrode technique in multicellular isolated PV specimens from male Wistar rats at postnatal days 1-, 7-, 14-, and 21- and also from 60-day-old animals, which were considered as mature. The application of Ap4A caused significant reduction of action potential duration in PV specimens from rats of all ages. Ap4A also caused significant resting membrane potential hyperpolar-ization in quiescent PVs specimens from 14-, 21-, and 60-day-old rats. In addition, Ap4A caused complete and significant suppression of ectopic automaticity caused by preliminary noradrenaline administration in PV from 21- and 60-day-old rats, but Ap4A was unable to alter spontaneous intrinsic activity in PV from neo-nate (1-day-old) rats. The Ap4A-caused attenuation of noradrenaline-induced ectopy in PV was accompanied by substantial resting membrane potential hyperpolarization in all cases. Our results allow suggesting that the release of Ap4A as a cotransmitter from autonomic nerve endings can reduce proarrhythmic ectopy caused by sympathetic stimulation of the PV myocardium in vivo.