VASOACTIVE-INTESTINAL-PEPTIDE - ELECTROPHYSIOLOGIC ACTIVITY IN THE NEWBORN HEART

We performed intracardiac electrophysiologic studies of the effects of vasoactive intestinal peptide (VIP, 0.125 mu g/kg/min) on sinus and a trioventricular (AV) nodal function, intracardiac conduction, and myoc ardial refractoriness in two groups of neonatal dogs (aged 6-16 d). Gr oup I consisted of eight neonates in whom VIP was administered after b ilateral vagotomy and beta-blockade with propranolol. Group II consist ed of five neonates studied after vagotomy and propranolol, plus total chemical sympathectomy (6-hydroxydopamine). In both groups, VIP resul ted in a significant shortening of sinus cycle length. AV nodal conduc tion time, measured as the AH interval (the time from the onset of the atrial electogram to the onset of the His bundle electrogram in the H is electrode catheter) during atrial pacing, also shortened after VIP. His-Purkinje conduction time and atrial effective refractory periods were unchanged by VIP. In other experiments, the direct chronotropic e ffect of VIP was evaluated in five isolated neonatal canine hearts usi ng a modified Langendorff technique. In these hearts, the spontaneous cycle length decreased from 403 +/- 88 to 293 +/- 69 ms, or -28 +/- 4% (mean +/- SD), after exposure to 0.1-0.5 nmol of VIP (p < 0.001). In nine other newborns (aged 4-16 days), the effect of selective (alpha(1 )- and alpha(2)-adrenergic receptor blockade on the positive chronotro pic effect of VIP was evaluated. The effect of VIP on sinus cycle leng th was not altered by the alpha(1)-adrenergic receptor blocker prazosi n or by the (alpha(2)-adrenergic receptor blocker yohimbine. These dat a indicate that, in the neonatal dog, VIP shortens sinus cycle length and AV nodal conduction time but has no effect on infra-nodal conducti on or myocardial refractoriness. We find no evidence that VIP's chrono tropic effect is modified by alpha-adrenergic receptor blockade. VIP m ay play a role in the neural modulation of heart rate and AV nodal con duction in the neonate.