Adenosine and hypoxia effects on atrioventricular node of adult and neonatal rabbit hearts

An isolated perfused heart model was used to assess the effects of hypoxia and adenosine on the adult and neonatal (1-5 days) rabbit atrioventricular (AV) node. The AV nodal function was assessed by the A-H interval at a constant atrial pacing cycle length and by the longest pacing cycle length resulting in Wenckebach periodicity. We defined the pacing cycle length at or below which the AV node demonstrated Wenckebach periodicity as the Wenckebach cycle length. Adenosine produced a smaller dose-dependent increase in A-H interval in neonates than in adults, but the increase in Wenckebach cycle length was similar in the two age groups. When the hearts were exposed to 5 min of hypoxia the increase of Wenckebach cycle length was greater for adults than for neonates. The change in Wenckebach cycle length in adults caused by hypoxia was significantly greater than that caused by 1 mM adenosine. In addition, in adults aminophylline could partially attenuate the increase in Wenckebach periodicity caused by adenosine, but aminophylline could not attenuate the increase in Wenckebach cycle length caused by hypoxia. We conclude that in the rabbit AV node 1) the adenosine effect in neonates is similar to that in adults; 2) neonates are relatively resistant to acute hypoxia compared with adults; and 3) the response to acute hypoxia in adults cannot be totally explained by the adenosine release theory.