An Investigation into the Presence of a Vagal Tachycardia and the Effect of Vasoactive Intestinal Polypeptide on Rat Heart Rate in vitro

The presence of the vagal tachycardia and the effect of vasoactive intestinal polypeptide in the isolated innervated rat atrium were investigated. The right vagus, or cardiac branch, were stimulated at 4, 8, 16 and 32 Hz, pulse duration 1 ms, 20 V, 30 s before atropine and for 1 min after atropine (3 µmol/l), experiments were carried out in the presence of atenolol (4 µmol/l). No significant vagal tachycardia was observed in the presence of atropine, the greatest increase in heart rate was at 16 Hz which was 3±1 beats/min (n = 12 rats) (p = 0.052). Baseline heart rates for the control, 226±11 beats/min (n = 12 rats) and atropine experiments, 210±8 beats/min (n = 12 rats), were not significantly different (p = 0.24). VIP (0.06, 0.12, 0.24 µmol/l) caused a maximum increase of 27±13 beats/min (n = 5 rats) after 6 µmol/l VIP which was not significant, two higher concentrations of VIP failed to increase heart rate further. These results show that the vagal tachycardia is not present and that VIP does not cause a significant tachycardia in the rat.