Effects of vasoactive intestinal polypeptide on heart rate in relation to vagal cardioacceleration in conscious dogs

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Abstract
Objective: The vagal cardiac accelerator (VCA) system takes part in the nervous control of the heart rate. In the present study we tried to adduce evidence that vasoactive intestinal polypeptide (VIP) contributes to vagally induced cardioacceleration. Methods: The effect of VIP on heart rate and arterial blood pressure was investigated after unmasking the inherent VCA activity by blocking the sympathetic accelerator and vagal decelerator influences on heart rate in conscious dogs. Results: Following intravenous administration of VIP (10 μg i.v.) the heart rate increased by 43.6±6.7 (28.1±4.7%), from 165.6±8.5 to 209.1±7.0 beats/min (PPn=11). After VCA activity was reflexly enhanced by α1-adrenoceptor stimulation with methoxamine, VIP increased heart rate by 36.9±7.3 (21.5±4.6%), from 179.8±5.2 to 216.7±5.8 beats/min (PPPr=−0.744, P=0.009) and after methoxamine (r=−0.689, P=0.019). The VIP-induced tachycardia is certainly not reflexly induced by the fall in arterial pressure, because intracoronary administration of VIP (0.5 μg i.c.) caused an appreciable increase in the heart rate by 63.7±13.0 (46.4±10.4%), from 143.0±8.1 to 208.7±12.0 beats/min (PPn=6). In addition, VIP (10 μg i.v.) also caused a tachycardia in vagotomized dogs with blocked β-adrenergic and muscarinic receptors. The administration of the VIP antagonists [d-p-Cl-Phe6,Leu17]-VIP (50–150 μg i.c.) and [Lys1,Pro2,5,Leu17]-VIP (20 μg i.c.) did not result in alterations in VCA activity nor did the VIP antagonists block the VCA reflex response to a rise in arterial pressure. However, none of the VIP antagonists reduced the VIP-induced tachycardia either. Conclusion: Vasoactive intestinal polypeptide is likely to play a part in the vagal cardiac accelerator system. However, conclusive evidence for its role as the terminal transmitter in the VCA pathway will have to wait for the availability of a specific cardiac VIP receptor antagonist.

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