The role of vagal pathway and NK1 and NK2 receptors in cardiovascular and respiratory effects of neurokinin A.

Neurokinin A (NKA) is a peptide neurotransmitter that participates in the regulation of breathing and the cardiovascular system. The purpose of the current study was to determine the cardiorespiratory pattern exerted by the systemic injection of NKA, to look at the contribution of neurokinin NK1 and NK2 receptors, and to establish the engagement of the vagal pathway in mediation of these responses. The effects of intravenous injections of NKA (50 μg/kg) were studied in anaesthetized, spontaneously breathing rats in the following experimental schemes: in neurally intact rats; and vagotomized at either midcervical or supranodosal level. Intravenous injections of NKA in the intact rats evoked sudden and short-lived increase in the respiratory rate concomitant with drop in tidal volume, followed by a prolonged depression, coupled with continuous augmentation of the tidal volume. Respiratory alterations were accompanied by transient tachycardia and prolonged hypotension. Midcervical vagotomy eliminated respiratory rate response and augmentation of tidal volume. Section of supranodosal vagi abrogated all respiratory reactions. NK2 receptor blockade abolished respiratory changes without affecting cardiovascular effects, whereas NK1 receptor blockade significantly reduced hypotension and increase in heart rate with no impact on the respiratory system. These results indicate that NKA induced changes in the breathing resulting from an excitation of the NK2 receptors on the vagal endings. A fall in blood pressure triggered by NKA occurs outside of the vagus nerve and is probably mediated via its direct action on vascular smooth muscles supplied with NK1 receptors.