Abstract
5-hydroxytryptamine (5-HT), injected intravenously (close to the right atrium), caused apnoea, bradycardia, hypotension and a reduction in the tidal volume in anaesthetized rats. The latency of the apnoea response was 0.61 seconds. 5-HT also produced similar effects when injected into the left ventricle (intra-arterial). The latency of the apnoea response was 0.33 s by this route. The apnoea and bradycardia evoked by intravenous and intra-arterial injections of 5-HT were abolished by bilateral vagotomy and reduced or abolished by vagal cooling. The blocking temperatures varied considerably in different animals, particularly with respect to the intravenous route of drug administration. Intravenous capsaicin caused apnoea, bradycardia and hypotension and these effects were blocked by vagal section or cooling. In contrast intra-arterial injections caused apnoea but no bradycardia and the apnoea was resistant to vagal section. The study shows that there are at least two populations of receptors; one which is chemosensitive to 5-HT and capsaicin and which is supplied by the pulmonary circulation, and one which is chemosensitive to 5-HT and which is accessible via the systemic circulation. The receptors and their vagal afferent connections may be non-myelinated since 5-HT and capsaicin are known to stimulate this type of ending in other species, and because respiratory responses were only blocked at low temperatures. In addition capsaicin also activates a non-vagal receptor, accessible from the systemic circulation, and which mediates apnoea but not bradycardia.
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Mitchell, H.W., Tomlin, J. & Ward, R.J. Reflex changes in respiration and heart rate evoked by intravenous and left ventricular injection of 5-HT and capsaicin in anaesthetized rats: a comparison of mechanisms. Lung 162, 153–163 (1984). https://doi.org/10.1007/BF02715643
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DOI: https://doi.org/10.1007/BF02715643