Diverging respiratory effects of serotonin and nicotine in vagotomised cats prior to and after section of carotid sinus nerves.

Respiratory effects of intravenous serotonin and nicotine were investigated prior to and after bilateral neurotomy of the carotid sinus nerves (CSNs) in eight pentobarbitone/chloralose-anaesthetised, bilaterally vagotomised and superior laryngeal nerves-sectioned cats. Injection of 188 nmol kg(-1) serotonin (hydrogen oxalate salt, 50 microg x kg(-1)) prior to and after CSNs section induced an expiratory apnoea of, respectively, 7.9 +/- 1.25 s and 8.3 +/- 1.6 s duration (mean +/- S.E.M.) in, respectively, five and three of those cats. In all cats, the serotonin challenge produced a period of accelerated breathing (P < 0.05) both prior to and after section of CSNs. Injection of a 433 nmol nicotine bolus (hydrogen tartrate salt, 200 microg) increased tidal volume by 25 +/- 8% in cats with intact CSNs (P < 0.01), but decreased it by 13 + 10% (P < 0.05) after CSNs section. Nicotine, but not serotonin, transiently increased mean arterial blood pressure in our cats, which rise was delayed by CSNs cut. Results of this study indicate that the respiratory response to serotonin occurs beyond carotid body chemoreceptors in vagotomised cats, and suggest that the volume response to intravenous nicotine depends qualitatively on carotid body chemoreceptor input in this experimental model.