How does lobeline injected intravenously produce a cough?

In order to examine, whether the lobeline-induced cough is a true reflex or a voluntary effort to get rid of its irritating sensations in the upper respiratory tract, we systematically studied the cough response to lobeline, of subjects who were unable to make conscious discriminations i.e. were either comatose (n = 4) or anaesthetized (n = 5). 8 μg/kg lobeline injected into the right atrium of one and 29 μg/kg intravenously (i.v.) into another evenly and spontaneously breathing comatose subject produced a cough after 4 s and 12 s, respectively. Cough was repeatable and showed a dose response relationship i.e., its latency decreasing and its duration/intensity increasing with the dose. In a third subject, capable only of weak spontaneous respiration, a relatively high dose injected into the right atrium (44 μg/kg) generated a pronounced cough-like respiratory movement superimposed on the artificial ventilation and also during the apnoea after disconnecting the pump. No respiratory response was evoked in a fourth subject who had no evidence of brainstem reflexes. In five normals, cough was elicited with a mean dose of 35 ± 5 μg/kg i.v. (latency 14 ± 2 s; duration 10 ± 3 s). After thiopental anaesthesia, injecting 41 ± 7 μg/kg produced a cough within 13 ± 2 s that lasted for 12 ± 2 s. It may be noted that neither the later dose nor the latency or duration of cough that it produced were significantly different from the pre anaesthesia values (P > 0.05). These two sets of results show unequivocally that the lobeline-induced cough is evoked reflexely; its magnitude in the conscious state could vary by subjective influences. We discuss the likelihood of its origin from juxtapulmonary capillary receptors.