ABSENCE OF VISCEROSOMATIC INHIBITION WITH INJECTIONS OF LOBELINE DESIGNED TO ACTIVATE HUMAN PULMONARY C-FIBERS

1. Activation of pulmonary C fibres (J receptors) in animals produces inhibition of spinal motoneurones. Intravenous bolus injections of lob eline are believed to activate pulmonary C fibres (J receptors) in hum an subjects and to produce characteristic sensations and cardiorespira tory responses. This study quantified the respiratory sensations evoke d by such injections and then used a range of suprathreshold doses of lobeline and tested for the presence of reflex or descending inhibitio n of motoneuronal output. 2. Injections of lobeline produced dose-depe ndent sensations of respiratory discomfort referred to the throat and upper chest beginning within about 10 s and often associated with coug hing. As the dose increased the latency for the sensations decreased w hile their duration and intensity increased. Reflex changes in blood p ressure, heart rate and ventilation also occurred. 3. Injections of lo beline at doses sufficient to evoke respiratory discomfort lasting 25- 32 s (37-73 mu g kg(-1)) increased the size of the H reflex in soleus with an onset latency of about 10 s and lasting about 20 s. 4. The siz e of EMG responses evoked in upper limb muscles by transcranial magnet ic stimulation of the motor cortex increased shortly after injections and remained elevated for about 30-35 s. 5. Injections of lobeline dur ing sustained voluntary contractions of the elbow flexors at submaxima l or maximal levels did not impair the ability to produce force. 6. Wa lking was not disrupted by repeated suprathreshold doses of lobeline. 7. It is concluded that injections of lobeline sufficient to evoke car diorespiratory reflexes and sensations of severe respiratory discomfor t are not associated with functionally important inhibition of motor p erformance. The results cast doubt on the ability of the J reflex to l imit exercise in humans.