Action of Lobeline on Pulmonary Artery Mechanoreceptors of the Cat

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Abstract
1. The effects of lobeline (15 to 45 µg/kg) and of local changes of pulmonary artery pressure on the afferent activity of single neurone preparations of pulmonary artery mechanoreceptors have been studied in the intact cat. Pulmonary artery pressure was monitored via a double lumen balloon catheter passed into the pulmonary artery. 2. Pulmonary artery mechanoreceptors were distinguished from systemic mechanoreceptors by applying two simple tests to all mechanoreceptors found by random selection of single fibers from the right cervical vagus. The afferent activity of pulmonary mechanoreceptors closely followed induced changes of pulmonary artery pressure, whereas other arterial mechanoreceptors did not. Pulmonary artery pressure was increased by the rapid injection of 2 ml dextran into the right atrium and was reduced by inflation of the intravascular balloon. 3. In the absence of a rise in pulmonary artery pressure, lobeline increased the afferent activity in two phases. The early response, which was seen in 17 of the 18 fibers studied, occurred within one to two seconds after injection and was characteristically a variable and moderate response. This effect is caused probably by the diffusion of lobeline to sensory endings within the pulmonary artery wall. The delayed response occurred about eight to ten seconds later. The activity of afferent fibers was intensified conspicuously and frequencies up to 300/sec were recorded. It is probable that this is due to the action of lobeline carried by the vasa vasorum to the intramural receptors. 4. By combining the results following balloon inflation and dextran injection, the relationship between impulses per heart beat and mean pulmonary artery pressure was established for a number of pulmonary artery mechanoreceptors in the intact animal. 5. Pulmonary artery mechanoreceptors were insensitive to doses of veratridine and relatively insensitive to doses of phenyldiguanide which, in the intact cat, produce equivalent reflexogenic responses as lobeline (45 µg/kg). 6. Other intrathoracic sensory endings studied, including atrial and ventricular stretch receptors, Hering-Breuer receptors, and systemic mechanoreceptors, were completely insensitive to the doses of lobeline used in these experiments. 7. These results support earlier experiments which suggested that when lobeline is injected, its reflex effect from the pulmonary vascular bed, namely depression of ventilation, is due to its action on a pressure- or stretch-sensitive area in the region of the bifurcation of the pulmonary artery.