Stimulation of group III and IV muscle afferents reflexly decreases total pulmonary resistance in dogs.

Although previous investigations have shown that stimulating group III and IV afferents reflexly decreased transverse tension from the trachealis muscle, these measurements provided no functional information about airway caliber. We therefore electrically stimulated gracilis muscle afferents in paralyzed, chloralose anesthetized dogs while recording total pulmonary resistance breath by breath. In addition, we recorded compound action potentials to determine which afferents were stimulated by current intensities of 3, 5, 20, 70 and 200 times motor threshold. We found that stimulating (20 Hz) the nerves at 3 times threshold, a current intensity which activated only group I and II afferents, had no effect on total pulmonary resistance, whereas stimulating the nerves at 5, 20 and 70 times threshold, current intensities which activated group I, II and III afferents, significantly decreased this variable. Stimulating the nerves at 200 times threshold, a current intensity which activated group IV as well as group I, II and III afferents, decreased total pulmonary resistance significantly more than did stimulating the nerves at 5, 20 or 70 times threshold. In addition stimulating the nerves at 200 times threshold but at frequencies of 2 and 5 Hz significantly decreased total pulmonary resistance. The decrease in total pulmonary resistance evoked by electrically stimulating the nerves at 200 times threshold was unaffected by propranolol but was abolished by atropine methylnitrate. We conclude that stimulating group III and IV gracilis muscle afferents in dogs reflexly decreases total pulmonary resistance, an effect due to the withdrawal of a tonic cholinergic input to the airways.