Thromboxane A2 mimetic, U46,619, and slowly adapting stretch receptor activity in the rabbit

Pharmacology of airway afferent nerve activity

Afferent nerves in the airways serve to regulate breathing pattern, cough, and airway autonomic neural tone. Pharmacologic agents that influence afferent nerve activity can be subclassified into compounds that modulate activity by indirect means (e.g. bronchial smooth muscle spasmogens) and those that act directly on the nerves. Directly acting agents affect afferent nerve activity by interacting with various ion channels and receptors within the membrane of the afferent terminals. Whether by direct or indirect means, most compounds that enter the airspace will modify afferent nerve activity, and through this action alter airway physiology.

Ipratropium bromide protects against bronchoconstriction during bronchoscopy

Pulmonary function is reportedly impaired by fiberoptic bronchoscopy. We investigated the effect of two anticholinergic agents, intramuscular atropine and inhaled ipratropium bromide, on bronchoconstriction in 29 patients who were undergoing diagnostic bronchoscopy. The patients were divided into three groups; the first received 0.5 mg of atropine intramuscularly; the second took four puffs of 0.02 mg ipratropium bromide aerosolized by a metered-dose inhaler, and the third inhaled four puffs of a placebo. Fifteen minutes later a standardized topical anesthetic, lidocaine, was administered, and a bronchoscopic examination was performed. Pulmonary function was measured before and 15 minutes after each step. Pulmonary function was not affected by the treatment with anticholinergics or the placebo. In the placebo and the atropine groups, the topical anesthesia produced significant reductions in forced expiratory volume in 1 second (FEV1) and peak expiratory flow rate (PEFR); further reductions in these values were observed after bronchoscopy. In the group treated with ipratropium bromide there were no significant changes in FEV1 and PEFR after topical anesthesia. Bronchoscopy induced significant reductions in FEV1 and PEFR, but the changes were significantly smaller than those seen in the placebo and atropine groups. The results suggest that the deleterious effect of bronchoscopy on pulmonary function is due to topical lidocaine anesthesia and to the bronchoscopic examination itself. Inhaled ipratropium bromide protects against these deleterious effects, whereas intramuscular atropine does not.

Effects of the thromboxane A2 mimetic, U46,619, on pulmonary vagal afferents in the cat

Release of thromboxane A2 (TxA2) or infusion of the TxA2 mimetic U46,619 in the cat elicits pulmonary hypertension and rapid shallow breathing (Shams et al., Respir. Physiol. 71: 169-183, 1988). The vagus nerve mediates the observed respiratory, but not the circulatory, effects (Shams and Scheid, J. Appl. Physiol. 68: 2042-2046, 1990). To identify the type of lung vagal afferent fibers involved in this respiratory response to TxA2, we have recorded the functional single-unit activity and its response to infusion of U46,619 in fine strands of the vagus nerve in the artificially ventilated cat and rabbit. The fibers were classified as originating from slowly adapting (SAR) or rapidly adapting (RAR) stretch receptors by their response to sustained pulmonary inflation (intrapulmonary pressure of 20-25 cmH2O) or as C-fibers, by their response to a bolus injection of phenylbiguanide. C-fibers responded variably to lung inflation. U46,619 infusion caused only a small increase in SAR or RAR activity along with increases in end-inspiratory tracheal airway pressure (Paw), but evoked a marked increase in the firing rate of C-fibers, independent of their response to lung inflation. This increase in C-fiber activity was unrelated to the increase in Paw, which accompanied the infusion of U46,619. Since these responses remained the same after indomethacin they appear to be due to a direct action of U46,619, and not to be mediated by prostanoids that might be released by U46,619. These data suggest that C-fibers are indeed involved in the respiratory effects of TxA2. Since the effects exerted on C-fibers by U46,619 were unrelated to increased Paw, TxA2 is likely to stimulate the nerve endings directly, rather than via smooth muscle contraction. On the other hand, the small stimulating effect of U46,619 on SAR and RAR may be mediated by bronchoconstriction.