PULMONARY SEROTONIN 5-HT3-SENSITIVE AFFERENT-FIBERS MODULATE RENAL SYMPATHETIC-NERVE ACTIVITY IN RATS

Cardiopulmonary reflexes with vagal afferents may control volume homeo stasis by influencing renal nerve activity. Such reflexes can be stimu lated mechanically and chemically, e.g., by serotonin (5-HT). We have demonstrated that stimulation of epicardial 5-HT3 receptors inhibits r enal sympathetic nerve activity (RSNA) by a cardiorenal reflex. We now tested the hypothesis that pulmonary 5-HT3-sensitive vagal afferent f ibers participate in the control of renal nerve activity. Two sets of experiments were performed. First, the responses of multifiber RSNA, h eart rate (HR), and blood pressure (BP) to the 5-HT3-receptor agonist phenylbiguanide (PEG; 10 mu g iv) were recorded in the presence of int act pulmonary afferents. Abdominal afferents were removed by subdiaphr agmatic vagotomy. Cardiac afferents were blocked by intrapericardial i njection of 10% procaine. Second, the responses of 25 single vagal pul monary afferent C fibers to PEG were assessed. PEG decreased BP, HR, a nd RSNA (-90 +/- 8%). When cardiac afferents were blocked by procaine, BP and HR failed to decrease in response to PEG; however, the RSNA de crease was still -48 +/- 8%. Single fibers generally responded to PEG by a slight increase in firing rate. A distinct subset of fibers (5 of 25) showed an activity increase of >15 Hz that preceded changes in BP and HR. The decreased RSNA in the absence of cardiac and abdominal va gal afferents and the strong response of 20% of pulmonary single fiber s to intravenous PEG suggest that pulmonary fibers play a role in a 5- HT3 serotenergic reflex. Thus pulmonary serotonin could influence the neural control of renal function.