CHEMOSENSITIVITY OF NOCICEPTIVE, MECHANOSENSITIVE AFFERENT NERVE-FIBERS IN THE GUINEA-PIG URETER

The mechanosensitivity and chemosensitivity of afferent fibres were in vestigated in an in vitro preparation of the guinea-pig ureter. Electr ophysiological recordings were obtained from 5 U-1 flow mechanical thr eshold, contraction-sensitive) and 74 U-2 units (high threshold). U-2 units had significant higher levels of spontaneous activity, lower con duction velocities, higher mechanical thresholds (U-1: 7 mmHg; U-2: 39 mmHg), less pronounced phasic responses and longer latencies in the r esponse to distensions than the U-l units. For chemical stimulation, g uinea-pig urine (> 800 mosmol/L), bradykinin and capsaicin were applie d intraluminally, The responses of U-l units mainly corresponded io th e contractions induced by the chemical stimulation. The vast majority of the U-2 units were excited by urine, bradykinin (threshold: 0.1-1 m u M) and capsaicin (threshold: 0.03-0.3 mu M). The responses to urine could be mimicked by high concentrations of potassium ions (> 200 mM), but not by an equiosmolar solution of NaCl, urea and mannitol. Chemic al stimulation could also result in a transient sensitization of the U -2 units to mechanical stimuli. In the anaesthetized guinea-pig, pseud o-affective responses could be evoked by ureteric distension (threshol d: 30-60 mmHg) and serosal application of capsaicin. Intraluminal appl ication of urine in vivo did not evoke any reactions, suggesting that the responses of the U-2 units to urine might be due to an impaired ba rrier function of the urothelium in vitro. The data are in agreement w ith the hypothesis that U-2 units are Visceral polymodal nociceptors, Since the U-1 units were also able to encode at least noxious mechanic al stimuli, their involvement in visceral nociception cannot be exclud ed.