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Department of Internal Medicine, University of California School of Medicine, Davis 95616, USA.

1. Activation of sympathetic visceral afferents during mesenteric ischaemia induces visceral pain and reflexly excites the cardiovascular system. The present study investigated the differential responses of ischaemically sensitive and insensitive sympathetic C fibre afferents to graded distension of the gastrointestinal tract. 2. Single-unit activity of C fibre afferents innervating the stomach, duodenum and jejunum was recorded from the right thoracic sympathetic chain of anaesthetized cats. Ischaemically sensitive and insensitive C fibre afferents were identified according to their response to 5-20 min of ischaemia. The functional characteristics of the stimulus-response relationships of afferents were determined by distension of a balloon placed in the corresponding segment of gastrointestinal tract. 3. The results show that ischaemically insensitive C fibre afferents had a lower threshold in response to distension (13 +/- 5 mmHg, n = 10). The discharge frequency of these afferents was saturated within a low pressure range of distension. However, ischaemically sensitive C fibre afferents had a high threshold (86 +/- 12 mmHg, n = 10) and a larger peak response to mechanical distension in the noxious range (60-180 mmHg). There were no differences between ischaemically sensitive and insensitive C fibre afferents with regard to their testing activity or responses to bradykinin (10 micrograms I.A.). 4. This study demonstrates that the gastrointestinal system is innervated by low and high threshold sympathetic C fibre afferents, the latter having the distinct ability to encode nociceptive information such as excessive distension and ischaemia.