Mucosal plexus and electrolyte transport across the rat colonic mucosa.

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Abstract
1. Histological and functional studies were performed on a preparation of rat colonic mucosa from which the myenteric and submucosal plexus were removed. This preparation, referred to as the mucosa preparation, was used to investigate the potential influence of the mucosal plexus on electrolyte transport. Two neuropharmacologically active agents were used: sea anemone toxin (ATX II) to stimulate the fibres of the mucosal plexus and tetrodotoxin (TTX) to block the fibres of the mucosal plexus. 2. The morphology of the neuronal network of the mucosal plexus was visualized after the epithelium was removed and whole mount preparations of the lamina propria and circular muscle layer of muscularis mucosae were stained histochemically for acetylcholinesterase activity. Several levels of organization within the mucosal plexus were seen. Each crypt is encircled by a thin bundle of fibres near the top. These thin fibres connect with thicker bundles of fibres that encircle groups of two to five crypts in a broad band. These bundles of fibres are in turn connected to larger bundles of fibres which lie in a flat plane just below the crypts along the circular muscle layer of muscularis mucosae. In addition perikarya and ganglia were revealed within the mucosal plexus. 3. The base-line net transport of Na+ and Cl- across the mucosa preparation was completely inhibited by ATX II (10-6 M). This effect of ATX II on net Na+ and Cl- transport was accompanied with an increase in the short-circuit current (Isc), transmural conductance, and open-circuit potential difference across the mucosa preparation. The effect of ATX II on Isc was dose dependent with a half-maximal effective concentration at 5 .times. 10-8 M-ATX II and a maximal effective concentration of 10-7 M. ATX II was effective only when added to the serosal solution. 4. Net Na+ and Cl- transport was restored by TTX (10-6 M) to base-line values in ATX II-treated tissue. In addition the value of all three electrical parameters rapidly returned to the values measured before the addition of ATX II. TTX was effective in antagonizing the effects of ATX II only when added to the serosal solution. 5. The results suggest that the regulation of electrolyte transport across the epithelium is at least one function of the mucosal plexus. Stimulation of the neurones within the mucosal plexus leads to the inhibition of electrolyte absorption. The neurones of the mucosal plexus appear to be spontaneously inactive and must be continuously stimulated in order to inhibit electrolyte absorption. This stimulation may be achieved with neuro-pharmacologically active agents or physiologically by the spontaneously active neurones extending from the submucosal plexus. Once this inhibitory influence is blocked the epithelium quickly returns to a state at or near maximal absorption suggesting that this absorptive state is the functional intrinsic set point of the epithelium.