SUBMUCOSAL PLEXUS AND ELECTROLYTE TRANSPORT ACROSS RAT COLONIC MUCOSA

Electrolyte transport across 2 preparations of mucosa from rat colon descendens was compared to determine what influence the submucosal plexus has on electrolyte transport. One preparation consisted of the mucosa, muscularis mucosae and the submucosal tissue and is referred to as the mucosa-submucosa preparation. The 2nd preparation obtained by further blunt dissection of the mucosa-submucosa preparation consisted of only the mucosa and the circular muscle layer of muscularis mucosae and is referred to as the mucosa preparation. Histological studies showed that the submucosal tissue and the longitudinal layer of muscularis mucosae could be removed leaving only the mucosa and the circular layer of muscularis mucosae. The extensive neuronal network of the submucosa was shown when the submucosal tissue and longitudinal muscle layer of muscularis mucosae, which were removed, were stained histochemically for acetylcholinesterase activity. Both preparations absorbed Na+ and Cl- when short-circuited. Na+ and Cl- absorption were significantly higher in the mucosa preparation. The increase in Na+ and Cl- transport in the mucosa preparation was accompanied with a decrease in the short-circuit current (Isc), the open-circuit potential difference (p.d.) and the transmural tissue conductance (Gt) when compared to the mucosa-submucosa preparation. Tetrodotoxin (TTX) dose-dependently decreased Isc and p.d. in the mucosa-submucosa preparation when added to the serosal solution. The 1/2 maximal effective concentration of TTX was 5 nM and maximal effective concentration 100 nM. TTX (1 .mu.M) had no effect on Isc or p.d. when added to the mucosal solution. The decrease in Isc and p.d. caused by TTX in the mucosa-submucosa preparation was accompanied with an increase in Na+ and Cl- absorption. TTX caused only a small decrease in Isc and p.d. in the mucosa preparation. There was no measurable change in Na+ and Cl- transport in the mucosa preparation. Spontaneously active neurons from the submucosal plexus evidently have an inhibitory influence on the mucosa. Physical removal of the submucosal plexus or pharmacological blockade of the neurons within the mucosa-submucosa preparation by TTX led to enhanced absorption, suggesting that the set point of the mucosa for electrolyte transport is at or near a maximal absorptive state. Regulation or modulation of the mucosa may therefore occur by mechanisms that lower this set point, causing an inhibition of absorption of electrolytes.