Interaction between sympathetic nerve fibres and epithelial transport in the rat jejunal mucosa in vivo

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Abstract
The aim of this study was to elucidate the mechanisms behind the absorptive response to mesenteric nerve stimulation (MNS). Segments of rat jejunum were mounted with intact blood circulation in an ''in-vivo chamber'', allowing parallel registration of net fluid transport rate (NFT) and electrical parameters of the tissue. All segments secreted fluid spontaneously, and the secretion rate was inversely correlated to the corresponding potential difference (PD) across the segment. The secretion was not inhibited by atropine. The nicotinic antagonist hexamethonium (Hx) inhibited the secretion and changed the relationship between PD and NFT. MNS reversed the secretion into an absorption (absorptive response) and decreased PD and short-circuit current (SCC) (electrogenic response). Total tissue resistance (Rt) was not significantly changed. The absorptive response increased at high control secretion rates and the electrogenic response increased at high control PD lvels. The electrogenic response was inversely related to the absorptive response; however, the presence of the Hx reversed this correlation. Guanethidine, a sympatholytic agent, eliminated all effects of MNS. Phentolamine eliminated the absorptive response but not the electrogenic response, and the remaining electrogenic response was not sensitive to propranolol (a .beta.-adrenergic antagonist). Hx induced similar but smaller effects than MNS, and after Hx treatment the MNS effects were reduced. We conclude: (1) Fluid secretion in our model involves a mechanism other than electrogenic Cl- secretion. (2) MNS activates sympathetic nerve fibres, which enhance fluid absorption and decrease PD and SCC. (3) The absorptive and electrogenic responses involve partly separate transport mechanisms. (4) Both responses are partly due to the inhibition of intramural secretory nervous pathways. (5) Some sympathetic nerve fibres interact directly with the epithelium.

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