Neural control of electrogenic transport in the rat jejunum; interaction between intramural and adrenergic mechanisms

The aim of the study was to test the hypothesis that extrinsic adrenergic neurons modulate electrogenic ion transport in the jejunum by inhibition of intramural secretory nerve activity. Isolated pieces of rat jejunum were mounted in the Ussing chamber. Noradrenaline (NA) was released from sympathetic nerve endings in the tissue by electrical stimulation of the mesenteric nerves or by the indirect sympathomimetic agent tyramine. Intramural neurons were activated by electrical field stimulation (EFS). Mesenteric nerve stimulation induced a frequency-related decrease in the transepithelial potential difference (PD) and short circuit current (SCC), effects which were abolished by the alpha-adrenergic antagonist phentolamine. A similar response was induced by the serosal application of tyramine and by exogenous NA. Tetrodotoxin (TTX), a nerve conduction inhibitor, had no significant effect on the response to NA, but attenuated the response to tyramine. Both NA and tyramine markedly attenuated the secretory response to EFS, but there was no significant effect of catecholamines on the secretory response to serosal theophylline. The results suggest at least two mechanisms behind the adrenergic modulation of electrogenic chloride secretion: (I) a ganglionic or presynaptic modulation of secretory nerve activity via innervated alpha-receptors, and (2) a modulation at the level of the enterocytes via non-innervated alpha-receptors. The latter effect does not seem to be mediated by the cAMP system.