A submucosal mechanism for catecholamine-induced increases in fluid absorption in rabbit ileum in vitro.

1. The effects of clonidine and dopamine on water movements across the mucosal and serosal surfaces of rabbit ileum have been investigated using a high-resolution method for monitoring water flows in vitro. 2. Theophylline (10 mM) and carbamyl choline (10 microM) caused a reduction in fluid inflow across the mucosal surface and a smaller decrease in fluid outflow across the serosal surface. Addition of the alpha 2-adrenergic agonist clonidine or dopamine fully reversed the theophylline, or carbamyl choline-induced decrease in mucosal inflow in a dose-related manner. 3. The effects of clonidine on mucosal inflow are blocked by the alpha 2-adrenergic antagonist, yohimbine. Yohimbine was much less effective than pimozide or d-butaclamol in blocking the effect of dopamine on mucosal inflow. These findings support the view that there are separate alpha 2-adrenergic and dopaminergic receptors. 4. The hydraulic conductance (Lp) of the serosal surface was measured directly from the change in serosal exit flow following addition of 2 mosmol kg-1 of polyethylene glycol (molecular mass 20,000 Da) to the serosal bathing solution. Theophylline reduced the Lp by 35%. Clonidine (1 microM) added to theophylline-treated tissues increased the Lp by 66%. This effect was prevented by yohimbine (1 microM). 5. The effects of theophylline, clonidine and dopamine on the permeability of the mucosal and serosal surfaces of the tissue to [3H]mannitol were measured. These showed that theophylline increased the rate of labelled mannitol loss across the mucosal surface but reduced the mannitol permeability across the serosal surface. This latter effect was reversed by clonidine and dopamine. 6. Changes in transepithelial electrical potential difference (PD), short-circuit current and resistance were monitored. Theophylline caused a rapid increase in PD and short-circuit current and a slower increase in resistance. Clonidine (5 microM) reversed the effects on PD and resistance but was without significant effect on short-circuit current. The results suggest that a major component of secretagogue-induced reduction in fluid transport in vitro is due to mechanical changes in the submucosa, probably induced by modulation of neurotransmitter release within the tissue.