Effects of acetylcholine and forskolin on the non-electrolyte permeability of the perfused rabbit mandibular gland

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Abstract
Previous studies have suggested that the permeability of exocrine glands to non-electrolytes may change according to the nature and intensity of the stimuli evoking secretion. The purpose of this study was to define the nature of these permeability changes using a method that distinguishes diffusion from solvent drag. Isolated rabbit mandibular salivary glands were perfused with solutions containing14C-labelled non-electrolytes and stimulated with acetylcholine. Diffusive permeability coefficients (P) and solvent-drag filtration coefficients (1-σ) were estimated from the relationship between salivary non-electrolyte concentration and salivary flow rate. Filtration coefficients for urea, ethanediol, glycerol, erythritol and sucrose increased with acetylcholine concentration while, with the exception of urea, the diffusive permeabilities remained virtually unchanged. The effect of increasing acetylcholine concentration can best be explained by postulating an increase in the effective channel radius of the water secretion pathway from 0.40 nm to 0.45 nm together with a small increase in the fraction of the total water flow passing through larger non-selective pores. Forskolin had little effect on either of the permeability parameters except for a small increase in the diffusive permeability to ethanediol.