Transepithelial transport of nonelectrolytes in the rabbit mandibular salivary gland

The characteristics of nonelectrolyte secretion by the rabbit mandibular salivary gland have been investigated in an in vitro, perfused preparation. The concentrations of 14C-labeled nonelectrolytes were measured in saliva samples collected over a range of flow rates during the secretory response of the gland to continuous acetylcholine infusion. Of the nine nonelectrolytes studied, the two particularly lipid-soluble molecules, ethanol and antipyrine, appeared in the saliva at approximately the same concentration as in the perfusate, regardless of the secretory flow rate. The more polar molecules (urea, ethanediol, thiourea, glycerol, erythritol, mannitol and sucrose) appeared at saliva/perfusate concentration ratios (phi) which showed a strong dependence on flow. With the exception of thiourea, this could be attributed to the combined contributions of diffusion and solvent drag. For the polar nonelectrolytes, estimates have been obtained of both the permeability coefficients of the gland (P) and the solvent-drag filtration coefficients (1 - sigma). The relation between 1 - sigma and molecular radius suggests that small polar nonelectrolytes and the bulk of the secreted water cross the epithelium via aqueous channels that are approximately 0.8 nm in width. The location of the channels remains uncertain because tissue space measurements indicate that the nonelectrolytes most affected by solvent drag have access to both transcellular and paracellular pathways.