TRANSEPITHELIAL FLUID SHIFT GENERATED BY OSMOLARITY GRADIENTS IN UNSTIMULATED PERFUSED RAT SUBMANDIBULAR GLANDS

The effects of osmotic gradients on transepithelial water movements we re examined in unstimulated perfused submandibular glands of the rat. Osmotic gradients were applied transepithelially by adding sucrose to or removing it from the perfusate. An infusion of hypotonic perfusate shifted fluid from the interstitium to the lumen (luminal fluid shift) transiently, whereas an infusion of hypertonic perfusate shifted flui d from the lumen to the interstitium (interstitial fluid shift) transi ently. The amount of fluid shifted from lumen to interstitium increase d as the luminal fluid osmolarity was raised or as the perfusate osmol arity was reduced. Thus, fluid movements across the salivary epitheliu m were shown to be simply dependent on the osmolarity difference betwe en lumen and interstitium. To estimate the effective pore radius of th e epithelium, non-electrolyte solutions (urea, dimethylurea, diethylur ea, mannitol, sucrose and maltotriose) were also used as luminal solut ions. The results from non-electrolyte experiments showed that the eff ective pore radius of the passage for non-electrolytes was slightly la rger than 0.38 nm. Solutes smaller than mannitol were less effective i n opposing the interstitial fluid shift, and the value of effective po re radius in this report was similar to that of the secretory water pa thway that has been measured in solvent drag studies (0.4-0.45 nm). Th ese findings suggest that the passage for nonelectrolytes may be a wat er transport pathway in salivary epithelium.