OSMOTIC FLOW TRANSIENTS DURING ACETYLCHOLINE STIMULATION IN THE PERFUSED RAT SUBMANDIBULAR-GLAND

Osmotic stress was applied to the perfused rat submandibular gland dur ing steady-stale fluid secretion. Alterations of perfusate osmolarity, by addition or withdrawal of sucrose or NaCl, caused transient change s in secretory rate during continuous stimulation with 1 mu M acetylch oline (ACh). Hyposmotic perfusates transiently increased, and hyperosm otic perfusates transiently reduced, the secretory rate. The transient s were attributed to changes in osmotic flow resulting from changes in the instantaneous transepithelial osmotic gradient. The time course o f the change in interstitial osmolarity was determined by using a Cl- electrode to record the changes in interstitial Cl- concentration foll owing a step change in perfusate Cl- concentration. From the calculate d changes in interstitial osmolarity and the resulting changes in secr etory rate, the osmotic water permeability of the secretory pathway wa s estimated to be greater than 15.0 +/- 1.2 mu l (mosmol l(-1))(-1) mi n(-1) (g wet weight)(-1) (9.8 x 10(-6) +/- 0.8 x 10(-6) l atm(-1) s(-1 ) g(-1)). The transepithelial gradient required to sustain steady stat e, ACh-evoked secretion would therefore be less than 16 mosmol l(-1) N aCl, which is consistent with previous micropuncture data indicating t hat the luminal fluid is approximately isosmotic.