Osmotic flow transients during acetylcholine stimulation in the perfused rat submandibular gland

Osmotic stress was applied to the perfused rat submandibular gland during steady-state fluid secretion. Alterations of perfusate osmolarity, by addition or withdrawal of sucrose or NaCl, caused transient changes in secretory rate during continuous stimulation with 1 microM acetylcholine (ACh). Hyposmotic perfusates transiently increased, and hyperosmotic perfusates transiently reduced, the secretory rate. The transients were attributed to changes in osmotic flow resulting from changes in the instantaneous transepithelial osmotic gradient. The time course of the change in interstitial osmolarity was determined by using a Cl- electrode to record the changes in interstitial Cl- concentration following a step change in perfusate Cl- concentration. From the calculated changes in interstitial osmolarity and the resulting changes in secretory rate, the osmotic water permeability of the secretory pathway was estimated to be greater than 15.0 +/- 1.2 microliter (mosmol 1-1)-1 min-1 (g wet weight)-1 (9.8 x 10(-6) +/- 0.8 x 10(-6) l atm-1s-1g-1). The transepithelial gradient required to sustain steady state, ACh-evoked secretion would therefore be less than 16 mosmol l-1 NaCl, which is consistent with previous micropuncture data indicating that the luminal fluid is approximately isosmotic.