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Lowering of interstitial fluid pressure in rat submandibular gland: a novel mechanism in saliva secretion

Section for Physiology, Department of Biomedicine, University of Bergen, Bergen, Norway

Submitted 19 August 2005 ; accepted in final form 3 November 2005

The submandibular gland transports fluid at a high rate through the interstitial space during salivation, but the exact level of all forces governing transcapillary fluid transport has not been established. In this study, our aim was to measure the relation between interstitial fluid volume (V_i) and interstitial fluid pressure (P_if) in salivary glands during active secretion and after systemically induced passive changes in gland hydration. We tested whether interstitial fluid could be isolated by tissue centrifugation to enable measurement of interstitial fluid colloid osmotic pressure. During control conditions, V_i averaged 0.23 ml/g wet wt (SD 0.014), with a corresponding mean P_if measured with micropipettes of 3.0 mmHg (SD 1.3). After induction of secretion by pilocarpine, P_if dropped by 3.8 mmHg (SD 1.5) whereas V_i was unchanged. During dehydration and overhydration of up to 20% increase of V_i above control, a linear relation was found between volume and pressure, resulting in a compliance (V_i/P_if) of 0.012 ml·g wet wt^–1·mmHg^–1. Interstitial fluid was isolated, and interstitial fluid colloid osmotic pressure averaged 10.4 mmHg (SD 1.2), which is 64% of the corresponding level in plasma. We conclude that P_if drops during secretion and, thereby, increases the net transcapillary pressure gradient, a condition that favors fluid filtration and increases the amount of fluid available for secretion. The reduction in P_if is most likely induced by contraction of myoepithelial cells and suggests an active and new role for these cells in salivary secretion. The relatively low interstitial compliance of the organ will enhance the effect of the myoepithelial cells on P_if during reduced V_i.

interstitial fluid volume; micropuncture; pilocarpine