Capillary tone: cyclooxygenase, shear stress, luminal glycocalyx, and hydraulic conductivity ( L p )

Control of capillary hydraulic conductivity ( L p ) is the physiological mechanism that underpins systemic hydration. Capillaries form the largest surface of endothelial cells in any species with a cardiovascular system and all capillaries are exposed to the flow‐induced force, shear stress ( τ ). Vasoactive molecules such as prostacyclin (cyclooxygenase product, COX ) are released from endothelial cells in response to τ . Little is known about how COX activity impacts capillary L p . The purpose here was to assess L p in situ following an acute Δ τ stimulus and during COX 1/ COX 2 inhibition. Mesenteric true capillaries ( TC ) of Rana pipiens (pithed) were cannulated for L p assessment using the modified Landis technique. Rana were randomized into Control and Test groups. Two capillaries per animal were used (perfusate, 10 mg·mL −1 BSA /frog Ringer's; superfusate, frog Ringer's or indomethacin (10 −5  mol·L −1 ) mixed in frog Ringer's solution). Three distinct responses of L p to indomethacin ( TC 2) were demonstrated ( TC 1 and TC 2 medians: Test Subgroup 1, 3.0 vs. 1.8; Test Subgroup 2, 18.2 vs. 2.2; Test Subgroup 3, 4.2 vs. 10.2 × 10 −7  cm·sec −1 ·cm H 2 O −1 ). Multiple regression analysis revealed a relationship between capillary L p and systemic red blood cell concentration or hematocrit, plasma protein concentration, and Δ τ (Test Subgroup 1, R 2  = 0.59, P  <   0.0001; Test Subgroup 2, R 2  = 0.96, P  =   0.002), but only during COX inhibition. Maintaining red blood cell and plasma protein levels within a normal range may control barrier function in a healthy state. Recovering barrier function may be an unrecognized benefit of transfusions during blood loss or edema formation.