Diabetic patients are often subjected to glucose concentrations that may exert significant osmotic effects on endothelial cells. It is possible that this may contribute to the increase in vascular permeability reported in diabetic patients and animals. Previous work has shown that large osmotic stresses do not increase permeability in mesenteric microvessels unless the attachment of the endothelial cells to the basement membrane are disturbed (Kajimura, et al. 1997). To determine whether small but physiologically relevant osmotic stresses due to increased glucose could increase permeability, the effect of 20 mM osmolarity Ringer’s solution on the hydraulic conductivity of mesenteric microvessels in vivo was investigated.
Frogs (Rana temporaria) were anaesthetised by submersion in MS222 (1 mg ml-1) and anaesthesia maintained by constant superfusion of the mesentery with frog Ringer solution containing MS222 (0.2 mg ml-1). Animals were humanely killed after each experiment by destruction of the brain. Lp (mean ± S.E.M. X 10-7cm.s-1 .cmH2O-1) was measured during perfusion and superfusion with normal or hyperosmotic Ringer solutions in 20-30 µm diameter mesenteric microvessels, using a modification of the Landis Michel method (Michel, et al. 1974). A mesenteric microvessel was cannulated with a bevelled glass micropipette connected to a manometer and perfused with 1 % bovine serum albumin (BSA) in frog ringer (5 mM D-glucose) containing rat red cells as flow markers. The vessel was subsequently perfused and superfused with either 20 mM D-glucose or 20 mM mannitol Ringer’s solution. A glass rod was used downstream from the cannulation site to occlude the vessel for 5-10 seconds. Hydraulic conductivity, Lp was then calculated from the radius of the vessel (r), velocity of the marker cells (dl/dt) and the length (l) between the marker cell and the occlusion site. The hyperosmotic Ringer’s solution significantly increased the hydraulic conductivity 4.0 ± 2.9 fold above baseline values (from 2.3 ± 2.2 to 13.4 ± 11.6). This was true for both hyperosmotic glucose (3.9 ± 4.3 fold, from 1.3 ± 0.6 to 6.1 ± 7.7), and mannitol (4.2 ± 2.0 fold, from 3.7 ± 3.4 to 11.9 ± 6.7 x10-7cms-1.cmH20-1).
These results show that hyperosmotic solutions applied to both sides of the vessel wall can increase hydraulic conductivity in mesenteric microvessels in vivo.
This work was supported by the British Heart Foundation.