Recently, it has been shown that VEGF transiently raises the hydraulic conductivity (Lp) in mesenteric microvessels of frog (Bates & Curry, 1996) and rat. However, when the morphological correlate of the permeability increase was investigated using serial section reconstruction of the vessel walls, frog and rat microvessels showed differing morphologies but a similar Lp increase (Neal & Michel, 1998). In frog mesenteric microvessels the change in permeability occurred along with transcellular openings through the endothelial cells. In rats the route through the endothelium appeared to be vacuolar, where vacuoles became connected with the luminal and abluminal plasma membranes of the endothelial cells. In both cases the route was transcellular. The intravascular pressures during experimentation were 20-30 cmH2O; this represents a higher relative microvascular pressure in frogs than in rats. It is possible that higher pressures result in formation of transcellular gaps, whereas lower pressures merely induce formation of vacuolar channels. Here we investigate whether changing the perfusion and occlusion pressure in mesenteric microvessels alters the permeability response to VEGF. Single mesenteric capillaries or venular capillaries of pithed frogs (Rana temporaria) were perfused through sharpened glass micropipettes with 1 % BSA (bovine serum albumin) in frog Ringer solution at either 15, 30 or 40 cm H2O. Lp was calculated from fluid filtration measurements using the single vessel micro-occlusion technique (Michel et al. 1974). Baseline Lp was measured during perfusion with 1 % BSA. The perfusate was then switched to one containing 1 nM VEGF in 1 % BSA in frog Ringer solution by refilling the micropipette through an indwelling fine tube. As soon as the new solution could be seen to enter the microvessel a rapid series of occlusions were made in order to measure the transient increase in Lp caused by the 1 nM VEGF.
When frog mesenteric capillaries and venular capillaries were perfused and occluded at 15 cmH2O prior to and during perfusion with 1 nM VEGF, Lp was increased from a mean ± S.E.M. value of 3.9 ± 0.9 to a peak of 36.0 ± 12.9 Ω 10-7 cm s-1 cmH2O-1 (n = 10). When similar micro-vessels were perfused and occluded at pressures above 30 cmH2O the increase in Lp was significantly attenuated and only rose from 1.6 ± 0.2 to 5.2 ± 1.0 Ω 10-7cm s-1 cmH2O-1 (n = 10, P < 0.05, Mann-Whitney U test).
Clearly the VEGF-induced increase in Lp was attenuated at greater perfusion and occlusion pressures. However, we have not shown whether the increased perfusion velocity and therefore shear rate at higher pressure can affect the magnitude of the VEGF-induced permeability increase.
This work was funded by The Wellcome Trust (58083) and the British Heart Foundation (FS98023).
- Bates, D.O. & Curry, F.E. (1996). Am. J. Physiol. Heart Circul. Physiol. 271, H2520-2528.
Michel, C.C., Mason, J.C., Curry, F.E., Tooke, J.E. & Hunter, P.J. (1974). Q. J. Exp. Physiol. Cogn. Med. Sci. 59, 283-309.
Neal, C.R. & Michel, C.C. (1998). J. Physiol. 506.P, 24P.This work was funded by The Wellcome Trust (58083) and the British Heart Foundation (FS98023).
E., Tooke, J.E. & Hunter, P.J. (1974). Q. J. Exp. Physiol. Cogn. Med. Sci. 59, 283-309.
Neal, C.R. & Michel, C.C. (1998). J. Physiol. 506.P, 24P.