The chronic effect of vascular endothelial growth factor on individually perfused frog mesenteric microvessels

Abstract

1. Hydraulic conductivity (Lp) of the wall of perfused microvessels has previously been shown to be chronically increased 24 h after a 10 min perfusion with vascular endothelial growth factor (VEGF). In order to investigate this further, Lp and the effective oncotic pressure difference (f3DeltaPi) acting across the vessel walls was measured before exposure to VEGF and 24 h later after the mesentery had been replaced in the abdominal cavity. 2. Acute 10 min perfusion with VEGF did not chronically change f3DeltaPi despite chronically increasing Lp 6.8 +/- 1.2-fold. This suggests that pathways formed 24 h after perfusion with VEGF which increase hydraulic conductivity of the capillary walls have the same reflection coefficient as those present before VEGF. 3. Acute 10 min perfusion with VEGF significantly increased the diameter of vessels after 24 h by 48 +/- 13%. To determine whether this was due to changes in the compliance of the vessel wall, the distensibility of microvessels was measured before and 24 h after perfusion with VEGF. The distensibility was increased 45 +/- 15% by VEGF but this was not great enough to account for the increase in diameter. 4. The chronic increase in Lp could be attenuated by inhibition of nitric oxide synthase with L-NAME. In addition, the chronic increase in permeability was correlated with the acute response to VEGF (r = 0.71, P < 0.01) suggesting that the acute and chronic changes may be related. 5. These results show that VEGF chronically increases Lp without affecting the oncotic reflection coefficient. This may be due to reduced pore path length, or increased small pore numbers, which are properties of fenestrated capillaries. They also show that VEGF increases microvascular distensibility and diameter.

Figure 1. Acute and chronic effect of VEGF on L_p

Hydraulic conductivity of 12 control (▪) and 22 c were first perfused with BSA (day 1, ), then with 1 nm VEGF (day 1, , are the peak values recorded during perfusion with VEGF), then 24 h later perfused again with BSA (day 2, ). **P < 0.01 compared with baseline on day 1.

Figure 2. Relationship between the acute and the chronic increase in L_p

The peak of the initial transient increase in L_p during perfusion with 1 nm VEGF (Peak L_p day 1), is plotted against the subsequent chronic increase in baseline L_p 24 h later during perfusion with 1% BSA. The relationship is described as: Day 2 L_p = 0.5219 × Peak day 1 + 5.929 (r = 0.71, P < 0.01, n = 21).

Figure 3. Measurement of reflection coefficient in a control vessel (A) and before and after VEGF (B)

A, pressure-filtration rate plot of a single control vessel on day 1 (▴) and day 2 (□). Neither L_p nor σΔΠ were changed (day 1: σΔΠ = 17.6 cmH₂O, L_p = 3.5 × 10⁻⁷ cm s⁻¹ cmH₂O⁻¹; day 2: σΔΠ = 16.6 cmH₂O, L_p = 2.1 × 10⁻⁷ cm s⁻¹ cmH₂O⁻¹). B, pressure-filtration rate plot of a single vessel before perfusion with VEGF (day 1, ▴) and 24 h after perfusion of VEGF (day 2, □). L_p was increased 12.8-fold, but σΔΠ was unaltered (day 1: σΔΠ = 21.2 cmH₂O, L_p = 1.2 × 10⁻⁷ cm s⁻¹ cmH₂O⁻¹; day 2: σΔΠ = 21.0 cmH₂O, L_p = 15.4 × 10⁻⁷ cm s⁻¹ cmH₂O⁻¹).

Figure 4. Chronic effect of 10 min perfusion of 1 nm VEGF on distensibility and diameter

Distensibility (left-hand axis and bars) and diameter (right-hand axis and bars) of control vessels (perfused with 1% BSA) on day 1 (□) and day 2 (▪), and vessels before perfusion with 1 nm VEGF () and 24 h after perfusion with VEGF (). *P < 0.05 and **P < 0.01 compared with day 1.

Figure 5. Effect of 100 μm l-NAME on chronically increased permeability

L_p measurements on a single microvessel before and during perfusion with 100 μm l-NAME, 24 h after perfusion with VEGF. The baseline on day 1 is shown. Perfusion of the vessel with l-NAME was started at time = 0 s.

Figure 6. Effect of arginine analogues on chronically increased permeability

The effect of arginine analogues on permeability of experimental (□) or control (▪) microvessels. Hydraulic conductivities of the walls of vessels perfused with 1% BSA (Base, Day 1), and 1 nm VEGF (Peak, Day 1). Twenty-four hours later the vessels were again perfused with 1% BSA (Base, Day 2), and then perfused with 100 μm l-NAME (experimental group, □, n = 6) or D-NAME (control group, ▪, n = 5). * Significantly lower than baseline on day 2, P < 0.05.

Figure 7. Relationship between effective oncotic pressure difference and the increase in hydraulic conductivity

The regression line is shown (continuous line) σΔΠ = 0.8L_p0/L_p+ 19.7 cmH₂O. Dashed line shows the predicted regression line if the reflection coefficient of the new pores was zero, i.e. large unrestricting gaps.