Microvascular Research Laboratories Research Interests

Control of Vascular Permeability

The primary function of the cardiovascular system is to deliver nutrients to cells, and remove metabolic wastes. Water and nutrients move from the blood to the tissue across the walls of capillaries and venules. The rate at which this happens depends on a number of factors, one of which is the permeability of the capillary wall. The endothelial cells which form the capillary wall control its permeability, in a way which is not very well understood. Permeability research in the MVRL centres on how endothelial cells regulate capillary permeability

 

Development of highly permeable new microvessels is critically important in a number of very common, and potentially lethal diseases. Solid tumours cannot grow more than 1mm in diameter without the development of new blood vessels, and yet we do not understand how these vessels are formed, or why they are highly permeable. Other conditions such as diabetes, psoriasis, atherosclerosis and arthritis are also associated with high permeability. We carry out experiments that will help us understand how permeability is regulated, and hopefully this will lead to new strategies for drug design, and novel therapies for these conditions, as well as helping us to understand how our bodies function normally.

 

One area of our  research concentrates on the mechanisms by which Vascular Endothelial Growth Factor causes increased transvascular solute and solvent flux in individual capillaries and postcapillary venules. We are now investigating the mechanisms by which the chronic increase in permeability, associated with new vessel growth, are brought about. Specifically we are interested in the role of VEGF induced calcium influx in this increased permeability, and the signal transduction pathways through which VEGF exerts its effects. The research involves acute and chronic measurement of permeability of single isolated perfused mesenteric microvessels, determination of the intracellular calcium concentration of the endothelial cells forming these vessels, and the physiological, ultrastructural, and molecular changes brought about by growth factors which result in elevated permeability.

How do we measure  Microvascular Permeability?.

The movie at this link  shows schematically how we measure permeability of a single capillary. The movie starts with a lower power view of a mesentery with the gut attached.  The gut is the green outer part of the diagram, and the arteries (in red) and veins (in blue) run through the thin connective tissue (the mesentery). Coming off the arteries and  veins are smaller arterioles and venules. These branch down to capillaries which connect to two sides of the arterial system. The movie zooms in on a single capillary.  Capillaries are endothelial lined tubes, usually only one endothelial cell thick, with no smooth muscle surrounding them. The capillary is about 1/50th of a millimetre wide  (20 microns). To measure permeability, we cannulate the capillary with a very fine needle made from glass. The needle (or micropipette) has a bevelled tip, just like a  hypodermic needle, but the tip is only 12-15 microns wide and exceedingly sharp. The micropipette is attached at the back end to a constant, set pressure, so a stream of fluid  comes out of the end of the micropipette.

The micropipette is filled with a perfusate, which is a physiological solution containing salts, serum albumin, and a low  concentration of red blood cells. You can see the red blood cells flowing along the capillary. To measure permeability we gently lower a glass rod onto the capillary some  distance down stream from the cannulation site. This blocks the flow out of the end of the capillary and causes the pressure in the capillary to equilibrate with the pressure in  the micropipette. Now the pressure inside the capillary is greater than outside, so fluid is forced across the capillary wall. As the fluid flows out it is replaced by fluid in the  pipette. This means there is a flow of fluid from the pipette into the capillary and across the capillary wall. The red blood cells in the capillary will move with the fluid flow,  seeming to flow along the capillary as the column of fluid in front of them shrinks by filtration. From the speed of the red cells  (dl/dt) and the cross sectional area of the  capillary (calculated from its radius r) we can calculate the rate of fluid flow across the capillary wall (Jv). From this,  the surface area of the vessel (A, calculated from the radius  and the length), and the pressure inside the vessel (which we have set) we can determine the permeability of the wall to water (the hydraulic conductivity). We then remove  the glass rod and allow the perfusate to flow freely. We can add drugs or markers to the vessel either by refilling the pipette while in the vessel, or dripping them on the  outside of the mesentery.

Angiogenesis and Vascular permeability.
We are currently intersted to find out how increased vascular peremability is regulated during angiogenesis. To this end we have developed a model for the measurement of permeability in rat mesentery during angiogenesis caused by VEGF expression in the surrounding fat cells. This results in increased blood vessel growth of previously characterised vessels, and hence increased permeability. The angiogenesis can be measured by recording the meentery on videotrap and then staining for dividing cells.
 

You can  also see a video of an actual permeability measurement.