A Micropipette Which Allows in Situ Perfusion of Arterioles and Capillaries

In order to investigate capillary physiology, a glass micropipette system was developed that allowed in situ perfusion of microvessels as well as rapid changes of perfusion solutions. Theta tube (WPI, Inc.; 1.5-mm o.d. glass stock capillary tubing which is divided into two hemicylindrical sides by a central glass septum) was pulled to a smaller diameter of approx 300-600 μm and inserted into the shank of a sharpened cannulating micropipette tip constructed from large-bore glass stock (1.6 mm i.d.). The resulting dead volume between the end of the Theta supply tube and the tip of the outer cannulating tip was approximately 90 nl. The perfusate was driven in a circuit from a pressurized feed reservoir down one side of the Theta supply tube pipette and back through the second side into a reservoir maintained at a lower pressure. The pressure gradient between the two reservoirs established a high-volume flow rate and subsequently a short perfusate transit time from the feed to the collection reservoir. The average pressure in the two reservoirs determined the pressure which drove the perfusate from the cannulating tip. At normal pressures and flows, the time required to change perfusion fluid composition at the pipette tip was less than 1 min, and discharge hematocrit of a red blood cell suspension was indistinguishable from the hematocrit measured in the feed reservoir. This pipette perfusion system should facilitate the investigation of a variety of physiological issues, including the causes of the normally low and variable capillary hematocrit and the effect of variations in the composition of suspending medium used to perfuse the capillaries and, further, would permit selected sites in the terminal vasculature to be perfused with materials which will label specific capillary networks in situ.