Variations in permeability along individually perfused capillaries of the frog mesentery

When single capillaries in the exposed transilluminated frog mesentery were perfused with solutions containing patent blue V (molecular radius 0-7 nm), unbound T1824 (molecular radius 1-3 nm) and T1824-albumin (molecular radius 3-6 nm), the coloured molecules initially appeared to enter the extracapillary tissues more easily at some places than at others along the length of the vessels. Three patterns of extravascular distribution were observed: (1) High frequency unevenness: patent blue V and unbound T1824 first appeared outside the capillary as spots, bars or bands set at intervals 30-80 mum apart along the length of the vessel. (2) Low frequency unevenness: at later times, the extravascular dye was seen to be unevenly distributed on a much larger scale, with lengths of vessel of up to 400 mum surrounded by little extravascular dye separating similar lengths around which much dye had accummulated. This pattern of distribution was seen for all three solutes. (3) Arteriovenous gradient: perfusions with all three solutes resulted in more extracapillary colouration accumulating around the venous ends of the capillaries. Reasons are given for believing that the distribution of extracapillary solute reflected the distribution of permeability along the length of the capillary. The variations in permeability were attributed to an uneven distribution of small pores along the capillary rather than to presences of large pores.