The permeability of single capillaries to potassium ions.

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Abstract
Methods for determining the diffusional permeability to K+ of single capillaries in the frog mesentery are described. By means of micropipettes, injections or infusions were delivered into a single capillary. The subsequent concentration variations in and about the capillary were followed with K+-sensitive microelectrodes. A theoretical analysis is provided which gives a quantitative frame of reference for evaluating the observed time-concentration curves in terms of capillary permeability. The advantage of single capillary studies is that the surface area through which diffusion occurs is known as is the concentration difference across the capillary membrane. Different techniques (3) are the single injection method which represents an application of the indicator diffusion technique where a high-K+ bolus is injected into a single capillary; the sack method which determines the rate of K+ disappearance from within and immediately outside an occluded capillary segment, after a brief increase in intracapillary K+ concentration; and the interstitial diffusion method which records time and spatial distribution of K+ in the interstitial space after a step-change in intracapillary K+ concentration. The methods gave an average K+ permeability of the capillary membrane of 67 .times. 10-5 cm/s (SD: 23, n = 26) at room temperature. These figures are clearly higher than those previously reported in the mammalian capillary using whole-organ techniques. In terms of the Pappenheimer pore model, this estimate of capillary permeability is consistent with the behavior of a membrane with a thickness of 1.0 .mu.m which possesses equivalent pores with a radius of 110 .ANG., a fractional pore area of 0.3% and a pore density of 8 .mu.m-2.