Techniques and applications of extracellular space determination in mammalian tissues

Abstract

This review summarizes the ways in which the extracellular space (ECS) may be estimated in mammalian tissues, and briefly describes some of the uses to which the EC confinement of certain molecules (markers or tracers) may be put in the elucidation of physiological functions. The introductory section is followed by a description of the more commonly used marker molecules and their functional characteristics, and of factors likely to lead to the spurious over- or under-estimation of the ECS. Certain alternative methods are also described, in particular those based on morphological and electrical criteria which seek to demonstrate small, functionally important, changes in the size of specialized regions of the ECS (e.g. lateral cellular interspaces) without necessarily being required to provide a quantitatively precise estimate of their size. Section III describes the results of measurements of ECS in various mammalian tissues (muscle, gastro-intestinal tract, nervous tissue, crystalline lens, placenta, lung and kidney) and some applications of EC markers to investigation of cellular function (e.g. uptake of metabolic substrates and epithelial transport) and, in outline, characterization of capillary permeability. The available literature in this field is very extensive, and in the interests of brevity the reader is, where appropriate, referred to previous reviews covering specialized aspects of ECS determination and related topics.

Asymmetric calcium influx and efflux at maternal and fetal sides of the guinea‐pig placenta: kinetics and specificity.

“…The problems generally encountered in selecting an extracellular marker for the purpose of studying cellular uptake in vitro and in vivo have been extensively reviewed recently (Law, 1982). For the single-circulation, paired-tracer dilution technique, reference and test molecules should ideally be of equal charge, similar size and diffusibility, and should not be subject to intravascular separation (Taylor effect).…”

Rapid transcapillary exchange and unidirectional neuronal uptake of noradrenaline in the perfused rabbit heart.

“…These experiments abandoned the use of an intravascular reference tracer, since the capillary extractions of noradrenaline and D-mannitol (extracellular reference) were similar and above 60 %. This procedure has been used to quantify cellular uptake in various organs (see review, Bustamante, Mann & Yudilevich, 1981) Cumulative 1251I-labelled albumin area (%) Law, 1982). At tracer concentrations of L-noradrenaline a 'virtual volume of distribution' could only be estimated and the value of 0 79 ml g-1 for noradrenaline (Table 1) corresponds well to the total tissue water, suggesting that the amine is distributed within the cells rather than bound to surface membranes.…”

Extracellular volume and blood volume in chronically catheterized fetal sheep.

“…In support of this, the ratio of the inulin space to the mannitol space in the fetus (0 95 + 0 01) was closer to unity than in nephrectomized adult dogs (0 74; Swan, Madisso & Pitts, 1954). As well, hyaluronidase increases the inulin space in some adult tissues (Law, 1982). Another explanation for the good agreement between sodium and saccharide spaces in the fetus relates to the relative sizes of the ECV and intracellular compartments.…”

“…Assuming the liver has extracellular and total water contents similar to the fetus as a whole, about 20 ml, or 25 %, of the difference between inulin and mannitol spaces is explained. The remaining 75 % is probably accounted for by the smaller size of mannitol, allowing greater penetration of tissues (Law, 1982). As well, mannitol is small enough to cross the sheep placenta (Boyd, Haworth, Stacey & Ward, 1976;Basso, Fernandez, Althabe, Sabini, Piriz & Belitzky, 1977).…”

“…(1979) is perhaps surprising when one considers that in adult animals saccharide spaces are generally lower than the sodium space. This is because sodium is not completely excluded from cells (Cheek et al 1957) and the saccharides penetrate dense connective tissue poorly (Law, 1982). However, Friis-Hansen (1961) noted that in the newborn there was good agreement between different extracellular indicators.…”