Capillary permeability and tissue exchange of water and electrolytes in the stomach

Techniques and applications of extracellular space determination in mammalian tissues

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.

The permeability of fish gills with comments on the osmotic behaviour of cellular membranes

The permeability of eel gills to ions, lipid-soluble and -insoluble substances has been measured under steady-state conditions. The results obtained support the multi-pathway model for transcapillary transport (Stray-Pedersen and Steen 1975). The gills are 25 times more permeable to THO than to Na+ or K+. Metabolic inhibitors do not affect the transfer of substances across the gills. Exposure to EDTA or MS-222 changes the permeability properties to resemble those of an aquous barrier. The gills are 10 times less water permeable than the capillaries of the rete mirabile.

The capillary permeability of the rete mirabile of the eel, Anguilla vulgaris L

Direct measurements of transcapillary exchange during steady-state conditions have been performed in the rete mirabile of the eel. The capillaries were found to be more than 30 times as permeable to THO (PTHO = 33.2 times 10(-5) cm sec-1), ethanol and antipyrine than to K+. The mutual relationship of the permeabilities for K+, Na+, urea and sucrose were similar to that between the corresponding free diffusion coefficients in water. The permeability characteristics did not change when the perfusate contained metabolic inhibitors. Additional of 1 mM/l of albumin lead to a significant transcapillary osmotic flow. NaCl, urea and sucrose, however, did not cause osmotic flow. Based on the quantitative values for permeability and volume flow the capillary pores should be about 1300 A and have a frequency of 1.3 times 10(6) per cm2 of the capillary surface, occupying an area of about 0.1% of the total area. The results obtained in the present investigation raise questions as to the validity of the Pappenheimer single-pore theory for transcapillary transport as well as to the fine structure of the capillary pores. As an alternative to Pappenheimer's theory a multi-pathway model is proposed in order to explain transcapillary transport.

Heart capillary permeability to lipid-insoluble molecules

1. A study has been made, in the isolated, beating dog heart perfused with blood, of the transcapillary exchange of the following substances: [(3)H]water, (22)Na, (86)Rb, (36)Cl, [(14)C]urea, [(3)H]glycerol, [(3)H]glucose, [(14)C]sucrose and [(3)H]inulin.2. The method used to study the exchange was the ;indicator diffusion' technique. It consists in a rapid arterial injection of a mixture containing a diffusible and a non-diffusible molecule, followed by a rapid split collection of the venous outflow, up to 30 sec. The fractional extraction, E, of the diffusible substance was obtained by comparing the relative concentrations of both tracers in injected medium and in each venous sample.3. E for [(3)H]water was the highest (0.90 +/- 0.3), and it did not vary with flow. All other molecules had values for E that decreased as flow increased.4. Capillary permeability constant, P, was estimated from PS = - F ln (1 - E), in which S is the surface area of exchange and F is the blood perfusion rate. To test the validity of the equation, E was measured at different blood perfusion rates. It was found that the equation did not apply at relatively low flows for the more diffusible substances.5. The average values of P estimated for inulin, sucrose, glucose, glycerol and urea were 0.27, 0.8, 1.0, 1.5 and 3.1 x 10(5) (cm/sec), respectively. The ratio P/D (in which D is the free diffusion in water constant) was the same for all these substances. This can be interpreted as showing that if pores exist in the capillary endothelium, they must be larger than 80-100 A diameter. It is concluded that the pores could actually be the intercellular slits as previously suggested by electron microscope studies. Endothelial cell participation in the exchange appears to be small except for [(3)H]water.

Intestinal capillaries. I. Permeability to peroxidase and ferritin

Horseradish peroxidase (mol. diam. approximately 50 A) and ferritin (mol. diam. approximately 110 A) were used as probe molecules for the small and large pore system, respectively, in blood capillaries of the intestinal mucosa of the mouse. Peroxidase distribution was followed in time, after intravenous injection, by applying the Graham-Karnovsky histochemical procedure to aldehyde-fixed specimens. The tracer was found to leave the plasma rapidly and to reach the pericapillary spaces 1 min post injection. Between 1 min and 1 min 30 sec, gradients of peroxidase reaction product could be demonstrated regularly around the capillaries; their highs were located opposite the fenestrated parts of the endothelium. These gradients were replaced by even distribution past 1 min 30 sec. Ferritin, followed directly by electron microscopy, appeared in the pericapillary spaces 3-4 min after i.v. injection. Like peroxidase, it initially produced transient gradients with highs opposite the fenestrated parts of the endothelium. For both tracers, there was no evidence of movement through intercellular junctions, and transport by plasmalemmal vesicles appeared less efficient than outflow through fenestrae. It is concluded that, in the blood capillaries of the inintestinal mucosa, the diaphragms of the endothelial fenestrae contain the structural equivalents of the small pore system. The large pore system seems to be restricted to a fraction of the fenestral population which presumably consists of diaphragm-free or diaphragm-deficient units.

Fractional extraction and transcapillary exchange during continuous and instantaneous tracer administration

The relation between extraction fraction, E, measured by the multiple-tracer, instantaneous intra-arterial injection method and by single-tracer, continuous injection was studied by experiments on gastric wall, liver, and skeletal muscle of anesthetized dogs. E was calculated from ratios of diffusible to nondiffusible tracer concentrations in successive samples of venous blood. Initial extraction, E(0), was the same by instantaneous or continuous injection. E diminished with time due to back-diffusion. In liver, E(0) for water- 3 H, mannitol- 3 H, and glucose- 3 H remained close to unity, indicating that transcapillary transport was flow-limited. E(0) for 22 Na, 36 Cl, and mannitol- 3 H in gastric wall, and for 22 Na and 86 Rb in skeletal muscle were less than unity and decreased with increasing blood flow, indicating diffusion limitation. E for 86 Rb in skeletal muscle eventually reached a plateau level, due to the large "sink" for 86 Rb in cell K. Both E(0) and plateau E for 86 Rb in muscle increased during exercise. Permeability surface products PS were calculated from measured E's and blood flows, Q, by the relation PS = -Q ln (1 - E). Their values suggest that E(0) reflects relative capillary permeabilities to different tracers, but only for capillaries of fastest arteriovenous transit. Plateau E's, such as for 86 Rb in skeletal muscle, represent all the capillaries, but are influenced also by extracapillary barriers.