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Tight junctions between epithelial cells are believed to control the paracellular diffusion of substances across epithelia. Epithelia in which tight junctions are poorly developed display a high paracellular electrical conductance, while those with extensive tight junctions show lower conductance values. We describe here a particular epithelium, that of the proximal tubules of the Necturua kidney, in which the development of the tight junctions varies in parallel with a change of paracellular electrical conductance. In control conditions, tight junctions between epithelial cells of the proximal tubules are more developed than in tubules undergoing saline diuresis, a situation which increases the conductance across the paracellular shunt pathway.

Freeze-fracture combined with quantitative electron microscopy of the intact human erythrocyte (RBC) and ghost revealed significant differences in their intramembranous particle coefficients. External (E) fracture-faces of unfixed ghost membranes were found to contain 40% fewer particles than those of intact unfixed RBC. The particle distribution of the intact RBC membrane depended on the use of glutaraldehyde fixation and glycerol cryoprotection. Whereas glutaraldehyde- and glycerol-treated cells disclosed 70% fewer E-face particles than did intact unfixed cells, poly-L-lysine-treated, intact, unfixed RBC showed no such differences. Treatment with a combination of poly-L-lysine and glutaraldehyde, however, increased the amount of E-face particles while reducing those of the protoplasmic (P) face. The poly-L-lysine effect varied with its concentration and was unaffected by previous application of neuraminidase. Nor did the lectin phytohemagglutinin induce particle rearrangement in intact cells. Our data demonstrate that the processes of glutaraldehyde fixation and glycerol cryoprotection modify the RBC membrane by decreasing the number of E-face particles present. In addition, the combination of poly-L-lysine and glutaraldehyde alters the affinity of some particles for one half of the membrane, suggesting that in freeze-fractured RBC, chemical bonds formed at the extracellular surface of the membrane can influence particle partitioning.

When a marked stimulation of the reninangiotensin system induced in the rat by bilateral adrenalectomy and salt-depletion is abruptly blocked by a 48-hr substitution treatment which desoxycorticosterone acetate (DOCA) and saltload, one observes: 1) an absolute increase of a form of renin revealed only after prior acidification (pH, 2.5) of kidney extract before usual incubation at the optimum pH of 6.5 and 2) the simultaneous appearance of many granules containing crystalline cores in the epithelioid cells of the juxtaglomerular apparatus. These observations are compatible with the existence of two forms of renal renin and show that after renin-depletion of the kidney, newly synthesized renin is mainly in the acid-activated form. Storage of this renin the epitheloid cells may possibly take the form of secretory granules with crystalline cores. It is speculated that the acid-activated from of renin may be either a renin proenzyme or a protein-bound form.