Studies carried out during the preceding project period have shown that epithelial cells can be isolated under relatively mild conditions from the urinary bladder of the toad, Bufo marinus, and that these cells remain viable as judged by the ability of these cells to consume oxygen, to exclude vital dyes, and to respond with a increase in the rate of oxygen consumption and cyclic AMP formation in the presence of vasopressin. Since these cells, once isolated from the epithelium, assume spherical shapes, their volume can be accurately and readily measured by determining their diameter with an eyepiece micrometer of an inverted microscope. Thus, it has been possible to measure quite accurately the rate of water movement across the plasma membrane of the cell using only a very small cell sample. We wish to pursue this methodology further in the coming project period, not only to establish the effect of vasopressin on the hydraulic conductivity of the plasma membrane, but also to study the permeability coefficients of the isolated cell membrane for urea, sodium, and chloride. The results of studies performed on isolated toad bladder epithelial cells will be compared to those obtained in the intact toad bladder epithelium. Moreover, these studies will be extended to epithelial cells isolated from different parts of rabbit kidney and to epithelial cells maintained for several passages in cell culture. BIBLIOGRAPHIC REFERENCES: Petrucelli, II, R.J., R.G. Abramson, and P. Eggena. Organic acid transport across the toad bladder. (1975. Abstracts Am. Soc. Nephrol.) Eggena, P. ADH action on osmotic water permeability of isolated toad bladder cells. (1976. Abstracts Am.Soc. Clin. Invest.)