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Volume changes due to osmotic flow in the distal portion of proximal tubules of Necturi were measured by the split oil drop technique. In agreement with previous findings no volume flow was induced by NaCI concentrations close to 60 raM. The tubule wall was found to be permeable to plasma electrolytes, which have an apparent reflection coefficient of 0.69. The mean apparent hydraulic conductivity was 0,33 X I0 -n cmS/dyne sec, comparable with other epithelia. A number of lipid-insoluble nonelectrolytes of widely varying molecular size had apparent reflection coefficients of about 0.5. In view of the insensitivity to molecular size it seems likely that apparent reflection coefficients determined from tubular volume changes depend primarily on the porosity of the intercellular barrier closest to the lumen and give little information about the subsequent fate of the test substances.T h e hydraulic conductivity of the proximal tubule of Necturus kidney m a y be determined by measuring the flow of solution induced by an osmotic pressure gradient. In previous experiments of this type we used mannitol to create the osmotic pressure difference since the rate of mannitol leakage out of the tubule h a d been found to be small (1, 2). These earlier results were based on sets of single observations since the stop flow perfusion method that was used yields only a single d a t u m for each tubule. T h e subsequent development of the split oil drop technique for the measurement of fluid movement by Gertz (3) has m a d e it possible to observe the time course of fluid movement and thus to make more accurate measurements. It was found that the isonatric point, at which there is no net N a movement, was 60 rnM NAG1, in agreement with previous observations. T h e tubule was therefore perfused with 60 inM NaC1 solutions, to which a series of lipid insoluble nonelectrolytes had been added, in order to 5x7

Cell water and ionic content were measured in aortic smooth muscle from rats and ground squirrels during 48 h of incubation in oxygenated Krebs solution held at low temperatures. Cells from the ground squirrel, a hibernator, maintained sodium and potassium contents near normal levels during incubation at 7 degrees C. In sharp contrast, cells from the rat lost potassium and gained sodium with half times of 14 and 11 h, respectively. The [K] in cell water for the rat decreased 138 mM while [Na] in cell water increased about 148 mM. Cells from the hibernator tended to lose 10 mM K and gain 13 mM Na. Vascular smooth muscle of both the rat and squirrel was able to maintain K content at 17 degrees C. Tissue content of the divalent cations Ca and Mg did not change in vessels from either animal. The present findings suggest that aortic smooth muscle cells from a hibernator have adapted in such a way as to maintain ionic gradients upon prolonged exposure to low temperatures.