LASSITER, WILLIAM puncture from surface tubules, as previously described (9, I 0). To prevent retrograde flow, tubules were blocked by injection of a droplet of mineral oil, and the rate of "39

Anesthetized rats and hamsters were given Ca45 intravenously, and fluid was subsequently collected by micropuncture from glomeruli and surface tubules in the rats, and from loops of Henle in the hamsters. In nondiuretic animals, fluid:plasma calcium ratios averaged 0.71 in the glomerulus; 0.76 in the proximal tubule; 2.0 in the loop of Henle; 0.47 in the distal convolution; and 0.9 in ureteral urine. In mannitol diuresis, the calcium ratio of glomerular fluid was unchanged, but ratios as low as 0.21 were noted in the proximal tubule. In this circumstance, the average proximal ratio was 0.61, and the distal ratio 0.07. These results indicate active transport of calcium out of all major parts of the nephron, with the bulk of calcium reabsorption occurring in the convoluted portion of the proximal tubule. Furthermore, the pattern of tubular reabsorption of calcium is similar to that of sodium, suggesting that the two are related.

Fluid was collected by micropuncture from individual renal tubules of anesthetized rats and its pH determined with the quinhydrone microelectrode. The single glomerular sample and early proximal fluid were isohydric with arterial blood, but later proximal fluid usually showed progressive acidification. The maximum proximal fall in pH was 0.43 u in nondiuretic rats, 0.56 u during profuse glucose or mannitol diuresis, and 0.78 u in rats previously loaded with ammonium chloride and undergoing glucose diuresis. Fluid from the early distal convolution was usually acidified relative to arterial blood but was not significantly different from late proximal fluid. Progressive acidification probably also occurred in the distal convolution. The pH decreased further in the collecting ducts, much more so in the nondiuretic state than during diuresis. The quantitative importance of proximal reabsorption of HCO3– and, by inference, H+ secretion is emphasized. It is suggested that the pH of tubular fluid may increase in the thin descending limb of the loop of Henle, especially in a kidney elaborating a concentrated urine, because of increased concentration of HCO3–.

Renal tubular transport of water, osmotically active solute, and PAH were studied with micropuncture methods in anesthetized rats loaded with a volume of 0.9% sodium chloride solution equivalent to 10% of their body weight. Inulin clearance and urine flow were greatly elevated above values in nondiuretic animals, but there was little change in PAH clearance. F/P inulin-C14OOH ratios indicated that 42% of the filtered water and solute was reabsorbed along the proximal convolution, a lower fraction than is found in nondiuretic and hypertonic saline-loaded rats. This indicates that the rate of solute and water reabsorption in the proximal tubule did not increase in proportion to the increased filtered load, and it was, in fact, similar to that in nondiuretic rats. Intravenous administration of vasopressin and aldosterone had no demonstrable effects on solute or water reabsorption. F/P PAH/inulin ratios demonstrated that PAH secretion occurred along the proximal tubule, but probably in no other portion of the nephron.

Fluid was collected by micropuncture from proximal and distal convolutions of anesthetized rats and analyzed for inulin, sodium, urea, and total osmotically active solute. The proximal fluid/plasma (F/P) sodium ratio was not significantly different from unity in antidiuretic animals, but was as low as 0.78 during mannitol diuresis. The distal F/P sodium ratio averaged 0.62 in antidiuresis, and 0.24 during osmotic diuresis. The data are interpreted to indicate active sodium transport by both proximal and distal convolutions. The F/P ratios for inulin, urea, and total osmotically active solute are in general agreement with previous studies.