William E. Lassiter

Microperfusion study of calcium transport in the proximal tubule of the rat kidney.

SummaryMicroperfusion experiments on the proximal convolution of the rat kidney in situ led to the following observations:1.The concentration of Ca++ in tubular fluid at which the net flux of Ca++ is zero is about 2.66 mEq/L. This corresponds to a TF/P of 0.78, very similar to the equilibrium Na+ TF/P of 0.76. The active pump potential for Ca++ under these conditions is ECa++=3 mV.2.The rate of Ca++ reabsorption in the proximal convolution varies linearly with concentration up to a tubular fluid concentration of 15 mEq/L. Parathyroid hormone deficiency or excess has no influence on Ca++ reabsorption in the proximal convolution.3.The ratio of net outward transport rates of Na+ and Ca++ at a TF/P of 1, is the same as the ratio of the concentrations of these ions in tubular fluid.4.The ratio of net influxes of Na+ and Ca++ into Na+-free and Ca++-free perfusion solutions is the same as the ratio of the plasma concentrations of these ions.5.Because of the striking similarities in the transport characteristics of Na+ and Ca++, it is postulated that similar or identical mechanisms are involved in the transtubular transport of the two ions.

Micropuncture Study of Inulin Absorption in the Rat Kidney

By means of a microinjection technique, inulin-carboxyl-C14 or inulin-methoxy-H3 was injected into single proximal tubules of the rat at various urine flow rates. Urine collected separately from the two kidneys showed negligible amounts of inulin activity on the noninjected side, thus demonstrating directly that there is no significant reabsorption of inulin by the renal tubular epithelium under these conditions.

Influence of ionic calcium on the water permeability of proximal and distal tubules in the rat kidney

SummaryThe influence of Ca++ on the water permeability of proximal and distal tubules of the nondiuretic rat kidney has been studied, utilizing a micro-perfusion method. Addition of Ca++ to the perfused solution had no effect on the permeability of proximal tubules, but decreased the permeability of distal convolutions from a mean value of 3.0 to 1.9 × 10−8 cm3/cm2 sec cm H2O. Ca++ thus has an effect on water permeability of the nephron which is opposite to the effect of ADH, and both agents act on the distal convolution, but not on the proximal tubule.

Micropuncture Study of Water Reabsorption and PAH Secretion in Urea Diuresis in Rats.

Summary Inulin and PAH concentrations and osmolalities were determined in samples of tubular fluid collected from surface convolutions of rat kidneys during urea diuresis. F/P inulin ratios averaged 1.6 for the proximal tubule, 2.9 for the distal convolution and 21.7 for the ureteral urine, all significantly lower than in nondiuretic rats. Net PAH secretion appeared to be confined to the proximal tubule and could not be demonstrated in other parts of the nephron.

Phosphate Reabsorption in the Distal Convoluted Tubule

Phosphate excretion by the mammalian kidney is subject to regulation by a number of factors including, but not limited to, dietary phosphorus intake, parathyroid hormone (PTH), and extracellular volume expansion. The primary quantitative importance of the proximal tubule in this regulation is well known, but the role of more distal nephron segments has been less clearly defined. It has long been recognized that the fraction of filtered phosphate delivered to distal convolutions accessible to micropuncture on the surface of the rat kidney may exceed fractional excretion, especially in phosphate retaining states, suggesting that further phosphate reabsorption occurs in the distal tubule and/or collecting duct (1). Early microinjection studies, however, failed to reveal significant distal phosphate reabsorption (2,3). Since the composition of the final urine reflects the pooled contributions of both superficial and deep nephrons, nephron heterogeneity, with more avid reabsorption in proximal tubules of juxtamedullary nephrons, was suggested as an alternative to distal reabsorption to explain the low urinary phosphate excretion (4). These two postulated mechanisms, distal reabsorption and nephron heterogeneity, need not of course be mutually exclusive.

The Renal Excretion of Nitrogen Containing Metabolites

Urea accumulates in the renal papilla of mammals. This accumulation plays a decisive role in the concentrating mechanism of the kidney. In our attempts to understand the mechanism through which urea accumulates in the papilla, we have found that nature has provided us with two tools, with which this mechanism can be investigated. One is that the urea concentration and distribution in the renal papilla in many mammals changes with the nitrogen content of the diet. The other is that there are two groups of mammals which behave quite differently in this respect. In the first group the amount of urea excreted in the urine relative to electrolytes has a pronounced effect upon the concentrating ability of the kidney. In the other, it has little or no effect.

Micropuncture Study of Net Transtubular Movement of Urea and Water in Rats Expanded with Isotonic Saline

Summary Net transtubular movement of urea and water was studied in rats made diuretic by infusion of a volume of isotonic saline solution equivalent to 10% of their body weight. The major site of urea reabsorption was the proximal tubule and there was little urea loss from more distal parts of the nephron. There was no evidence of addition of urea to tubular fluid flowing through the loop of Henle. Throughout the nephron the percentage of filtered water remaining varied exponentially with transit time.