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A B S T R A C T Studies were designed to compare the segmental analysis of sodium reabsorption along the nephron during volume expansion with either 10% body weight Ringer's or 0.6% body weight hyperoncotic albumin. Total kidney and nephron glomerular filtration rate increased similarly with both, but urinary sodium excretion (12.7 vs. 4.0 Aeq/min, P <0.001) and fractional sodium excretion (5.0 vs. 1.6%, P < 0.001) increased to a greater extent with Ringer's. Fractional reabsorption of sodium in the proximal tubule was diminished in both groups but to a significantly greater extent during Ringer's (P < 0.005). Absolute reabsorption was inhibited only in the Ringer's group. Delivery of filtrate out of the proximal tubule was greater in the Ringer's studies, 45 vs. 37 nl/min (P < 0.001).However, both fractional and absolute sodium delivery to the early and late distal tubule were not significantly different in the two groups. Fractional reabsorption in the collecting duct decreased from 96% in hydropenia to 31% during Ringer's but fell only slightly to 80% in the albumin studies. Absolute collecting duct reabsorption was also greater in the albumin studies, 0.55 vs. 0.21 neq/min (P < 0.001), which could totally account for the difference in urinary sodium excretion between the two groups. 22Na recovery in the final urine after end distal microinjections was 71% during Ringer's infusion and 34% during albumin (P < 0.001). From these data we conclude that: (a) Ringer's solution has a greater inhibitory effect on proximal tubular sodium reabsorption, and (b) in spite of this effect, differences in mucosal to serosal collecting duct sodium transport are primarily responsible for the greater natriuresis during Ringer's infusion.

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Aqueous vasopressin was infused to bicarbonate- and glucose-loaded dogs and to nonloaded antidiuretic dogs in doses of 50 mU/kg per min or 50 mU/kg per h. Both doses caused a marked increase in sodium, chloride, and water excretion. The larger dose raised the fractional excretion (sodium clearance (C-Na)/glomerular filtration rate (GFR) times 100) of these ions from 2% or less to in excess of 20%. Blocking the pressor effects of these doses of vasopressin with sodium nitroprusside did not alter the marked natriuretic and chloriuretic effect. The maximal rate of bicarbonate and glucose reabsorption was not depressed by vasopressin infusion; fractional phosphate excretion, however, was markedly increased. Inhibiting distal hydrogen ion secretion by inducing selective aldosterone deficiency failed to uncover a vasopressin-induced inhibition of proximal bicarbonate reabsorption that might have been masked by increased distal bicarbonate reabsorption. There was no significant change in GFR, renal plasma flow, or filtration fraction. The distribution of cortical renal blood flow (measured by the radioactive microsphere technique) shifted toward the inner cortex after vasopressin administration. Vasopressin, in pharmacologic doses, is a potent diuretic that most likely exerts this effect by directly inhibiting sodium reabsorption at a point in the nephron distal to the proximal tubule.

Various methods have been used to measure total renal blood flow. Of these the most commonly employed are the clearance of paraamino-hippurate (PAH) and techniques that make use of the electromagnetic flowmeter. PAH clearance (corrected for extraction ratio) is a cumbersome method requiring catheterizaiion of the renal vein. It would be desirable to have a method which is simple anld less traumatic but equally reliable. Neutze et al.(1) have shown that total renal blood flow can be determined using a radioactive microsphere (MS) technique, andl that values so obtained correlate well with renal blood flow as determined by using the electromagnetic flowmeter. Neutze and co-workers measured renal blood flow by multiplying cardiac output by the ratio of counts in the kidney to total body counts. Thus, their technique involved determination of cardiac output and total body counting and is obviously no less cumbersome than any of the other commonly used techniques.Using the Fick principle we reasoned that total renal blood flow could be obtained by the ratio of radioactivity in the kidney to that in arterial blood sampled immediately after the injection of microspheres. This report describes this technique and the results obtained using it as compared to PAH clearance.Methods. Twelve female dogs were anesthetized with sodium pentobarbital and catheters 3were placed in the left ventricle (for microsphere injection), the femoral artery (for blood sampling), and the femoral vein (for PAH infusion). The left kidney was exposed through a flank incision and the renal vein catheterized via the ovarian vein. The ureters were cannulated by a suprapubic approach. Renal plasma flow was measured by the clearance of PAH using Wolf's equation (2) : RPF = ( U -R ) X V / ( A -R ) ( A , R and U represent PAH concentration in arterial, renal venous and urine samples, respectively; V is the urine flow expressed in ml/min) .Radioactive microspheres 15 k 5 pm in size, labeled with s5Sr or l4ICe were injected into the left ventricle within 10 sec; a total amount of 15 pCi (400,000 MS) was injected. Immediately following the administration of the microspheres, peripheral arterial blood was drawn at a constant rate using a Sage infusion pump over a 1-min period and the volume withdrawn noted; usually 30 ml were removed. At the end of the experiment the left kidney was removed. The blood sample and the left kidney were digested in concentrated HCl, the volume of each sample was measured and 1 ml aliquots were counted in a Beckman gamma counter; in this manner the total counts in, each sample were determined.Since the concentration of MS following the injection will be the same in any artery (1 ), if the removal rate of the blood from the femoral artery equalled the rate of renal blood Aow, the total amounts in the kidney and the femoral blood sample would be the same; thus renal blood flow can be calculated using the following formula:Renal blood flow = (Total Kidney Counts/ Total Femoral Blood Counts) X Blood Removal rate (ml/min) .