Effects of glucagon on glomerular filtration rate and urea and water excretion.

Clearance experiments were performed in anesthetized male Wistar rats to reevaluate the renal effects of glucagon (Gluc) on glomerular filtration rate (GFR) and solute and water excretion. After an 80-min control period, these effects were evaluated in the last 80 min of a 2-h intravenous Gluc infusion. Gluc induced significant increases in GFR (+20%), urine flow rate (+150%), free water reabsorption (+50%), urea synthesis and urea excretion (+66%), and nonurea solute excretion (+67%). In addition, fractional urea excretion (FEurea) increased by 43% (P less than 0.01). Additional experiments showed that increases in either urea excretion or urine flow rate (induced by appropriate infusion of urea or half-dilute saline), similar to those seen after Gluc, could not account for the increased FEurea. All significant effects of Gluc were also observed during infusion of antidiuretic hormone or during water diuresis. The tubular effects of Gluc could be explained by a reduction in proximal reabsorption. The dose of Gluc required to induce all the effects described above was 12 ng.min-1.100 g body wt-1, a dose producing an approximately 10-fold supraphysiological peripheral plasma concentration but a "physiological" level for the liver. Infusion of 1.2 ng induced almost no change in renal function, and infusion of 120 ng induced no greater effects than 12 ng. These results suggest 1) that Gluc, a hormone liberated after protein ingestion, exerts coordinated effects on liver and kidney to increase simultaneously urea synthesis and excretion and to promote water conservation and 2) that these effects could, at least in part, be indirect and depend on the Gluc-induced stimulation of hepatocyte metabolism.

Arginine synthesis by the proximal convoluted tubule in rats with chronic renal failure.

The high plasma level of citrulline (Cit) is one of a number of abnormalities in the plasma amino acid pattern in chronic renal failure (CRF). Synthesis of arginine (Arg) from citrulline in the kidney is the major source of Arg for the body. In order to evaluate the renal activity of Arg synthesis in CRF, we studied arginine production in proximal convoluted tubules (PCT) isolated from male Sprague-Dawley rats 1 month after 5/6 nephrectomy and from sham-operated rats (n = 6 of each). PCT segments were incubated in a sealed chamber with 50 or 200 microM of [L-ureido 14C]-Cit (simulating in vivo plasma concentrations in healthy rats or rats with CRF, respectively). Arginase and urease were added to the medium to hydrolyze Arg into 14CO2 + NH3. 14CO2 was trapped in KOH and counted. Results showed that: (1) in CRF, Arg production per unit tubular length is increased in proportion to hypertrophy of PCT (x 1.5); (2) in CRF, as in the healthy kidney, Arg production increases with Cit concentration (x 2.5 from Cit 50 to 200 microM). Taking into account the hypertrophy and the elevation in Cit concentration, the increase in Arg production per unit length (x 3.6) is not sufficient to compensate for the reduction in nephron number. Most likely, a greater length of maximal tubule is recruited for renal Arg synthesis in CRF.

Vasopressin is involved in renal effects of high-protein diet: study in homozygous Brattleboro rats.

The present study was designed to test the possible role of vasopressin in the renal response to dietary protein. This possibility was suggested by the similarity of effects on renal function and morphology of chronic high-protein intake and chronic stimulation of urine concentration. Adult male Brattleboro rats, genetically unable to produce vasopressin, were fed high-protein (32% casein = HP, n = 8) or low-protein (10% casein = LP, n = 9) diet for 7 wk. Renal function was evaluated by clearance techniques based on 24-h urine collections in metabolic cages. The response to a single injection of the vasopressin analogue 1-desamino-8-D-arginine vasopressin (DDAVP) was also tested. Kidney weight and height of the different renal zones were assessed at the end of the study. HP diet increased urea excretion nearly sevenfold. Water intake increased by 57% (P less than 0.001) and urine flow rate by 71% (P less than 0.01). Urine osmolality rose from 104 to 181 mosmol/kgH2O (P less than 0.001). At variance with what occurs in rats with endogenous vasopressin (Sprague-Dawley; Bouby, N., et al. Kidney Int 34: 4-12, 1988), HP diet increased creatinine clearance per unit body weight by only 14% and did not change free water clearance, renal mass, and height of inner stripe of outer medulla. However, the rise in urine osmolality and drop in free water clearance after DDAVP were significantly greater in HP- than in LP-fed Brattleboro rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Quick isolation of rat medullary thick ascending limbs. Enzymatic and metabolic characterization.

This paper describes a rapid and simple method for isolation of medullary thick ascending limbs (MTAL) from rat kidney. The technique takes advantage of the fact that MTAL represents a high fraction of the inner stripe (IS) tissue in the outer medulla, and that this nephron segment is more resistant than others to mechanical and enzymatic disruption. Special attention was given in the design of each step of the isolation procedure in order to improve purity and yield of the preparation. Major steps are the following: careful dissection of the IS; cutting IS tissue into small pieces of regular size (approximately equal to 1 mm3); mild and brief enzymatic hydrolysis in a 65 U/ml collagenase solution; separation of long MTAL segments from other tubule fragments and cells, and washing of the collagenase solution, on a nylon sieve (100 microns opening). This technique does not require lengthy centrifugations and provides about 6 mg fresh tissue (= 1 mg protein) from two rat kidneys in 2 h. Light microscopy and transmission electron microscopy show a good purity (at least 95%) and good preservation of TAL ultrastructural morphology. Adenylate cyclase responsiveness to arginine-vasopressin (AVP), glucagon (GLU) and salmon calcitonin (SCT) of the MTAL suspension is similar to that reported for single microdissected rat MTAL. Viability of the MTALs was demonstrated by the ability to accumulate cyclic AMP in presence of AVP, GLU, SCT and forskolin. Normal oxygen consumption was 45.1 +/- 2.4 (SEM) microliter . mg protein-1 . h-1 (n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)