Susan C. Opava-Stitzer

Free water clearance curves during saline, mannitol, glucose and urea diuresis in the rat

1. Free water clearances were measured during infusion of hypotonic saline, glucose, urea, and mannitol in Brattleboro rats. For each solute the free water clearances were plotted using either V or (CH2O + CNa) as the distal tubular delivery term.

Renal Na+ -K+ - ATPase in Weanling and Adult Spontaneously Hypertensive Rats

Abstract The interrelationships among plasma renin activity (PRA, ng AI/ml plasma/hr), aldosterone concentration (ng%), and renal Na+-K4-ATPase activity (μmole PO4/mg protein/hr) were studied in 9 weanling normotensive spontaneously hypertensive rats (SHR), 9 adult hypertensive SHR, and 9 weanling and 9 adult normotensive Wistar-Kyoto rats (WKY). All groups were placed on a normal (0.4% sodium) diet. PRA and plasma aldosterone, measured in samples drawn from the ether-anesthetized rat, were higher in weanling SHR (15.2 ± 2.0, 37 ± 4.2) than in WKY. PRA measured in samples collected from a separate group of unanesthetized weanling SHR was also greater than in age-matched WKY. In adult SHR, PRA (6.1 ± 0.9) and plasma aldosterone (20.0 ± 2.7) were decreased. During the weanling period Na+-K+-ATPase activity in SHR was not only greater than in age-matched WKY but was also increased compared to adult normotensive and hypertensive rats (137 ± 9 weanling SHR, 89 ± 7 weanling WKY, 73 ± 11 adult SHR, 84 ± 17 adult WKY). Thus, during the weanling period the renin-angiotensin-aldosterone (R-A-A) system and renal Na+-K+-ATPase activity are activated in SHR. The elevation of Na+-K+-ATPase activity may be due to increased aldosterone levels. It was noted, however, that plasma aldosterone was similar in adult WKY and weanling SHR, while Na+-K+-ATPase activity was higher in SHR. These findings involving R-A-A and renal Na+-K+-ATPase activity prior to the elevation of blood pressure suggest that the kidneys may play a role in the initiation of hypertension in SHR.

Role of water balance in the enhanced potassium excretion and hypokalaemia of rats with diabetes insipidus.

1. The role of water balance in the hypokalaemia of rats with diabetes insipidus (DI rats) was studied. 2. After a 3‐day balance study DI rats had a lower muscle potassium content, and plasma [K+], and the urinary excretion of potassium in response to oral KCl loading was reduced when compared to normal rats. The hypokalaemia was found to be associated with elevated concentrations of potassium in renal medulla and papilla when compared to values in normal Long‐Evans rats. 3. During a 9‐day balance study urinary potassium excretion was higher than that of normal rats on days 1‐3, but not different on days 4‐9; this transient elevation was observed in DI rats on normal, high and low potassium diets. On a low potassium diet the urinary potassium excretion of DI rats fell to minimal levels, making unlikely the existence of a renal defect in potassium handling. 4. Muscle potassium content and plasma [K+] were normal after 9 days in metabolism cages. This spontaneous reversal of the hypokalaemia of DI rats was associated with increased water content of renal medulla and papilla, and decreased potassium concentration in these zones. 5. The effect of acute mild dehydration on potassium handling of DI rats was evaluated. Water deprivation for 1‐8 hr was sufficient to raise the urinary potassium excretion of DI rats above that of DI rats drinking ad lib. Renal tissue [K+] was significantly increased after 8 hr of dehydration. Water deprivation also enhanced the response of DI rats to an oral KCl load. Two days of chronic dehydration in the form of water rationing also significantly enhanced the urinary potassium excretion of DI rats. 6. These data suggest that chronic mild dehydration may be responsible for the modest potassium deficiency observed in DI rats via alterations in renal tissue [K+] and consequently in urinary potassium excretion. Correction of dehydration during prolonged periods in metabolism cages may account for the spontaneous reversal of the hypokelaemic condition.

Effects of Endogenous Antidiuretic Hormone (ADH) on Macrophage Phagocytosis

Although several studies have indicated that antidiuretic hormone (ADH) enhances the phagocytic function of the reticuloendothelial system (RES) in shock syndromes, it remains unknown what influence ADH exerts upon the individual phagocytic components of this system. The present investigation was designed to evaluate the effects of endogenous ADH on the phagocytic activity of peritoneal macrophage cells. As a phagocytic stimuli, fluorescent methacrylate microbeads were injected intraperitoneally into Brattleboro (ADH deficient) and normal Long Evans rats in the presence and absence of exogenous ADH. Peritoneal cells were harvested 19-22 hr after the administration of the microbeads and the percent phagocytosis was determined in macrophage cells using a fluorescence-activated cell sorter (FACS II). Our results indicate that the percentage of peritoneal macrophages ingesting the fluorescent methacrylate microbeads was significantly reduced in the absence of ADH (Brattleboro rats: 5.4 +/- 0.6% versus Long Evans rats: 16.8 +/- 2.3%; p less than 0.001). In addition, our data demonstrate that exogenous administration of ADH significantly enhanced macrophage phagocytosis in Brattleboro (14.7 +/- 2.2%) and normal Long Evans (49.6 +/- 4.5%) rats. These data suggest, for the first time, that endogenous ADH might play a modulatory role in the phagocytic activity of a specific component of the RES, namely, the macrophage cell.

SODIUM AND POTASSIUM BALANCE IN THE BRATTLEBORO RAT

Since its discovery in 1961,l it has become apparent that the Brattleboro (DI) rat is a useful model for the study of a variety of physiological problems in addition to the obvious one of the role of antidiuretic hormone (vasopressin, ADH) in urine concentration and water balance. In the study of the control of extracellular fluid volume and electrolyte balance, in particular, the DI rat offers a unique opportunity to observe the spontaneous interaction of homeostatic mechanisms when a single disturbance, namely the absence of ADH, has been introduced. This review will attempt to present a comprehensive description of the state of sodium and potassium balance in the DI rat. Although some data exist on the handling of other electrolytes 51, 54 these have not been studied extensively. Mention will also be made of the multiple factors that may influence electrolyte balance in the Brattleboro rat. New data, obtained by Opava-Stitzer and Fernhdez-Repollet, will be included when relevant.

Renal Hemodynamics and Urinary Concentrating Capacity in Protein Deprivation: Role of Antidiuretic Hormone

The role of antidiuretic hormone (ADH) in the renal concentration defect and hemodynamic changes in protein malnutrition was evaluated in rats with diabetes insipidus (DI) after 2 weeks of low protein feeding. Free water reabsorptive capacity (TcH2O), glomerular filtration rate (GFR), and renal plasma flow (RPF) were measured in the protein deprived rats and in DI rats fed a normal protein diet. The effect of urea supplementation of the low protein diet on renal concentrating capacity was also evaluated. In addition, the renal hemodynamic response to acute administration of ADH was measured and correlated with changes in plasma renin concentration and renal renin content (RRC). Protein deprivation in DI rats resulted in reduced urine osmolality and urea excretion, differences which were reversed by urea supplementation. Protein deprivation did not affect free water reabsorptive capacity but did reduce GFR and RPF. Acute ADH administration significantly increased GFR and RPF in protein-deprived rats; these changes were associated with a reduction in RRC and release. These results suggest that dietary protein restriction does not directly affect the tubular capacity to generate and reabsorb free water. The hemodynamic changes seen in protein deprivation are not mediated by ADH and may be secondary to increased intrarenal angiotensin II.

Effect of Lithium and Antidiuretic Hormone on Plasma Renin Concentration in Diabetes Insipidus Rats (Brattleboro Rat Model)

are reduced, all of which might stimulate renin secretion. Conversely, ADH might reduce plasma renin levels in the Dr rat indirectly, secondary to its effects on water balance, as well as by a direct effect on renin secretion. To differentiate between direct and indirect effects of ADH on renin secretion, plasma renin concentration was measured in Dr rats, before and during ADH administration, with or without LiCl treatment. Using different doses of ADH, combined with LiCI treatment, it was possible to raise the titer of antidiuretic hormone in the plasma of the Dr rat while blocking to a varying extent the action of ADH on tubular water reabsorption.

EFFECT OF POTASSIUM ON PLASMA RENIN CONCENTRATION IN THE PRESENCE AND ABSENCE OF ADH (BRATTLEBORO RAT MODEL)

Rats with hereditary hypothalamic diabetes insipidus (so-called DI rats) have elevated plasma renin leve1s.l. * Although the mechanism responsible for this condition has not been elucidated, it seems reasonable to postulate that the absence of ADH and/or the hypokalemia previously reported in these rats might contribute to the elevation of plasma renin concentration (PRC). Evidence in favor of this hypothesis emerges from studies in which both A D H S and potassiumB have been shown to inhibit renin release. In an attempt to examine the relative roles of ADH and potassium in the regulation of renin secretion, PRC was measured in DI rats maintained on a potassium-free, normal potassium, or high potassium diet in the presence and absence of ADH treatment. Male and female DI rats were used in all experiments. Body weights ranged from 250-350 g. As depicted in TABLE 1, balance studies and PRC determinations were performed during a control period in four groups of untreated DI rats fed with a normal diet. Twenty-four-hour urine samples were collected for the measurement of urine volume, osmolality, and sodium and potassium concentrations. Plasma renin concentration was measured by radioimmunoassay of angiotensin I (AI, New England Nuclear). Following the control period, subgroups of six rats were placed for three weeks on the respective dietary regimen and/or ADH treatment; balance studies and PRC measurements were performed as previously described. Our data indicate that in the absence of ADH treatment PRC of DI rats maintained on a K+-free diet (97 f 11 ng AI/ml/h) was not significantly different from that of DI rats maintained on a normal diet (98 2 20 ng AI/ ml/h). As illustrated in FIGURE 1, ADH treatment significantly diminished PRC of DI rats maintained on a low or a normal potassium intake. Additionally, no significant difference between PRC of ADH-treated rats maintained on a normal or a low K+ diet (48 f 6 vs. 71 -c 13 ng AI/ml/h) was detected. FIGURE 2 depicts the effect of a high potassium diet on PRC of DI rats in the absence of ADH treatment. It is evident from the results that a high K+ intake significantly reduced PRC of DI rats (74 f 7 vs. 44 f 6 ng AI/ml/h, p < 0.01). This value of PRC was also significantly lower than that observed in control DI rats maintained concurrently on a normal potassium intake (109 * 15, p < 0.01). As shown in FIGURE 3, PRC was found to be significantly lower in rats treated with both ADH and a high K+ diet (34 f 3 ng AI/ml/h) than in rats treated with ADH alone (53 f 5 ng AI/ml/h, p < 0.01), suggesting an additive effect of potassium and ADH on PRC. As summarized in TABLE 2,