Knox, Franklyn G.

Prostaglandin E2, when infused directly into the renal interstitium, enhances sodium reabsorption by the superficial proximal convoluted tubules of anesthetized Sprague-Dawley rats. The present study was designed to investigate the role of angiotensin II in the prostaglandin E2-induced stimulation of proximal sodium reabsorption. Micropuncture at the superficial late proximal tubule demonstrated a significant increase in the fractional reabsorption of sodium from 39.9 +/- 2.3% in control conditions to 51.8 +/- 3.0% (n = 9, P less than 0.01) during the renal interstitial infusion of prostaglandin E2. The stimulatory effect of prostaglandin E2 on proximal sodium reabsorption was markedly attenuated by pretreatment with saralasin. During intravenous saralasin infusion, prostaglandin E2 did not significantly change the fractional reabsorption of sodium from 42.2 +/- 5.8 to 45.4 +/- 6.0% (n = 7, NS). In summary, the stimulatory effect of renal interstitial infusion of prostaglandin E2 on proximal sodium reabsorption was attenuated by pretreatment with saralasin. Therefore renal interstitial infusion of prostaglandin E2 may enhance proximal sodium reabsorption, at least in part, through stimulation of angiotensin II production in the rat kidney.

The objective of this study was to test the hypothesis that changes in renal perfusion pressure (RPP) are not fully transmitted to the renal interstitium in spontaneous hypertension in comparison with normotensive states. Okamoto spontaneously hypertensive and normotensive Wistar-Kyoto rats were used in this study. Renal interstitial hydrostatic pressure (RIHP) was measured directly and continuously via a polyethylene matrix that was implanted chronically in the left kidney 3 wk before RIHP measurement. When RPP was allowed to increase from 136 +/- 0.5 to 162 +/- 1.3 mmHg in male spontaneously hypertensive rats, RIHP was not significantly changed from 3.7 +/- 0.9 to 4.6 +/- 1.1 mmHg, and fractional excretion of sodium (FENa) increased significantly from 0.26 +/- 0.12 to 0.65 +/- 0.15% (P less than 0.05). When RPP was allowed to change from 104 +/- 0.9 to 127 +/- 1.3 mmHg in male Wistar-Kyoto rats, RIHP increased markedly from 4.0 +/- 0.3 to 7.2 +/- 0.4 mmHg (P less than 0.05), and FENa was significantly elevated from 0.27 +/- 0.08 to 2.02 +/- 0.55% (P less than 0.05). In conclusion, spontaneously hypertensive rats have a blunted increase in RIHP and pressure natriuresis response in comparison with Wistar-Kyoto rats. Thus, in Okamoto spontaneously hypertensive rats, the effect of RPP on RIHP is attenuated, leading to a blunted pressure natriuresis response.

Recent studies suggest that the autonomic nervous system, when activated by respiratory alkalosis, may attenuate the renal effects of atrial natriuretic factor (ANF). We evaluated the renal responses to infusion of ANF during acute exposure to hypocapnia in the presence or absence of the renal nerves in anesthetized rats. In this study, renal function during hypocapnia was assessed and was compared with renal function during normocapnia produced by normal ventilation as well as by hyperventilation. Rats were divided into three experimental groups (n = 8): 1) a normally ventilated normocapnic (NV-N) (control) group; 2) a hyperventilated normocapnic (HV-N) group; and 3) a hyperventilated hypocapnic (HV-H) group. The innervated (Inn) right kidney served as a control for the contralateral denervated (DNX) kidney. In Inn and DNX kidneys of HV-H rats, an infusion of ANF (12 micrograms.kg-1.h-1) produced a smaller increase in urine flow rate (delta V: 9.8 +/- 3.9 and 1.3 +/- 4.2 microliters/min) and in fractional Na excretion (delta FENa: 1.35 +/- 0.52 and 0.73 +/- 0.58%) compared with NV-N rats (delta V: 37.6 +/- 4.9 and 59.9 +/- 9.7 microliters/min; delta FENa: 3.24 +/- 0.37 and 3.88 +/- 0.65%). No differences were observed in delta V and delta FENa in Inn kidney between HV-H and HV-N groups; however, the attenuated natriuretic and diuretic responses to ANF in DNX kidney of HV-H rats were also observed in comparison with HV-N rats (delta V: 27.4 +/- 4.3 microliters/min; delta FENa: 2.94 +/- 0.48%). ANF induced natriuresis in DNX kidney to the same degree in NV-N and HV-N rats.(ABSTRACT TRUNCATED AT 250 WORDS)

This study evaluated the effect of acute hypoxia on renal handling of phosphate in rats in the presence and absence of parathyroid hormone (PTH). Hypoxia causes respiratory alkalosis in spontaneously breathing humans and animals. Respiratory alkalosis has been reported to induce a blunted phosphaturic response to PTH. In this study, to avoid the confounding effect of hypocapnia accompanying the hypoxia on phosphate excretion, the rats were ventilated mechanically, and arterial PCO2 levels were controlled. Rats were divided into two main groups depending on the arterial PO2 levels: a hypoxic group (n = 16) and a normoxic group (n = 18). Hypoxia was produced by ventilating with 10% oxygen, and hypocapnia was produced by hyperventilation. In response to PTH, the hypoxic rats without hypocapnia showed a greater increase in fractional excretion of phosphate (FEPi; 37.7 +/- 2.6%, mean +/- SE) compared with normoxic rats (27.4 +/- 2.5%, P < 0.02). During hypocapnia, there was no difference in FEPi between hypoxic and normoxic groups (21.2 +/- 1.5 and 19.5 +/- 1.2%, respectively), and both groups showed a significantly blunted phosphaturic response to PTH compared with normocapnia (P < 0.05 and P < 0.01, respectively). Urinary adenosine 3',5'-cyclic monophosphate (cAMP) increased similarly after PTH infusion between each group. To test whether the phosphaturic effect of PTH in hypoxia and the blunted phosphaturic effect of PTH in hypocapnia are due to steps beyond the generation of cAMP, the phosphaturic response to cAMP infusion was evaluated in 1) hypoxic and normocapnic rats (n = 6), 2) normoxic and normocapnic (control) rats (n = 6), and 3) normoxic and hypocapnic rats (n = 7).(ABSTRACT TRUNCATED AT 250 WORDS)

The objective of these experiments was to test the hypothesis that renal interstitial hydrostatic pressure (RIHP) plays an important role in the natriuretic effect of atrial natriuretic factor (ANF) in anesthetized Wistar rats. Three groups of male Wistar rats were used in this study. Two groups were infused with different doses of ANF, and the third group was a time control. In all groups, one kidney was acutely decapsulated, and the contralateral kidney was used as control. Renal decapsulation was used to control RIHP. In one group, 3 micrograms.kg-1.h-1 of synthetic ANF were infused intravenously (iv) and produced a plasma level of ANF (PANF) of 810 +/- 186.5 pg/ml. This pharmacological dose of ANF produced a significant increase in RIHP of the control kidney from 9.5 +/- 0.8 to 11.1 +/- 1.3 mmHg (P less than 0.05) but not in the decapsulated kidney [from 7.1 +/- 0.6 to 8.1 +/- 0.9 mmHg, not significant (NS)]. However, the changes in fractional excretion of sodium (FENa) and urine flow rate (V) as a result of ANF infusion were similar in both kidneys. In the decapsulated kidney, FENa and V increased by 1.53 +/- 0.41% and 26.21 +/- 5.98 microliters/min, respectively, from control to ANF infusion periods. In the control kidney, FENa and V increased by 1.60 +/- 0.28% and 31.61 +/- 5.87 microliters/min, respectively, from control to ANF infusion periods. In the second group, 1 microgram.kg-1.h-1 iv of synthetic ANF was infused and produced 165.2 +/- 29.3 pg/ml of PANF.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphate deprivation causes a resistance to the phosphaturic effect of parathyroid hormone (PTH). The present study evaluated the role of the beta-adrenergic system in this resistance phenomenon. In clearance experiments performed on acutely thyroparathyroidectomized male Sprague-Dawley rats, the phosphaturic response to PTH was determined in the presence and absence of propranolol in rats fed a low-phosphate diet (LPD) for 0.5, 1, 2, 3, or 4 days. Fractional excretion of phosphate (FEPi) in control rats fed a normal-phosphate diet (NPD) increased from 4.37 +/- 1.6 to 38.5 +/- 3.4% in response to PTH infusion. Propranolol did not change FEPi in NPD animals in the absence or in the presence of PTH (2.0 +/- 1.1 vs. 36.7 +/- 1.6%). LPD resulted in a gradual decrease in the phosphaturic response to PTH infusion as compared with NPD animals. PTH increased FEPi to 24.2 +/- 6.0% after one-half day of LPD, but when the infusion was supplemented with propranolol, PTH increased FEPi to 38.0 +/- 4.7%, similar to that in NPD animals. In the group fed LPD for one day, PTH increased FEPi to 16.9 +/- 4.3%, whereas in the presence of propranolol FEPi was restored to a similar level as in the NPD group (36.0 +/- 5.9%). Two days of LPD markedly decreased FEPi in response to PTH to 7.9 +/- 3.8% as compared with NPD rats, and propranolol infusion did not change this value significantly. Three and 4 days of LPD induced complete resistance to the phosphaturic effect of PTH in the presence as well as in the absence of propranolol.(ABSTRACT TRUNCATED AT 250 WORDS)

The Okamoto spontaneously hypertensive rat (SHR) has exaggerated natriuretic and diuretic responses to acute volume expansion in comparison with the Wistar-Kyoto (WKY) rat. The objective of these experiments was to determine the role of renal interstitial hydrostatic pressure (RIHP) and atrial natriuretic factor (ANF) in these natriuretic and diuretic responses of the SHR and the WKY rat. Two groups each of SHRs and WKY rats were used in this study. In one group of each, both kidneys were acutely decapsulated, and in the remaining two groups the renal capsules were left intact (control). In both control SHR (n = 6) and WKY (n = 7) groups, volume expansion was associated with a significant increase in RIHP (from 5.0 +/- 0.5 to 7.1 +/- 0.6 mmHg in SHR group and from 6.0 +/- 0.5 to 9.4 +/- 0.3 mmHg in WKY rat group) and a high level of plasma ANF (PANF). The increase in RIHP was significantly lower in the control SHR group compared with that of control WKY rat group (2.1 +/- 0.4 vs. 3.5 +/- 0.3 mmHg; P less than 0.05), whereas PANF was significantly higher in the SHR group (848 +/- 151 pg/ml) compared with that of the WKY group (503 +/- 37 pg/ml) at the end of the volume-expansion period. When the increase in RIHP that occurs during volume expansion in the SHR is blunted by acute bilateral renal decapsulation, the natriuretic and diuretic responses are significantly attenuated despite levels of PANF that are similar to those measured in the control SHR group.(ABSTRACT TRUNCATED AT 250 WORDS)

The objective of this study was to investigate the possible causal role of renal interstitial hydrostatic pressure (RIHP) in the natriuretic and diuretic responses of the Wistar rat. The relationship between renal perfusion pressure (RPP), RIHP, and fractional excretion of sodium (FENa) was established in the acutely decapsulated kidney and the contralateral control kidney of the same rat. The renal response to acute saline volume expansion was also studied in control and decapsulated kidney. When RPP was allowed to increase from 100 +/- 1.2 to 123 +/- 1.3 mmHg in male Wistar rats (n = 10), RIHP and FENa increased significantly from 3.3 +/- 0.4 mmHg and 0.57 +/- 0.15% to 4.3 +/- 0.4 mmHg and 1.77 +/- 0.41% in the decapsulated kidney and from 4.1 +/- 0.4 mmHg and 0.86 +/- 0.17% to 6.9 +/- 0.5 mmHg and 2.56 +/- 0.38% in control kidney. During saline volume expansion, RIHP and FENa increased significantly from 6.3 +/- 0.5 mmHg and 1.30 +/- 0.43% to 9.8 +/- 0.5 mmHg and 7.53 +/- 0.88% in the decapsulated kidney. In the control kidney, RIHP and FENa were 8.3 +/- 0.6 mmHg and 1.81 +/- 0.35% during control period and increased significantly to 12.7 +/- 0.4 mmHg and 9.31 +/- 0.50% during acute saline volume expansion. We conclude that the renal capsule is essential for the full increase in RIHP and for the full expression of the natriuretic and diuretic responses of pressure natriuresis and acute volume expansion of Wistar rats.

Renal interstitial hydrostatic pressure (RIHP) has been implicated in the regulation of sodium excretion. Studies using vasodilators and other maneuvers to increase RIHP have found a significant correlation between RIHP and sodium excretion. Since correlative studies do not prove a cause-and-effect relationship, it is not known whether the rise in sodium excretion in these studies is the result of increases in RIHP or if RIHP is elevated as a result of decreases in sodium and water reabsorption and increases in intratubular pressure. Therefore, the purpose of the present study was to determine whether elevation of intratubular hydrostatic pressures in response to direct inhibition of tubule transport with loop diuretics results in increases in RIHP in dogs and rats. Intrarenal hydrostatic pressures, renal hemodynamics, and sodium and water excretion were examined in dogs during intravenous administration of furosemide (3 mg/kg bolus followed by 0.03 mg.kg-1 x min-1) or bumetanide (60 micrograms/kg bolus followed by 1 microgram.kg-1 x min-1). Furosemide administration increased urinary flow rate (V; 0.10 +/- 0.02 to 4.6 +/- 0.97 ml/min), urinary sodium excretion (UNaV; 16 +/- 5 to 549 +/- 123 mu eq/min), and proximal tubule hydrostatic pressure (PT; 21 +/- 1 to 28 +/- 1 mmHg) but had no effect on RIHP (7.2 +/- 0.6 to 7.4 +/- 0.7 mmHg) or peritubular capillary hydrostatic pressure (14 +/- 1 to 14 +/- 1 mmHg).(ABSTRACT TRUNCATED AT 250 WORDS)

Fluid and solute reabsorption by the proximal tubule is the result of both transcellular and paracellular flux. The role of transcellular transport has been extensively studied, but the importance of paracellular flux has not been as thoroughly investigated. The purpose of this review is to update concepts about the contribution of paracellular transport for reabsorption by the proximal tubule.