Blood flow distribution and tissue solute content of the isolated-perfused kidney

Summary Isolated renal perfusion experiments were performed in the dog kidney to evaluate solute composition of cortex, medulla, and papilla and intrarenal blood flow distribution after 1 to 4 h of normothermic blood perfusion. Tissue slices were assayed for sodium, potassium, urea and water content and external monitoring of¹³³Xe washout and radionuclide-labelled microphere distribution were utilized to determine intrarenal blood flow distribution. Between the first and fourth hours of renal perfusion, total renal blood flow increased secondary to a decrease in renal vascular resistance and urine osmolality decreased. Concurrently there was a reduction in medullary tonicity primarily due to a loss of both urea and sodium. During this same period of time, there was a redistribution of intrarenal blood flow from outer cortex to inner cortex as determined by the microsphere technique. In addition, the distribution of¹³³Xe was significantly altered: radioactivity of Component I decreased from 82±1% to 62±5% while radioactivity increased in Component II from 9±1% to 19±2%. Dissipation of the hypertonic medullary interstitium and alteration of intrarenal blood flow distribution may explain the observed loss of urinary concentration as well as other functional alterations encountered in the isolated perfused kidney.     

Functional heterogeneity of the descending limbs of Henle's loop

By using the in vitro microperfusion technique, were examined functions of the descending limbs of Henle's loop obtained from either the short-loop nephron (SDL) or the upper portion of the long-loop nephron of hamsters (LDLu). Morphological distinctions between these segments were confirmed by light and electron microscopic observations. Both segments were highly permeable to water. The LDLu was highly permeable to sodium and to chloride: efflux coefficients (10^–7 cm²·s^–1) for²²Na and³⁶Cl were 41.0±5.4 and 3.8±0.6, respectively. The SDL were less permeable to sodium and to chloride: efflux coefficient for²²Na and³⁶Cl were 2.9±1.4 and 0.9±0.2, respectively. In contrast, the SDL was more permeable to urea as compared to the LDL, efflux coefficients for urea being 5.1±1.4 vs 1.4±0.3, respectively. When composition of the perfusate was identical to that of the bathing fluid, no transepithelial voltage was demonstrated. The volume flux was very small or undetectable. From these observations, we propose that the internephron heterogencity must be taken into consideration for constructing a model of countercurrent system in the renal medulla.     

Role of vasopressin V2 receptors in modulation of the renal cortico-papillary NaCl gradient

In order to examine if modulation by vasopressin of NaCl transport in Henle's loops (via V2 receptors) can significantly modify medullary ionic hypertonicity, the effects of stimulation or inhibition of these receptors were studied in anaesthetized Wistar rats. Total electrolyte concentration in the medullary interstitium was continuously measured as tissue admittance (reciprocal impedance), using needle electrodes recording from the inner and outer medulla of the in situ kidney. Deamino-[Cys¹,D-Arg⁸]vasopressin (dDAVP], a V2 agonist, infused i.v. at 7.5 ng · min^–1 · kg^–1, significantly increased admittance by 9% and 8% in the inner and outer medulla, respectively. A slightly pressor i.v. infusion of natural arginine vasopressin (AVP) induced pressure natriuresis and did not affect medullary electrolyte concentration. Inhibition of V2 receptors with [d(CH₂)⁵,D-Phe², Ile⁴]-AVP, infused i.v. at 133 g · h^–1 kg^–1 in indomethacin-treated rats, decreased admittance (significant in the inner medulla). Neither of the three agents used caused significant changes in the renal blood flow (RBF) or clearance of inulin (C _in). The demonstration that changing activity of V2 receptors affects the corticopapillary NaCl gradient indicates that, at least in rodents, stimulation of loop salt transport by AVP may represent an additional mechanism enhancing urine concentration.     

Proximal tubular transport and urinary excretion of sodium after renal denervation in sodium depleted rats

The effect of unilateral renal denervation on renal handling of water, sodium and potassium was studied with clearance and micropuncture techniques in sodium depleted anaesthetized rats in the nondiuretic state. In clearance experiments renal denervation resulted in a +140 and +320% increase in urine flow and potassium excretion, but sodium excretion of innervated (I) and denervated (D) kidneys was similar (I: 12.0±2.0, D: 14.0±3.6 nM·min^–1·g^–1; NS). However, upon the loop diuretic furosemide (1 mg·kg^–1), a marked denervation natriuresis was observed (I: 2.8±0.9, D: 5.9±1.0 M·min^–1;P<0.05) and denervation diuresis and kaliuresis persisted, too (+95 and +60%, respectively). Micropuncture results revealed that fractional reabsorption of filtrate to late proximal puncture site was depressed by renal denervation from 62 to 49% while no change in time control rats was seen (64±2 vs. 64±1%; NS). In micropuncture experiments besides augmented urine flow (+82%) from D kidneys also a small denervation natriuresis was present (I: 21.6±6.4, D: 29.2±7.0 nM·min^–1;P<0.05). It is concluded that the lack or marked attenuation of denervation natriuresis in sodium depleted rats were the result of an almost complete compensatory distal reabsorption of the excess sodium (but not of water and potassium) leaving the proximal tubule after denervation. The distal adaptive response can be overcome by furosemide.     

Tubuloglomerular feedback and autoregulation of glomerular filtration rate in Wistar-Kyoto spontaneously hypertensive rats

Experiments were done in Wistar-Kyoto spontaneously hypertensive rats (SHR) to examine the efficiency of autoregulatory adjustments of kidney and nephron filtration rate (GFR) to acute changes in blood pressure (BP) over a broad blood pressure range. When BP of the SHR was reduced from 158±7 to 118 ±3 mm Hg by aortic clamping, kidney-GFR remained unchanged from 1.19±0.11 to 1.17±013 ml·min^–1·g^–1 kidney weight (KW), respectively. Single nephron GFR (SNGFR) measured at early distal tubule sites was similarly unchanged with the same BP change, 27.9±1.5 vs. 24.9±2.1 nl·min^–1·g^–1 KW (P>0.10). Proximal and distal estimates of SNGFR were significantly different from each other at high BP (7 nl·min^–1·g^–1,P<0.025), but were not different at low BP (2.0 nl·ml^–1·g^–1,P>0.10). Studies assessing tubuloglomerular feedback activity were done with orthograde perfusion of the loop of Henle using recollections of early proximal flow rate (EPFR) as an index of change of glomerular filtration rate. A change in perfusion rate from 0 to 45 nl·min^–1 induced a reduction in early proximal flow rate of 40.5 ±4.5%. Juxtaglomerular renin activity of superficial nephrons was 36.2±4.3 in the SHR, a value insignificantly different from 23.7±4.4 ng Angiotensin II amide·0.1 ml^–1·h^–1. 5 glomeruli^–1 in normal controls (P>0.05). The SHR appears to behave as a normal animal with respect to tubologlomerular feedback and autoregulatory renal vascular adjustments. Like normal rat models, the SHR demonstrated dependence on maintenance of distal filtrate delivery to achieve single nephron GFR autoregulation.Financial support for these studies and for Dr. Ploth were made available by funds from the Deutsche Forschungsgemeinschaft     

Vasoactive intestinal peptide stimulation of renal adenylate cyclase and antagonism by (4Cl-d-Phe6Leu17)VIP

The effect of vasoactive intestinal peptide (VIP) and related peptides [glucagon, secretin, PHI 1-27 (peptide with N-terminal histidine and C-terminal isoleucine)] on renal adenylate cyclase (AC) has been determined in several species. The largest stimulation (4.1±0.5-fold basal) of AC by 1 mol · l^–1 VIP was observed in feline cortical plasma membranes. In rabbit and guinea-pig, VIP increased AC activity 1.5±0.3- and 1.8±0.3-fold respectively but glucagon had no such action. Conversely in the rat glucagon stimulated AC some 3-fold over basal activity whereas VIP had little effect. In dog, cat and mouse both peptides were effective in increasing AC activity. For cat, half-maximal stimulation of cortical plasma membrane AC by VIP was seen at 27.0±9.0 nmol · l^–1 (SEN=9 animals). VIP also increased AC activity in both outer (red) and inner (white) medulla. In feline cortical membranes VIP and PTH (parathyroid hormone) when added in combination were fully additive. However for VIP and glucagon in combination there was no cumulative increase in AC activity, indeed the resultant activity was less than that attained by VIP alone. The VIP analogue (4Cl-d-Phe⁶Leu¹⁷)VIP at 10 mol · l^–1 produced a right shift in the VIP-dose response curve and increased the EC₅₀ from 17.2±5.8 nmol · l^–1 to 132.0±22.2 nmol · l^–1 VIP (SEN=4). There was no reduction in the maximum response elicited by VIP consistent with a competitive type of antagonism by this analogue. PHI-stimulated AC was also reduced by (4Cl-d-Phe⁶-Leu¹⁷)VIP resulting in a similar right shift in the dose response curve. However, this analogue of VIP had no effect on glucagon- or secretin-stimulated AC, indicated by no change in EC₅₀ values.     

Limb vasodilatory capacity and venous capacitance of trained runners and untrained males

Aerobically trained athletes possess enhanced vasodilatory capacity and venous capacitance in their exercising muscles. However, whether they also possess these characteristics in their non-specific exercising muscles is undetermined. This study examined vasodilatory capacity and venous capacitance of specific (legs) and non-specific exercising muscles (arms) of ten trained runners and ten active but untrained males aged 18–35 years. Venous occlusion plethysmography determined baseline and peak blood flow after 5 min of reactive hyperaemia. Forearm and leg venous capacitance were determined as the difference between baseline and 2 min of venous occlusion at 50 mmHg. During reactive hyperaemia, trained runners had higher leg (48.4±5.3 ml·100 ml tissue^–1·min^–1) and arm (40.8±2.1 ml·100 ml tissue^–1·min^–1) vasodilatory capacity compared to the untrained (leg: 37.3±2.5 ml·100 ml tissue^–1·min^–1; arm: 34.2±2.2 ml·100 ml tissue^–1·min^–1; P<0.05), and higher calf vascular conductance (0.51±0.06 ml·100 ml tissue^–1·min^–1·mmHg^–1 versus 0.35±0.03 ml·100 ml tissue^–1·min^–1·mmHg^–1; P<0.05). The trained also had higher venous capacitance in both arms (3.5±0.2 ml 100·ml^–1) and legs (4.8±0.1 ml·100 ml^–1) compared to the untrained (3.0±0.2 ml 100·ml^–1; 4.2±0.2 ml·100 ml^–1; P<0.05). These findings show that vasculature adaptations to running occur in both specific and non-specific exercising muscles.     

The protection of renal function from ischemic injury in the rat

The role of cell swelling in the development of tissue injury was investigated in an ischemic model of acute renal failure in the rat. One kidney was selectively flushed with a 10% solution of polyethylene-glycol M.W. 6,000 (PEG) in normal saline prior to the clamping of the vascular pedicle, and the contralateral kidney was flushed with normal saline. Both kidneys remained ischemic for 45 min with the test solutions within their extracellular space; the concentration of PEG was chosen to prevent cell swelling during the ischemic period.After 24 h of reflow, a striking difference in function was found. The endogenous creatinine clearance in the PEG-treated kidney was 2.41±0.18 ml · min^–1 Kg BW^–1 , whereas in the contralateral kidney it was reduced to 0.29±0.15 ml · min^–1 Kg BW^–1 (n=7,P<0.001).Microscopic examination at the end of the ischemic period revealed a widespread pattern of cell swelling in the saline-treated kidneys which was more pronounced in the inner cortex, whereas the PEG-treated kidneys were well preserved. After 24 h of reflow a comparable distribution of tubular necrosis was found in the saline-treated kidneys-again, tubular structures in the PEG-treated kidneys appeared mostly normal.These findings support the hypothesis that cell swelling is an early, important step in the development of tubular necrosis in ischemic renal injury.A portion of this work was submitted in abstract form to the 7th International Congress of Nephrology, Montreal, 1978     

Carbonic anhydrase independent bicarbonate reabsorption

The present study was designed to define the prerequisites of carbonic anhydrase independent bicarbonate reabsorption. In free flow experiments during systemic application of carbonic anhydrase inhibitor benzolamide (50 mg/kg B. W.) bicarbonate recovery in % of filtered load was found to be 74±8% in late proximal convoluted tubules, 39±6% in distal convoluted tubules and 32±4% in urine, indicating that most of carbonic anhydrase independent bicarbonate reabsorption occurs in tubule segments prior to distal convoluted tubules. In vivo continuous microperfusion experiments in proximal convoluted tubules demonstrated that luminal benzolamide (0.5 mmol/l) virtually abolishes net bicarbonate fluxes, when bicarbonate concentration in the luminal perfusate (25 mmol/l) is close to peritubular plasma concentration (24.4 mmol/l). In contrast, a significant downhill reabsorptive flux occurs, when perfusate bicarbonate concentration is 75 mmol/l and a significant downhill secretory flux is observed, when the perfusate is initially free of bicarbonate. The corresponding apparent permeabilities are 1.0±0.1·10^–6 cm²/s for influx and 1.6±0.4·10^–6 cm²/s for efflux of bicarbonate.Clearance studies reveal that carbonic anhydrase dependent and independent bicarbonate reabsorption are not saturable but depend on the rate of volume reabsorption in the kidney. In conclusion, passive movements of bicarbonate do occur in proximal convoluted tubules and most likely contribute to carbonic anhydrase independent bicarbonate reabsorption.This study has been supported by Österr. Fond zur Förderung der wiss. Forschung Nr. 4366. Part of this study has been presented at the 8th Int. Congr. Nephrol. Athens, 1981, and at the Herbsttagung of the Deutsche Physiologische Gesellschaft, Innsbruck 1981     

Effect of dehydration and rapid rehydration on renal function and on plasma renin and aldosterone levels in the black Bedouin goat

Bedouin goats in the extreme deserts of the Middle East are regularly subjected to severe dehydration and possess a capacity to rapidly rehydrate by drinking large volumes of water. Urine flow, glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) in the fully hydrated animals amounted to 0.74±0.4 ml · min^–1, 76±29 ml · min^–1 and 344±146 ml · min^–1 respectively. In goats that were dehydrated to a loss of about 30% of their initial body weight, urine flow dropped to 24% of the value recorded in the hydrated animals and GFR and ERPF dropped to half their level recorded in the hydrated phase. Na and K⁺ excretion decreased in the water depleted goats and further decrease was recorded following drinking. Following drinking the urine flow, GFR and ERPF of the recently rehydrated goats dropped to below the rates recorded in the dehydrated animals. During the 3 h of the continuous recording that followed the drinking, all three rates did not exceed the predrinking level. Plasma renin activity amounted to 0.37±0.32 ng AI·ml^–1·h^–1 in the hydrated animals. In dehydrated ones it amounted to 4.8±2.8 ng AI·ml^–1·h^–1 and a further increase was recorded following drinking. Aldosterone in the hydrated goats was 5.5±4.3 ng% and increased to 13.9±2.3 ng% in the dehydrated animal and amounted to 20.1±5.5 ng% 2 h following drinking. It is concluded that the kidney in the Bedouin goat plays a major role in conserving both water and solutes, not only when deprived of water but also following its rapid rehydration.     

Decreased renal haemodynamic response to inhibition of nitric oxide synthase in subtotally nephrectomized rats

To assess the renal haemodynamic response to manipulations of the nitric oxide (NO) system, we examined subtotally nephrectomized (SNX) rats and control rats (CON) 28 days after their operation. Bolus infusions of the NO synthase inhibitor N ^G-nitro-l-arginine (l-NA) were given intravenously at doses of 2 mg/kg and 10 mg/kg. Blood pressure was measured intra-arterially, glomerular filtration rate was measured by inulin clearance and fractional changes in renal blood flow (RBF) were determined by a Doppler flow probe. Both doses of l-NA caused a similar and dose dependent increase in mean blood pressure in both SNX and CON rats. In contrast, the decrease in RBF and the increase in the renovascular resistance index (RVRI) was less in SNX rats as compared to CON rats (RBF = –70.1±2.2% of baseline vs –52.7±5.2%, P<0.01; RVRI = +177±9% of baseline vs +243±24%, P<0.05). These changes were not affected by autonomic blockade (hexamethonium), or by blockade of the angiotensin II receptor (Losartan). The exogenous NO donor sodium nitroprusside (0.5 and 1.5 g · kg^–1 · min^–1) lowered mean blood pressure to a similar degree in SNX and CON rats; in contrast, RVRI decreased less in SNX rats (86.9±9.2% of baseline) than in CON rats (68.2±4.6%, P<0.05). We conclude that the reaction of the renal vasculature to manipulations of the NO system is altered in the SNX rats. The data suggest that in the remnant kidney, renovascular resistance is less dependent on endogenous NO and the vascular bed is less sensitive to exogenous NO.     

Osmotic work across inner medullary collecting duct accomplished by difference in reflection coefficients for urea and NaCl

To demonstrate that osmotic work can be accomplished across the inner medullary collecting duct (IMCD) by the difference in refelection coefficients for urea and NaCl, phenomenological coefficients for urea and NaCl transport were determined in isolated segments of the hamster IMCD perfused in vitro. Arginine vasopressin at 100 U/ml increased urea permeability from 11.5±2.9 to 31.7±4.2×10^–7 cm² s^–1 in the middle IMCD but not in the upper IMCD. Urea transport in the middle IMCD consisted of two components, transport with saturable kinetics and simple passive diffusion. Permeability to Na⁺ was very low (2×10^–7 cm² s^–1). Reflection coefficients as measured by the equiosmolality method, with raffinose being a reference solute, were 0.87±0.05 and 0.71±0.04 for urea and 1.03±0.07 and 0.91±0.04 for NaCl in the upper and the middle IMCD, respectively. Reflection coefficient for urea in the middle IMCD was 0.68 when determined by the zero volume flux method. When the middle IMCD was perfused with bicarbonate Krebs-Ringer (BKR) solution containing 200 mmol/l urea, the replacement of urea in the bathing fluid with equiosmolal NaCl caused large volume flux (3.81±0.45 nl mm^–1 min^–1) associated with dilatation of intercellular space. The existence of vasopressin in the bath was essential for this phenomenon. This effect was inhibited by 5×10^–4 M phloretin in the bath, suggesting that the vasoressin-stimulated urea transport is responsible for this phenomenon. From these observations, we conclude that transport parameters of the middle IMCD are appropriate for accomplishment of osmotic work across this segment in the absence of physicochemical osmotic gradients.     

Renal handling ofl-histidine studied by continuous microperfusion and free flow micropuncture in the rat

Renal tubular reabsorption ofl-histidine (His) was measured in vivo et situ by continuous microperfusion and free flow micropuncture of single proximal convoluted tubules of the rat kidney. The reabsorption is shown to be saturable. A permeability coefficient (P) of <29 m² · s^–1, a maximum reabsorption rate (J _max) of 2.75±1.05>J _max>1.97±0.86 (SEM) nmol · m^–1 · s^–1 and an affinity constant (K _m) of 13.8±4.2>K _m>10.9±4.0 (SEM) mol · l^–1 (lower values forP=29 m² · s^–1, higher values forP=0) were calculated from the microperfusion data. Using these constants and taking backflux of His and water reabsorption into account a good fit with the concentration profile of His along the proximal tubule — measured by free flow micropuncture — was obtained.Varying the buffered pH-values of the perfusion fluids (5.0 or 7.4) influenced neither the active reabsorption nor passive permeability of His. This indicates that the charge of the imidazol group of His does not play a significant role in His reabsorption. Further experiments showed that the addition of 20 mmol · l^–1 l-arginine — a strong inhibitor of the reabsorption system for dibasic ammino acids — did not have a significant effect on the reabsorption ofl-histidine. It is concluded, therefore, that His is reabsorbed by a system for neutral amino acids. Non ionic diffusion does not play an important role for His reabsorption.Part of this work was presented at the 51st meeting of the German Physiological Society in Kiel, 1979 [15]     

Renal balance of pterin cofactors in the rat

In rat kidney, the rate of urinary biopterin [biopterin (B), 7,8-dihydrobiopterin (BH₂), and 5,6,7,8-tetrahydrobiopterin (BH₄)] excretion as measured by Crithidia assay, was found to be at least 3 times greater than the rate at which it is filtered (GFR·P_Biopt.). At a renal blood flow of 6.43±1.34 ml/min per g kidney wt., biopterin concentrations in arterial and renal venous blood and plasma were similar [art. blood: 250±50 ng/ml, ren. venous blood: 247.3±50.9 ng/ml, art. plasma: 23.1±5.8 ng/ml, ren. venous plasma: 23.4±6.9 ng/ml (means±S.D.)]. ¹⁴C-BH₄ and³H-inulin, infused by means of a micropump into late proximal segments of single nephrons at concentrations of 10^–4–10^–6 mol, were excreted at similar fractional rates (inulin: 0.85–0.97, BH₄: 0.87–0.92, total recovery 1.00–1.09 and 0.99–1.11, respectively). Similar results were obtained with 6,7-dimethylpterin, but not with 6,7-dimethyl-5,6,7,8-tetrahydropterin. The latter is reabsorbed at a fractional rate of 0.20 at concentrations of 10^–7 and 10^–6 mol.In microperfusion studies in isolated proximal tubular segments in vivo et situ, no reabsorption of 6,7-dimethylpterin could be detected. In case of 6,7-dimethyl-5,6,7,8-tetrahydropterin (DMPH₄) at concentrations of 5·10^–5 and 10^–5 mol/l, however, a permeability constant of 2.39·10^–5 cm/s has been measured.From the fact that more biopterin leaves the kidney with urine plus venous blood than entered it is concluded that reduced biopterin is synthesized de novo in the kidney. With the exception of DMPH₄, all types of biopterin are not significantly reabsorbed, but rather, are excreted into the urine due to an anisotropic permeability characteristic of the nephron.Supported by Deutsche Forschungsgemeinschaft     

Radioactive microsphere distribution and single glomerular blood flow in the normal rabbit kidney

Summary Intracortical distribution of blood flow was studied in the rabbit kidney with 15 m labelled microspheres (M) injected into the left ventricule. M injection did not alter renal function. Thanks to arterial filling of left kidney with silicone rubber, efferent vascular patterns of the glomeruli could be precisely identified. Glomeruli of different populations were sampled by microdissection and their radioactivity measured. Assuming that intracortical distribution of M reflected distribution of flow to the glomeruli, individual glomerular blood flows (GBF) were determined. In hydropenic rabbits, GBF was higher in deep glomeruli providing vasa recta (G4) (193±14 nl·min^–1) than in most other superficial (G1 and G2) and deep glomeruli (G3) (190±27, 113±10 and 127±9 nl·min^–1 respectively; . The exact significance of GBF found in superficial glomeruli with straight ascending efferent arterioles supplying aglomerular suscapsular cortex (G1) was questioned because of the possible axial streaming of spheres. Afferent medulary blood flow was calculated to represent 9.0±0.9% of total renal blood flow.Attaché de Recherches I.N.S.E.R.M.     

Localisation and characterisation of functional vasoactive intestinal peptide receptors in feline kidney

Specific ¹²⁵I-labelled vasoactive intestinal peptide (VIP) binding was determined in feline renal cortical and medullary plasma membranes. For the cortex, Scatchard analysis of the data resulted in a curvilinear plot with a high-affinity site K _0.5 of 8.4±2.6 nmol l^–1 (SE, n=6) and a second low-affinity site K _0.5 204 ± 16 nmol l^–1 with binding site concentrations (B _max) of 385±44.5 and 2710±181.3 fmol mg protein^–1 respectively. Conversely a similar analysis of the results obtained for outer medullary membranes gave a single site with a K _0.5 of 1.2±0.2 nmol l^–1 (SE, n=4) and B _max of 157.8±24.7 fmol mg^–1. Inner medullary membrane binding data. Gave a single site of lower affinity (K _0.5= 62.5±21.6 nmol l^–1; n=3). Structurally related peptides, glucagon and secretin, were ineffective (up to 1 mol l^–1) in displacing VIP from specific sites in both cortex and medulla. Porcine PHI 1–27 (a peptide having N-terminal histidine and C-terminal isoleucine) and a VIP antagonist [4-Cl-D-Phe⁶Leu¹⁷]VIP both displaced ¹²⁵I-VIP from cortical and medullary membrane binding sites with IC₅₀ values of 43.0 nmol l^–1 and 1.3 mol l^–1 (cortex) and 132.0 nmol l^–1 and 1.5 mol l^–1 (medulla) respectively. The localisation of specific VIP binding sites in feline kidney was investigated further by in vitro autoradiography. A high density of binding sites was visible at the cortico-medullary boundary as well as in the outer stripe of the outer medulla and in radial structures projecting into the cortex. There was a moderate density of binding sites in the superficial cortex. In addition the distribution of tubular VIP-sensitive adenylate cyclase was determined in microdissected nephron segments. In the presence of 10 mol l^–1 GTP, 1 mol l^–1 VIP effected marked stimulations over basal adenylate cyclase activity in medullary collecting tubules (4.7-fold), the bright and granular portions of the distal convoluted tubule (4.1- and 2.2-fold respectively) and in the pars recta of the proximal tubule (2.7-fold). Thus VIP-responsive adenylate cyclase has a discrete localisation in the feline nephron, which appears to correlate with specific binding sites as defined by autoradiography.     

Effects of parathyroid hormone and N6,O2′-dibutyryl cyclic AMP on Ca2+ transport across the rabbit distal nephron segments perfused in vitro

Effects on Ca²⁺ transport of parathyroid hormone (PTH) and N⁶,O²-dibutyryl adenosine 3,5-cyclic monophosphate (DB-cAMP) were examined in the rabbit distal nephron segments including the cortical thick ascending limb of Henle's loop (CAL), the connecting tubule (CNT) and the cortical collecting tubule (CCT) by the in vitro perfusion technique. When PTH (10^–8 mol·l^–1) was added to the bath, efflux of Ca²⁺ (pmol·mm^–1·min^–1) was increased from 6.29±1.46 to 7.96±1.66 (P<0.02) in the CAL, and from 8.55±1.30 to 13.73±1.24 (P<0.001) in the CNT, respectively, without changes in influx of Ca²⁺. The effect of PTH on Ca²⁺ transport in the CAL, however, was abolished when phosphate concentration in the medium was reduced from 3.0 to 1.0 mmol·l^–1. When DB-cAMP (10^–3 mol·l^–1) was added to the bath, efflux of Ca²⁺ was also increased from 7.01±0.83 to 9.40±0.82, (P<0.05) in the CAL, and from 13.11±0.89 to 19.74±0.52 (P<0.005) in the CNT, respectively. By contrast, neither PTH nor DB-cAMP affected efflux of Ca²⁺ in the CCT. PTH did not affect the transepithelial voltage either in the CAL or in the CNT. But in the CNT, DB-cAMP decreased the voltage from –14.1 to –9.4 mV. The response of adenylate cyclase activity to PTH in the collagenase treated isolated nephron segments was also examined. Significant increases in adenylase cyclase activity were observed in the CAL as well as in the CNT with 10^–6 mol·l^–1 PTH. These data indicate that PTH stimulates Ca²⁺ transport across the CNT probably via activation of the adenylate cyclase-cyclic AMP system. The hormone may also stimulate Ca²⁺ transport across the CAL in a special condition where plasma phosphate concentration is elevated.     

The influence of long-term infusion of the calcium antagonist diltiazem on postischemic acute renal failure in conscious dogs

Summary The influence of long-term infusion of the calcium-entry blocker diltiazem on postischemic acute renal failure was investigated in conscious dogs monitored by implanted instruments. In 18 uninephrectomized beagle dogs on a salt-rich diet, an electromagnetic flow probe and an inflatable plastic cuff were placed around the renal artery. Acute renal failure was induced by inflating the cuff for 180 min in the conscious animal. Group A (n=5, control) received an intraaortic injection of 0.9% NaCl (5 ml/day) from the 3rd day before until the 7th day after ischemia and group B (n=6, posttreatment) an intra-aortic injection of diltizem (5 µg·min^–1·kg^–1) beginning at the end of ischemia until the 7th day. Group C (n=7, pre- and posttreatment) received diltiazem from the 3rd day before until the 7th day after ischemia. In group A, renal blood flow dropped from 149±16 (preischemic) to 129±29 ml·min^–1 on the 1st day after ischemia. In contrast, renal blood flow increased on the 1st postischemic day in both treatment groups by 29±15% (group B,P 0.05) and 14±13% (group C). In the following days, there was no significant difference in renal blood flow between groups A, B and C. In group B, the reduction of the glomerular filtration rate was similar to that in the control group. In group C, the glomerular filtration rate was significantly less reduced than in group A (34±1.8 preischemically to 17±5.4 on day 1,P 0.05 and 20±4.1 ml·min^–1 on day 7,P 0.05). Plasma renin activity increased in both diltiazem groups, more pronounced so in group B (from 3.7±1.0 on day 1 to 16.2±7.9 ng ATI·ml^–1·h^–1 on day 7,P 0.05). In contrast to groups A and B, the increase in fractional sodium excretion was less pronounced in group C. Likewise, the decrease in free water-reabsorption was less marked than in groups A or B. It was apparent that diltiazem, when administered pre- and post-ischemically, preserved glomerular filtration rate and renal blood flow. When diltizem was given solely postischemically there was an improvement in renal blood flow, but no significant influence on glomerular filtration rate. We therefore conclude that mainly tubular factors, in addition to the attenuation of postischemic vasoconstriction, are involved in the protective effect of diltiazem on postischemic acute renal failure in conscious dogs.Abbreviations ARF acute renal failure - C_osmol clearance of osmolarity - E_Na urinary excretion rate of sodium - FE_Na fractional excretion rate of sodium - GFR glomerular filtration rate - HR heart rate - NE norepinephrine - PAM mean arterial blood pressure - PRA plasma renin activity - RBF renal blood flow - RVR renal vascular resistance - T_H2_O free water reabsorption - V_U urine volume     

Effect of lower-body positive pressure on postural fluid shifts in men

Summary To quantify the effect of 60 mm Hg lower-body positive pressure (LBPP) on orthostatic blood-volume shifts, the mass densities (±0.1 g· l^–1) of antecubital venous blood and plasma were measured in five men (27–42 years) during combined tilt table/antigravity suit inflation and deflation experiments. The densities of erythrocytes, whole-body blood, and of the shifted fluid were computed and the magnitude of fluid and protein shifts were calculated during head-up tilt (60°) with and without application of LBPP. During 30-min head-up tilt with LBPP, blood density (BD) and plasma density (PD) increased by 1.6±0.3 g · l^–1, and by 0.8±0.2 g · l^–1 (±SD) (N=9), respectively. In the subsequent period of tilt without LBPP, BD and PD increased further to +3.6±0.9 g · l^–1, and to +2.0±0.7 g · l^–1 (N=7) compared to supine control. The density increases in both periods were significant (p<0.05). Erythrocyte density remained unaltered with changes in body position and pressure suit inflation/deflation. Calculated shifted-fluid densities (FD) during tilt with LBPP (1006.0±1.1 g · l^–1,N=9), and for subsequent tilt after deflation (1002.8±4.1 g · l^–1,N=7) were different from each other (p<0.03). The plasma volume decreased by 6.0±1.2% in the tilt-LBPP period, and by an additional 6.4±2.7% of the supine control level in the subsequent postdeflation tilt period. The corresponding blood volume changes were 3.7±0.7% (p<0.01), and 3.5±2.1% (p<0.05), respectively. Thus, about half of the postural hemoconcentration occurring during passive head-up tilt was prevented by application of 60 mm Hg LBPP.H. Hinghofer-Szalkay was a European Space Agency fellow on leave from the Physiological Institute, Karl-Franzens-University, A-8010 Graz, Austria.     

H+ ion secretion in proximal tubule of low-CO2/HCO 3 − perfused isolated rat kidney

Acidification in proximal tubule of the isolated rat kidney, perfused in vitro, was studied by stopped-flow microperfusion techniques, using Sb microelectrodes to measure luminal pH. The kidney was perfused with mammalian Ringer's solution at pH 7.4 buffered by 20 mmol/l phosphate and containing 7.5 g/100 ml bovine albumin, equilibrated with air. Final urine pH was 6.88±0.5. Steady-state pH in proximal segments was 6.81±0.03 (n=80), and acidification half-time (t/2) 7.25±0.33 (80) s, giving a net secretory H⁺ ion flux of 0.51±0.05 nmol·cm^–2·s^–1. This flux was about 70% of in vivo (blood perfused kidneys). During luminal perfusion with solutions at pH 6.2, back-flux of H⁺ was 0.82 ±0.08 nmol·cm^–2·s^–1, with an alkalinizationt/2 of 6.33 ±0.34 (34) s. The difference between acidification and alkalinizationt/2 was not significant. This is compatible with a pump-leak system of H⁺ transport. The back flux of H from the lumen was markedly reduced in low Na⁺ perfused kidneys in the presence of 10^–4 mol/l amiloride in the lumen, indicating that this process is mediated by the luminal Na/H exchanger. Observations in the presence of high K levels suggest that it may have also a charged component. 10^–4 mol/l acetazolamide added to the kidney perfusate reduced acidification to 0.5% of control, and 10^–6 mol/l SITS to 25% of control. Thus, despite the lowpCO₂ (0.1–0.4 kPa, or 1–3 mm Hg), the CO₂/Hco ₃ ^– buffer system still plays an important role in tubular acidification in this preparation.