Harald Sonnenberg

Methode zur Durchstrmung einzelner Nephronabschnitte

ZusammenfassungEs wird eine Mikroperfusionsapparatur beschrieben, mit der konstante Perfusionsraten zwischen 10 und 35 · 10−6 ml/min eingestellt werden können. Mit Hilfe dieser Apparatur können Teile eines Nephrons in situ kontinuierlich durchströmt werden. Die angegebene Technik erlaubt Tubulusperfusionen, ohne daß Beimischungen von Glomerulumfiltrat das Perfusat verändern, so daß Messungen lokaler, tubulärer Transportraten von Elektrolyten und Nichtelektrolyten oder Permeabilitäten von Wasser und gelösten Substanzen möglich sind.

Salt-sensitive hypertension in ANP knockout mice: potential role of abnormal plasma renin activity.

Atrial natriuretic peptide (ANP), a peptide hormone produced by the heart, exerts a chronic hypotensive effect. Knockout mice with a homozygous disruption of the pro-ANP gene (-/-) are incapable of producing ANP and are hypertensive relative to their wild-type (+/+) siblings. Previous studies showed that arterial blood pressure (ABP) was further increased in conscious -/- mice kept for 2 wk on 2% salt, but not in anesthetized -/- mice after 1 wk on 8% salt. To determine whether inconsistencies in observed effects of salt on ABP of -/- mice are due to duration of increased salt intake and/or the state of consciousness of the animals, we measured ABP from an exteriorized carotid catheter during and after recovery from anesthesia with ketamine-xylazine in adult +/+ and -/- mice kept on low (LS; 0.008% NaCl)- or high (HS; 8% NaCl)-salt diets for 3-4 wk. Conscious ABP ± SE (mmHg) of +/+ mice did not differ significantly on either diet (HS, 113 ± 3; LS, 110 ± 5). However, on HS diet -/- mice had significantly higher ABP (135 ± 3; P < 0.001) than both -/- (115 ± 2) and +/+ (110 ± 5) mice on LS diet. Anesthesia decreased ABP in all groups, but the genotype- and diet-related differences were preserved. Plasma renin activity (PRA, ng ANG I ⋅ ml-1 ⋅ h-1) in blood collected at termination of experiment was appropriately different on the 2 diets in +/+ mice (HS, 4.9 ± 1.9; LS, 21 ± 2.8). However, PRA failed to decrease in -/- mice on HS diet (HS, 18 ± 2.9; LS, 19 ± 3.7). Independent of genotype, concentration of endothelin-1 (ET-1, pg/mg protein) and endothelial constitutive NOS (ecNOS, density/100 μg protein) was significantly elevated in kidneys of mice fed on HS diet (ET-1 -/-, 31 ± 4.7 and +/+, 32 ± 4.1; ecNOS -/-, 160 ± 19 and +/+, 156 ± 19) compared with mice fed on LS diet (ET-1 -/-, 19 ± 1.9 and +/+, 21 ± 1.8; ecNOS -/-, 109 ± 13 and +/+, 112 ± 18). We conclude that, regardless of the state of alertness, -/- mice develop salt-sensitive hypertension after prolonged feeding on HS, in part due to their inability to reduce PRA, whereas the specific renal upregulation of ecNOS and ET-1 in response to HS intake may be an ANP-independent adaptive adjustment aimed at improving kidney function and counteracting the pressor effect of salt.

Chronic regulation of arterial blood pressure in ANP transgenic and knockout mice: Role of cardiovascular sympathetic tone

OBJECTIVE Atrial natriuretic peptide (ANP) lowers arterial blood pressure (ABP) chronically, in association with vasodilation of the resistance vasculature. The mechanism mediating the chronic relaxant effect of ANP is likely indirectly mediated by interactions with tonic vasoeffector mechanisms, inasmuch as the resistance vasculature is relatively insensitive to direct cGMP-mediated relaxation by ANP. On the basis of evidence that ANP has widespread sympatholytic activity, the current study investigated whether the chronic hypotensive effect of ANP is mediated by attenuation of tonic cardiovascular sympathetic tone. METHODS Total plasma catecholamine concentration and changes in basal ABP and heart rate (HR) following autonomic ganglionic blockade were measured as indices of underlying sympathetic nerve activity in hypotensive ANP-overexpressing transgenic mice (TTR-ANP), hypertensive ANP knockout mice (-/-) and the genetically-matched wild type (NT and +/+, respectively) control mice. Pressor and chronotropic responses to norepinephrine infusion were measured in ganglion-blocked mice of all genotypes, and norepinephrine receptor binding was assessed in representative tissues of -/- and +/+ mice, in order to determine whether peripheral adrenergic receptor responsiveness is altered by ANP-genotype. RESULTS Basal ABP was significantly lower in TTR-ANP and higher in -/- compared to their wild-type controls. Basal HR did not differ significantly between mutant and control mice. Autonomic ganglionic blockade reduced ABP and HR in all genotypes, however, the relative decrease in ABP was significantly smaller in TTR-ANP and greater in -/- mice than in their respective controls. Total plasma catecholamine was significantly higher in -/- than in +/+ mice but did not differ significantly between TTR-ANP and NT mice. Norepinephrine infusion during ganglionic blockade elicited quantitatively similar pressor and chronotropic responses in mutant and control mice. Tissue norepinephrine binding did not differ significantly between -/- and +/+ mice. CONCLUSIONS The present study shows that differences in endogenous ANP activity in mice, resulting in chronic alterations in ABP are accompanied by directional changes in underlying cardiovascular sympathetic tone, and suggests that the chronic vasodilator effect of ANP is, at least partially, dependent on attenuation of vascular sympathetic tone, possibly at a prejunctional site(s).

Proximal tubular reabsorption of some organic acids in the rat kidney in vivo

SummaryUsing a microperfusion technique proximal tubular reabsorption of sulfamerazine, phenobarbital, p-aminohippuric acid, uric acid, sulfaurea and N4-acetylsulfamerazine was studied as a function of intratubular pH in the rat kidney in vivo. Transtubular permeabilities were related to in vitro lipoid solubility measurements and degrees of non-dissociation of the acids. It was found that1.sulfamerazine and phenobarbital exhibit nonionic backdiffusion, the magnitude of which is dependent on lipoid solubility,2.p-aminohippuric acid, uric acid and sulfaurea do not show this backdiffusion, a finding explained by lack of lipoid solubility,3.N4-acetylsulfamerazine, despite relatively high lipoid solubility, is not reabsorbed in the proximal tubule.

A Humoral Component of the Natriuretic Mechanism in Sustained Blood Volume Expansion

A natriuretic and diuretic response to whole blood infusion in the rat, exaggerated and sustained by intravenous reinfusion of excreted urine, was shown to be associated with increased glomerular filtration and reduced tubular reabsorption. Cross-circulation of animals so responding (donor rats) with isovolemic recipients led to a modest natriuretic and diuretic response in the latter, not accounted for by altered physical composition of the blood nor by observed changes in filtration rate or arterial blood pressure. The recipient natriuresis was unchanged when nephrectomized donors were used and it occurred in experiments in which donor urine was simultaneously replaced by intravenously infused Ringer-Locke solution; the natriuretic property of the cross-circulating blood could therefore not have been due to reinfusion of urinary constituents, nor to accumulation of metabolites, nor to a factor of renal origin. A recipient natriuresis was also observed when the expanded and urine reinfused donor had been acutely adrenalectomized, ruling out an altered secretion of adrenal cortical or medullary hormones as a principal cause of this natriuresis; the data, however, do not exclude participation of reduced aldosterone secretion in the normal effector mechanism. In control experiments in which whole blood was exchanged for donor blood, a small delayed natriuresis did occur in the recipient; this could be completely prevented by administration of aldosterone. In similar exchange experiments with adrenalectomized donors, a small natriuresis developed in the recipient before blood administration but declined afterwards. These minor natriuretic effects probably resulted from altered mineralocorticoid content of the cross-circulating blood due to factors other than blood volume change. The larger natriuretic response seen in all recipients when the donor was volume expanded must have been due largely to a humoral natriuretic factor of other than renal or adrenal origin.

Chronic hypertension in ANP knockout mice: contribution of peripheral resistance.

Atrial Natriuretic Peptide (ANP) exerts a chronic hypotensive effect which is mediated by a reduction in total peripheral resistance (TPR). Mice with a homozygous disruption of the pro-ANP gene (-/-) fail to synthesize ANP and develop chronic hypertension in comparison to their normotensive wild-type (+/+) siblings. In order to determine whether alterations in basal hemodynamics underlie the hypertension associated with lack of endogenous ANP activity, we used anesthetized mice to measure arterial blood pressure (ABP) and heart rate (HR), as well as cardiac output (CO) by thermodilution technique. -/- (n = 7) and +/+ (n = 10) mice of comparable weight and age were used. Stroke volume (SV) and TPR were derived from CO, HR, and ABP by a standard formula. ABP (mm Hg) was significantly higher in -/- (132+/-4) (P < 0.0001) than in +/+ mice (95+/-2). CO (ml min(-1)), HR(beats min(-1))and SV (microl beat(-1)) did not differ significantly between -/- and +/+ mice (CO -/- = 7.3+/-0.5, +/+ = 8.3+/-0.6; HR -/- = 407+/-22, +/+ = 462+/-21; SV -/- = 17.6+/-1.1, +/+ = 17.6+/-1.7). However, TPR (mm Hg ml(-1) min(-1)) was significantly elevated in -/- mice (18.4+/-0.7) compared to +/+ mice (12.3+/-1) (P = 0.0003). Autonomic ganglion blockade with a mixture of hexamethonium and pentolinium was followed by comparable percent reductions in CO (-/- = 28+/-4, +/+ = 29+/-3), HR (-/- = 9+/-4, +/+ = 16+/-4) and SV(-/- = 21+/-4, +/+ = 15+/-6) in both genotypes. However, the concomitant decrease in ABP (%) in -/- (41+/-2) was significantly greater than in +/+ (23+/-4) mice (P = 0.0009) and was accompanied by a significant reduction in TPR. We conclude that the hypertension associated with lack of endogenous ANP is due to elevated TPR, which is determined by an increase in cardiovascular autonomic tone.

Effects of furosemide, acetazolamide, and mannitol on medullary collecting-duct function in the rat kidney

The microcatheterization technique was used to study transport of fluid, sodium, and potassium in the medullary collecting duct in rats (232–357 g) before and during administration of diuretic drugs. Series I received furosemide (1.5 mg initial dose + 1.5 mg/h), Series II acetazolamide (0.75 mg +0.75 mg/h), and Series III and IV mannitol at 0.3 g+0.3 g/h, and 0.3 g+1.0 g/h, respectively. All diuretics caused diuresis, natriuresis, and kaliuresis. The renal response to furosemide and acetazolamide was associated with increased hematocrit and decreased filtration rate, indicating depletion of blood volume. No such effect was seen in Series III and IV. In the collecting duct furosemide completely abolished the normal reabsorption of sodium and fluid and initiated net secretion of potassium. Partial inhibition of salt and water transport in the collecting duct was observed with acetazolamide and the low dose of mannitol (III). Both treatments resulted in potassium secretion. In Series IV the high rate of mannitol infusion was associated with complete inhibition of salt and water reabsorption from the medullary collecting system similar to that of Series I. The greater excretory response in this group compared to the furosemide series was due to increased delivery of tubular load to the collecting duct. It is concluded that a major site of action of furosemide is in the medullary duct, resulting in quantitative inhibition of salt and water reabsorption from this nephron segment. The partial transport inhibition during acetazolamide or modest mannitol diuresis can be explained by the presence of poorly absorbable solute in duct fluid. The mechanism of inhibition of reabsorption after the high rate of mannitol infusion remains undetermined.

Potassium Depletion and Salt-Sensitive Hypertension in Dahl Rats: Effect on Calcium, Magnesium, and Phosphate Excretions

Weanling male inbred Dahl rats (Jr salt-sensitive (S) and salt-resistant (R) strains) were placed on high (4%, HK) and low (0.2%, LK) potassium diets for 4 weeks. Both diets contained 8% sodium chloride, 2.5% calcium, 0.8% magnesium, and 2.0% phosphorous. Balance studies were carried out during the final week on the diets. Mean arterial blood pressure was determined, and dietary intake and urinary output of water, sodium, chloride, potassium, calcium, magnesium, and phosphate were monitored daily during this period. The data show that blood pressures of S rats were significantly higher than those of R rats on both HK and LK diets; however, reduced dietary potassium was associated with increased blood pressure in both strains. Urinary excretions of calcium and magnesium were higher, and urinary phosphate excretion was lower, in S compared to R rats. Decreased potassium intake was associated with increased excretion of calcium, magnesium and phosphate in both strains. The changes in calcium and magnesium excretion were significantly correlated to blood pressure across strains and diets. We conclude that the effects of a high salt diet on increasing blood pressure can be potentiated by lack of potassium, even in previously salt-resistant rats. Increased blood pressure is associated with increased divalent cation excretion. It is not yet known whether this is a cause-and-effect relationship.

Effect of adrenalectomy on medullary collecting-duct function in rats before and during blood volume expansion.

SummaryAdrenalectomized rats, kept on tap water for 3 days and infused with the glucocorticoid dexamethasone during the experiment, were compared to similarly treated sham-operated rats. Using the microcatheterization technique, reabsorption of fluid, sodium and potassium in the medullary collecting duct was studied before and after infusion of donor blood (2.3% of body weight). Before intravascular volume expansion sodium excretion in adrenalectomized rats was greater than in sham-operated ones. Extensive overlap between the two groups made this difference not statistically significant. However, the fraction of filtered sodium excreted was significantly greater after adrenalectomy, indicating the expected tubular transport defect. Fluid reabsorption from the medullary collecting-duct system was comparable in both series. Adrenalectomy did not inhibit net sodium reabsorption from the inner medullary duct, although reduction of Na transport in the outer medullary collecting system could be inferred. Renal excretion of potassium was not associated with net K secretion in the collecting duct in either group. During hypervolemia induced by intravenous infusion of donor blood, marked diuresis, natriuresis and kaliuresis were observed in all animals, associated with inhibition of net fluid and sodium reabsorption along the collecting system in both inner and outer medulla. Small, but statistically significant secretion of potassium became evident. The relatively reduced renal response in adrenalectomized animals could be attributed in part to a decreased filtered load compared to sham-operated rats. It is concluded: (1) that lack of mineralocorticoid does not prevent the normal fluid and sodium reabsorption from the lumen of the inner medullary collecting system, and (2) that the inhibition of this reabsorption consequent to hypervolemia is independent of changes in plasma aldosterone levels.

Regulation Of Sodium, Calcium And Vitamin D Metabolism In Dahl Rats On A High‐Salt/Low‐Potassium Diet: Genetic And Neural Influences

1. A dietary combination of high salt and low potassium (HSLK) exacerbates hypertension in Dahl salt‐sensitive (DS) rats and renders previously normotensive Dahl salt‐resistant (DR) rats hypertensive. In both strains, the severity of hypertension correlates with urinary calcium loss. However, the magnitude of excretory calcium losses is significantly greater in DS rats and is potentiated by chemical sympathectomy in both strains.