Limits to sustained energy intake. II. Inter-relationships between resting metabolic rate, life-history traits and morphology in Mus musculus.

Links between resting metabolic rate (RMR) and reproductive output have been previously sought at both inter- and intraspecific levels, but have only been found in some interspecific studies. We aimed to examine correlations between RMR measured both prior to breeding and at peak lactation with litter size and litter mass in Mus musculus. By manipulating the litter size of some females at birth, we aimed to establish the direction of causality in any correlation between litter size and RMR. Correlations between maternal morphology and RMR, litter size and litter mass were also examined. Neither pre-breeding RMR nor mass-independent pre-breeding RMR was correlated with litter size or litter mass. RMR at peak lactation, however, was positively correlated with litter size and negatively correlated with mean pup mass. After correcting for the effects of body mass, residual peak lactation RMR was not correlated with litter size or litter mass. Body size was the major morphological variable influencing litter mass, offspring mass and asymptotic food intake. Mammary tissue mass was correlated with litter size when only the data for mice raising unmanipulated litters were used. RMR at peak lactation was significantly related to the principal component of morphology dominated by carcass mass. This study confirms the findings of previous intraspecific and some interspecific studies that found no correlation between RMR and reproductive output after the effects of body mass had been removed.

Limits to sustained energy intake. I. Lactation in the laboratory mouse Mus musculus.

Laboratory mice (strain MF1) were used to determine whether sustainable rates of energy intake are limited during lactation. Mice raising natural-sized litters (N=71) reached an asymptote in their daily food intake between days 13 and 16 of lactation at 23.1 g x day(-1) and also between litter sizes of 9 and 15 pups (22.8 g x day(-1)). A second group of 37 females had their litter sizes manipulated at birth to raise more or fewer offspring than they gave birth to. When the litter size was increased, females did not increase their food intake to match their new litter size. However, when litter size was decreased, females decreased their asymptotic daily food intake during late lactation in relation to the extent of reduction in litter size. Therefore, it appeared that females were limited during late lactation and with large litter sizes. The milk energy exported amounted to 44% of the gross energy intake, and the estimated daily energy expenditure was therefore considerably lower than the sustained energy intake [8.0xRMR(gross), 6.6xRMR(assimilated)], and averaged 3.1xRMR, where RMR is resting metabolic rate. It was not possible to determine whether the apparent limit on sustained energy intake was acting centrally or peripherally because of the asymptotes in both food intake and milk energy output with increasing litter size.

Limits to sustained energy intake. III. Effects of concurrent pregnancy and lactation in Mus musculus.

To determine whether mice were limited in their capacity to absorb energy during late lactation, we attempted to increase the energy burden experienced by a group of female mice during late lactation by mating them at the postpartum oestrus, hence combining the energy demands of pregnancy and lactation. These experimental mice were therefore concurrently pregnant and lactating in their first lactation, and were followed through a normal second lactation. In a control group, females also underwent two lactations but sequentially, with the second mating after the first litter had been weaned. Maternal mass and food intake were measured throughout the first lactation, second pregnancy and second lactation. Maternal resting metabolic rate (RMR) was measured prior to the first mating and then at the peak of both the first and second lactations. Litter size and litter mass were also measured throughout both lactations. In the first lactation, experimental mice had a lower mass-independent RMR (F1,88=5.15, P=0.026) and raised significantly heavier pups (t=2.77, d.f.=32, P=0.0093) than the control mice. Experimental mice delayed implantation at the start of the second pregnancy. The extent of the delay was positively related to litter size during the first lactation (F1,19=4.58, P=0.046) and negatively related to mean pup mass (F1,19=5.78, P=0.027) in the first lactation. In the second lactation, the experimental mice gave birth to more (t=2.75, d.f.=38, P=0.0092) and lighter (t=-5.01, d.f.=38, P<0.0001) pups than did the controls in their second lactation. Maternal asymptotic daily food intake of control mice in the second lactation was significantly higher (t=-4.39, d.f.=37, P=0.0001) than that of the experimental mice and higher than that of controls during their first lactation. Despite the added burden on the experimental females during their first lactation, there was no increase in their food intake, which suggested that they might be limited by their capacity to absorb energy. However, control females appeared to be capable of increasing their asymptotic food intake beyond the supposed limits estimated previously, suggesting that the previously established limit was not a fixed central limitation on food intake. As RMR increased in parallel with the increase in food intake during the second lactation of control mice, the sustained energy intake remained at around 7.0xRMR.

Suppression of inducible nitric oxide generation by agmatine aldehyde: beneficial effects in sepsis.

The induction of inducible nitric oxide synthase (iNOS) serves an important immuno-protective function in inflammatory states, but ungoverned nitric oxide (NO) generation can contribute to a number of pathologic consequences. Delineation of the mechanisms that can downregulate iNOS-generated NO in inflammation could have therapeutic relevance. Here we show that agmatine, a metabolite of arginine, inhibits iNOS mediated nitric oxide generation in cytokine stimulated cell culture preparations. This effect was not cell type specific. Increased diamine oxidase (DAO) and decreased aldehyde dehydrogenase (AldDH) activities are also representative of inflammatory settings. Increasing the conversion of agmatine to an aldehyde form by addition of purified DAO or suppression of aldehyde breakdown by inhibition of AldDH activity increases the inhibitory effects of agmatine in an additive fashion. Inhibitors of DAO, but not monoamine oxidase (MAO), decreased the inhibitory effects of agmatine, as did the addition of AldDH or reacting aldehydes with phenylhydrazine. We examined rats given lipopolysaccharide (LPS) to evaluate the potential effects of agmatine in vivo. Endotoxic rats administered agmatine prevented the decreases in blood pressure and renal function normally associated with sepsis. Agmatine treatment also increased the survival of LPS treated mice. Our data demonstrate the capacity of agmatine aldehyde to suppress iNOS mediated NO generation, and indicate a protective function of agmatine in a model of endotoxic shock. How agmatine may aid in coordinating the early NO phase and the later repair phase responses in models of inflammation is discussed.

Polyamine transport system mediates agmatine transport in mammalian cells.

Agmatine is a biogenic amine with the capacity to regulate a number of nonreceptor-mediated functions in mammalian cells, including intracellular polyamine content and nitric oxide generation. We observed avid incorporation of agmatine into several mammalian cell lines and herein characterize agmatine transport in mammalian cells. In transformed NIH/3T3 cells, agmatine uptake is energy dependent with a saturable component indicative of carrier-mediated transport. Transport displays an apparent Michaelis-Menten constant of 2.5 microM and a maximal velocity of 280 pmol x min(-1) x mg(-1) protein and requires a membrane potential across the plasma membrane for uptake. Competition with polyamines, but not cationic molecules that utilize the y+ system transporter, suppresses agmatine uptake. Altering polyamine transporter activity results in parallel changes in polyamine and agmatine uptake. Furthermore, agmatine uptake is abrogated in a polyamine transport-deficient human carcinoma cell line. These lines of evidence demonstrate that agmatine utilizes, and is dependent on, the polyamine transporter for cellular uptake. The fact that this transport system is associated with proliferation could be of consequence to the antiproliferative effects of agmatine.

Ornithine decarboxylase, kidney size, and the tubular hypothesis of glomerular hyperfiltration in experimental diabetes.

In early diabetes, the kidney grows and the glomerular filtration rate (GFR) increases. This growth is linked to ornithine decarboxylase (ODC). The study of hyperfiltration has focused on microvascular abnormalities, but hyperfiltration may actually result from a prior increase in capacity for proximal reabsorption which reduces the signal for tubuloglomerular feedback (TGF). Experiments were performed in Wistar rats after 1 week of streptozotocin diabetes. Kidney weight, ODC activity, and GFR were correlated in diabetic and control rats given difluoromethylornithine (DFMO; Marion Merrell Dow, Cincinnati, Ohio, USA) to inhibit ODC. We assessed proximal reabsorption by micropuncture, using TGF as a tool for manipulating single-nephron GFR (SNGFR), then plotting proximal reabsorption versus SNGFR. ODC activity was elevated 15-fold in diabetic kidneys and normalized by DFMO, which also attenuated hyperfiltration and hypertrophy. Micropuncture data revealed an overall increase in proximal reabsorption in diabetic rats too great to be accounted for by glomerulotubular balance. DFMO prevented the overall increase in proximal reabsorption. These data confirm that ODC is required for the full effect of diabetes on kidney size and proximal reabsorption in early streptozotocin diabetes and are consistent with the hypothesis that diabetic hyperfiltration results from normal physiologic actions of TGF operating in a larger kidney, independent of any primary malfunction of the glomerular microvasculature.

Identification of human TGF-beta1 signal (leader) sequence polymorphisms by PCR-RFLP.

Links between disease susceptibility and genetically determined variation in human cytokine expression have recently been described. This has led to a demand for simple methods of identifying cytokine gene polymorphisms of potential clinical relevance. Here, we describe a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method for identifying two human transforming growth factor beta1 (TGF-beta1) signal (leader) sequence polymorphisms, T869C (Leu10Pro) and G915C (Arg25Pro). This permits simple and robust identification of TGF-beta1 leader sequence genotypes and demonstrates the physical linkage in cis between T869C (Leu10Pro) and G915C (Arg25Pro). The method does not require previously genotyped standards. The efficacy of enzyme digestion is internally controlled by the presence of conserved restriction sites.

Temporal adjustment of the juxtaglomerular apparatus during sustained inhibition of proximal reabsorption.

Tubuloglomerular feedback (TGF) stabilizes nephron function by causing changes in single-nephron GFR (SNGFR) to compensate for changes in late proximal flow (VLP). TGF responds within seconds and reacts over a narrow range of VLP that surrounds normal VLP. To accommodate sustained increases in VLP, TGF must reset around the new flow. We studied TGF resetting by inhibiting proximal reabsorption with benzolamide (BNZ; administered repeatedly over a 24-hour period) in Wistar-Froemter rats. BNZ acutely activates TGF, thereby reducing SNGFR. Micropuncture was performed 6-10 hours after the fourth BNZ dose, when diuresis had subsided. BNZ caused glomerular hyperfiltration, which was prevented with inhibitors of macula densa nitric oxide synthase (NOS). Because of hyperfiltration, BNZ increased VLP and distal flow, but did not affect the basal TGF stimulus (early distal salt concentration). BNZ slightly blunted normalized maximum TGF response and the basal state of TGF activation. BNZ sensitized SNGFR to reduction by S-methyl-thiocitrulline (SMTC) and caused the maximum TGF response to be strengthened by SMTC. Sensitization to type I NOS (NOS-I) blockers correlated with increased macula densa NOS-I immunoreactivity. Tubular transport measurements confirmed that BNZ affected TGF within the juxtaglomerular apparatus. During reduced proximal reabsorption, TGF resets to accommodate increased flow and SNGFR through a mechanism involving macula densa NOS.

Converting enzyme inhibition and the glomerular hemodynamic response to glycine in diabetic rats.

GFR normally increases during glycine infusion. This response is absent in humans and rats with established diabetes mellitus. In diabetic patients, angiotensin-converting enzyme inhibition (ACEI) restores the effect of glycine on GFR. To ascertain the glomerular hemodynamic basis for this effect of ACEI, micropuncture studies were performed in male Wistar-Froemter rats after 5 to 6 wk of insulin-treated streptozotocin diabetes. The determinants of single-nephron GFR (SNGFR) were assessed in each rat before and during glycine infusion. Studies were performed in diabetics, diabetics after 5 d of ACEI (enalapril in the drinking water), and weight-matched controls. Diabetic rats manifest renal hypertrophy and glomerular hyperfiltration but not glomerular capillary hypertension. ACEI reduced glomerular capillary pressure, increased glomerular ultrafiltration coefficient, and did not mitigate hyperfiltration. In controls, glycine increased SNGFR by 30% due to increased nephron plasma flow. In diabetics, glycine had no effect on any determinant of SNGFR. In ACEI-treated diabetics, the SNGFR response to glycine was indistinguishable from nondiabetics, but the effect of glycine was mediated by greater ultrafiltration pressure rather than by greater plasma flow. These findings demonstrate that: (1) The absent response to glycine in established diabetes does not indicate that renal functional reserve is exhausted by hyperfiltration; and (2) ACEI restores the GFR response to glycine in established diabetes, but this response is mediated by increased ultrafiltration pressure rather than by increased nephron plasma flow.

Nitric oxide and tubuloglomerular feedback.

Glomerular filtration is closely coupled to tubular reabsorption by a system of tubuloglomerular feedback (TGF). TGF operates within the juxtaglomerular apparatus (JGA) of each nephron, where changes are sensed in the salt content of fluid at the luminal macula densa and that information is transmitted to the glomerular microvasculature to elicit compensatory changes in single nephron glomerular filtration rate (GFR). Type I nitric oxide synthase (NOS) is expressed in the macula densa. Other NOS isoforms may be produced in the mesangium, and glomerular microvessels. These NOSs are strategically positioned to influence each step of the TGF process. However, micropuncture experiments using NOS antagonists have shown that nitric oxide (NO) does not mediate TGF. Instead, local NOS blockade causes the curve that represents TGF to shift leftward and become more steep. Changes in macula densa NO production may underlie the resetting of TGF, which is required in order to keep the TGF curve aligned with ambient tubular flow as tubular flow changes to accommodate physiologic circumstances. Also, macula densa NO production may be substrate limited and dissociated from NOS protein content. The importance of NO to TGF resetting and the substrate dependence of NO production have both been found during changes in dietary salt.

Absorption of visible spectrum radiation by the wing membranes of living pteropodid bats.

The wing membranes of bats present a large surface area upon which radiation might be taken up, increasing heat load to the animals. This, combined with the high amount of heat produced during flight, has been advanced as one hypothesis explaining the fact that bats are almost exclusively nocturnal. The proportion of short-wave (visible) radiation absorbed by bat wing membrane has previously been measured at between 0.7 and 0.92. These measurements were made on pieces of membrane taken from the wings of dead, mainly insectivorous bats from temperate regions. Here we examined the amount of light transmitted through and reflected off the wing membranes of four species of live pteropodid bats. There were significant differences in wing reflection between species. At 0.68, the average proportion of light absorbed into the wing membranes was lower than previously reported. This might be because we worked with live animals or because ours were tropical bats which are routinely exposed to tropical sun when roosting. Variation in wing tension strongly affected light absorption. It was predicted that the relaxed state of wing membrane through part of the wing beat cycle would increase the absorption of light into the wings of day-flying bats. The proportion of light absorbed into wings was shown to be an important factor in the heat balance of hypothetical bats flying during the day. Our results raise the predicted temperature at which bats flying during the day might experience hyperthermia by approximately 2 degrees C and suggest that variation in albedo of wings between species may make some species more susceptible to overheating than others.

Resetting protects efficiency of tubuloglomerular feedback.

Tubuloglomerular feedback (TGF) may effect long-term protection of total body salt and water or may govern minute-to-minute autoregulation of renal function. The task for which TGF is best suited depends on the orientation of ambient tubular flow relative to the inflection point of the TGF curve and on the tendency of TGF to reset in response to prolonged stimulation. Current data suggest that the TGF curve is coupled closely to ambient flow in individual nephrons such that the system is capable of compensating both negative and positive perturbations in tubular flow. This coupling is mediated by events within the juxtaglomerular apparatus that cause the TGF curve to reset laterally in response to sustained shifts in tubular flow. This resetting of TGF occurs within 30 to 60 minutes of an applied stimulus, suggesting that TGF is better suited to mediate dynamic autoregulation than to account for sustained vasoconstriction during proximal tubular injury.

Reduced proximal reabsorption resets tubuloglomerular feedback in euvolemic rats.

Inhibition of renal carbonic anhydrase reduces proximal reabsorption and activates tubuloglomerular feedback (TGF). The TGF response is saturable, with highest gain focused near the natural flow rate. Therefore, any large change imposed on ambient tubular flow should reduce the TGF response to subequent flow perturbations. However, TGF tends to align with ambient flow regardless of the rate of ambient flow, suggesting that TGF resets to accommodate changes in flow while maintaining feedback efficiency. We used micropuncture and videometric flow velocitometry to test for TGF resetting in free-flowing nephrons during systemic infusion of the carbonic anhydrase inhibitor benzolamide (BNZ, 5 mg x kg(-1) x h(-1)) in euvolemic rats. Late proximal flow (V(LP)) and the fractional compensation (C) of TGF for perturbations in V(LP) were assessed repeatedly before and during BNZ. Early on, BNZ reduced C, consistent with TGF saturation. Over the next 45-60 min, V(LP) increased gradually by approximately 5 nl/min as C recovered to pre-BNZ levels. BNZ also increased V(LP) by approximately 5 nl/min when TGF was rendered inoperative by intratubular wax block, but this increase occurred rapidly. These data demonstrate rightward resetting of TGF during reduced proximal reabsorption.

Effect of chronic salt loading on kidney function in early and established diabetes mellitus in rats.

Glomerular hyperfiltration and renal hypertrophy are among the events that characterize the early course of diabetes mellitus in rats and human patients. Previous studies from this laboratory demonstrated that salt restriction paradoxically reduces total renal vascular resistance (RVR) and increases glomerular filtration rate (GFR) in diabetic rats (J Am Soc Nephrol 1995;5:1761-7). In the present study we examined the converse condition by testing the effects of chronic salt loading on kidney function in moderately hyperglycemic insulin-treated rats with early and established streptozotocin diabetes. Salt loading was accomplished by adding 1% NaCl to the drinking water 1 day or 35 days after diabetes was induced. The high-salt diet appropriately increased salt excretion in diabetic rats and nondiabetic controls. GFR and renal plasma flow were determined by inulin and para-amino hippuric acid (PAH) clearance 7 days after salt loading was started. Diabetic rats receiving tap water exhibited hyperfiltration with no change in renal blood flow (RBF). In nondiabetic rats, salt loading caused a reduction in total RVR and proportional increases in RBF, GFR, and kidney weight (KW). Salt loading in early diabetes did not affect RVR, RBF, or KW and caused a paradoxical reduction in GFR. In established diabetes, salt loading reduced RVR and increased RBF, similar to results in nondiabetic rats, but as in rats with early diabetes, it did not increase GFR or KW. In summary, although the response in RVR and RBF to chronic salt loading depends on the duration of diabetes, the increase in GFR and KW as seen in nondiabetic rats is blunted in the early and established state of insulin-treated diabetes in rats. These findings further support the notion that the renal response to variation in salt intake is altered in insulin-treated diabetes in rats.

Chest tube removal after cardiac surgery.

Prompt remove of chest tubes by RNs has allowed earlier and more aggressive ambulation of our patients and, along with other interventions, has decreased length of stay by 1.5 days while improving quality of care. Proper education, both didactic and clinical, is the key component in preparing RNs to safely and effectively perform this procedure.

Validation of the labeled bicarbonate technique for measurement of short-term energy expenditure in the mouse.

The energy expenditure of free-living animals has been studied extensively by the doubly-labeled water (DLW) technique. This method provides a reasonably accurate estimate of daily energy needs. However, there is considerable interest in the energy demands of animals over much shorter timescales, for which the DLW technique is less useful. We examined the possibility of measuring the expenditure of small animals over these shorter timescales from the washout kinetics of a bolus dose of 13C labeled bicarbonate. The study involved 19 laboratory mice which were injected either i.p. or s.c. with 0.2 ml of 13C labeled bicarbonate in water. Mice were placed in a standard respirometry system, maintained at different temperatures to precipitate a 3 fold variation in metabolism. Samples of breath were collected from the chamber into vacutainers at one minute intervals for approximately 40 minutes to an hour. Samples were analyzed by admission to a mass spectrometer (VG Optima) via a GC interface which identified and admitted the CO2 peak. The log converted isotope elimination was linear (r2 > 98% in all cases) indicating a single pool was involved. We evaluated the pool size from a dilution series of the injectate in equilibrium with CO2 gas. Conventional compartmental analysis produced an estimate which on average across the 19 individuals provided a reasonable estimate of the CO2 production. Individual estimates were however imprecise and the overall correlation between isotope and calorimeter estimates had an r2 of only 15%. Reasons for this discrepancy are unclear. Nevertheless an empirical model, using the elimination gradient, pool size and route of isotope administration as predictors explained 86% of the variation in CO2 production. Elimination of a bolus dose of 13C labeled bicarbonate provides a useful tool for estimating the energy metabolism of mice over intervals between 15 and 40 minutes.

Renal response to blood pressure elevation in normal and glomerulonephritic rats.

Concurrent renal disease appears to augment greatly the adverse effects of systemic hypertension on renal function and the development of glomerulosclerosis. This study examined the effects of systemic hypertension and treatment of hypertension in groups of normal non-nephritic rats and rats submitted to 16 wk of glomerulonephritis induced by the administration of anti-glomerular basement membrane antibody. Hypertension was produced by application of a clip to the right renal artery and blood pressure was treated with an angiotensin-converting enzyme (ACE) inhibitor, quinapril. Glomerulosclerosis of two types developed: a diffuse type that is characteristic of anti-glomerular basement membrane glomerulonephritis, and a focal segmental glomerulosclerosis that is characteristic of systemic hypertension. Glomerulonephritis significantly reduced the capacity of ACE inhibitors to decrease systolic blood pressure in awake animals. In addition, glomerulonephritis produced significant effects on plasma angiotensin II concentrations, whereby ACE inhibition no longer lowered plasma angiotensin II levels and in fact produced an increase. Glomerular capillary hydrostatic pressure and hydrostatic pressure gradient correlated with systolic blood pressure and with the incidence of focal glomerulosclerosis in non-nephritic rats. However, in glomerulonephritis, systolic blood pressure no longer correlated with glomerular capillary pressure, and glomerular capillary pressure no longer correlated with the development of glomerulosclerosis, although systolic blood pressure did correlate with the degree of focal segmental glomerulosclerosis. Concurrent glomerulonephritis strongly conditions the effects of superimposed hypertension by altering the relationship between systemic blood pressure and glomerular capillary hydrostatic pressure and by decreasing the response of hypertension to therapy.

Increased tubular flow induces resetting of tubuloglomerular feedback in euvolemic rats.

As single-nephron glomerular filtration rate (SNGFR) and late proximal flow (VLP) increase during growth or following volume expansion, the tubuloglomerular feedback (TGF) function (defined as the decrement in SNGFR due to the process of TGF) shifts rightward in the plane defined by VLP and SNGFR as required to maintain the homeostatic efficiency of TGF. It is not known whether this resetting of TGF requires changes in the systemic hormonal milieu or results from prolonged activation of TGF itself. We employed micropuncture and videometric flow velocitometry (an optical technique for measuring flow in unobstructed nephrons) to address this issue in Inactin-anesthetized euvolemic rats. The fractional compensation (C) of TGF for perturbations [late proximal flow perturbation (VH) = +/- 5 nl/min] in VLP was assessed repeatedly before and during a sustained increase in flow imposed by adding 20 nl/min to early proximal flow (VEP). Augmenting VEP initially saturated TGF, thus suppressing C. Over the next 30 min, C recovered to 70% of its original value, suggesting a rightward resetting of the TGF function to match the increase imposed on VLP. Resetting was confirmed by documenting an evolving asymmetry of C about VH = 0 by testing C vs. VH for -12 < or = VH < or = 12 in increments of 4 nl/min. Beyond 30 min of augmented VEP, C gradually declined due to desensitization of TGF. A sustained increase in VLP is sufficient to include TGF resetting, independent of any change in the systemic neurohumoral milieu.

Agmatine, a bioactive metabolite of arginine. Production, degradation, and functional effects in the kidney of the rat.

Until recently, conversion of arginine to agmatine by arginine decarboxylase (ADC) was considered important only in plants and bacteria. In the following, we demonstrate ADC activity in the membrane-enriched fraction of brain, liver, and kidney cortex and medulla by radiochemical assay. Diamine oxidase, an enzyme shown here to metabolize agmatine, was localized by immunohistochemistry in kidney glomeruli and other nonrenal cells. Production of labeled agmatine, citrulline, and ornithine from [3H]arginine was demonstrated and endogenous agmatine levels (10(-6)M) in plasma ultrafiltrate and kidney were measured by HPLC. Microperfusion of agmatine into renal interstitium and into the urinary space of surface glomeruli of Wistar-Frömter rats produced reversible increases in nephron filtration rate (SNGFR) and absolute proximal reabsorption (APR). Renal denervation did not alter SNGFR effects but prevented APR changes. Yohimbine (an alpha 2 antagonist) microperfusion into the urinary space produced opposite effects to that of agmatine. Microperfusion of urinary space with BU-224 (microM), a synthetic imidazoline2 (I2) agonist, duplicated agmatine effects on SNGFR but not APR whereas an I1 agonist had no effect. Agmatine effects on SNGFR and APR are not only dissociable but appear to be mediated by different mechanisms. The production and degradation of this biologically active substance derived from arginine constitutes a novel endogenous regulatory system in the kidney.