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Leptin, the gene product of the ob gene, is important in the control of appetite in rodents and may have an important role in humans. The clearance of leptin from the circulation is unknown. As the leptin receptor is present in the kidney, we evaluated the role of the kidney in removing circulating leptin in humans. We measured leptin in aortic and renal vein plasma in 8 patients with intact renal function and 6 patients with impaired renal function who were undergoing elective cardiac catheterization. Renal blood flow was measured in all patients to calculate net mass balance across the kidney. In patients with intact renal function there is net renal uptake of 12% of circulating leptin, whereas in patients with renal insufficiency there is no renal uptake of leptin. In a separate cohort of 36 patients with end-stage renal failure on hemodialysis, peripheral leptin levels factored for body mass index was increased by > fourfold as compared to a group of healthy controls (N = 338). In addition, plasma leptin is not cleared by hemodialysis with a modified cellulose membrane. Additional studies are required to evaluate the role of leptin in mediating the anorexia of uremia.

Diabetic nephropathy is a common complication in patients with either type I or type II diabetes. The pathogenesis of diabetic nephropathy is thought to involve both metabolic and vascular factors leading to chronic accumulation of glomerular mesangial matrix. In this context, both transforming growth factor-beta (TGF-beta) and endothelin may contribute to these processes. To determine if diabetic patients demonstrate increased renal production of TGF-beta and endothelin, aortic, renal vein, and urinary levels of these factors were measured in 14 type II diabetic patients and 11 nondiabetic patients who were undergoing elective cardiac catheterization. Renal blood flow was measured in all patients to calculate net mass balance across the kidney. Diabetic patients demonstrated net renal production of immunoreactive TGF-beta1 (830 +/- 429 ng/min [mean +/- SE]), whereas nondiabetic patients demonstrated net renal extraction of circulating TGF-beta1 (-3479 +/- 1010 ng/min, P < 0.001). Urinary levels of bioassayable TGF-beta were also significantly increased in diabetic patients compared with nondiabetic patients (2.435 +/- 0.385 vs. 0.569 +/- 0.190 ng/mg creatinine, respectively; P < 0.001). Renal production of immunoreactive endothelin was not significantly increased in diabetic patients. In summary, type II diabetes is associated with enhanced net renal production of TGF-beta1, whereas nondiabetic patients exhibit net renal extraction of circulating TGF-beta1. Increased renal TGF-beta production may be an important manifestation of diabetic kidney disease.

Systemic and renal oxygen consumption and hemodynamics were studied in patients with normal renal function (NI; serum creatinine concentration (Screat), 1.0 +/- 0.04 mg/dL) and those with moderate chronic renal failure with diabetes mellitus Screat, 2.7 +/- 0.2 mg/dL) or without diabetes mellitus (Screat, 2.4 +/- 0.1 mg/dL). Patients with chronic renal failure were anemic and had normal systemic oxygen consumption (NI, 10,564 +/- 277; chronic renal failure, 9,669 +/- 362 mumol of O2/min) and elevated systemic oxygen extraction (NI, 22.9 +/- 1; chronic renal failure, 30.9 +/- 1.2%) (P less than 0.02). Cardiac output and index and arterial oxygen saturation were equivalent in normal patients and in patients with chronic renal failure. Patients with chronic renal failure had higher renal oxygen extraction (NI, 7.3 +/- 0.8; chronic renal failure, 13.9 +/- 1%), lower RBF (NI, 572 +/- 146; chronic renal failure, 197 +/- 20 mL/min/kidney), and lower renal oxygen consumption per kidney (NI, 391 +/- 101; chronic renal failure, 177 +/- 20 mumol of O2/min/kidney) than did normal patients (P less than 0.02). There was a linear relationship between hemoglobin and RBF (r = 0.47, P less than 0.02). Patients with chronic renal failure and diabetes had lower RBF (diabetes mellitus, 146 +/- 23; without diabetes, 242 +/- 28 mL/min/kidney) and renal oxygen consumption per kidney (diabetes mellitus, 131 +/- 21; without diabetes, 218 +/- 29 mumol of O2/min/kidney (P less than 0.03) but equivalent renal oxygen extraction when compared with patients without diabetes. Patients with chronic renal failure without diabetes mellitus had higher renal oxygen consumption when expressed per 100 mL of creatinine clearance (diabetes mellitus, 1,016 +/- 150; without diabetes mellitus, 1,453 +/- 175 mumol of O2/min/100 mL of creatinine clearance; P less than 0.03). There was a significant linear relationship (P less than 0.005, r = 0.38) between calculated creatinine clearance and renal oxygen consumption with a y intercept representing basal renal oxygen consumption (115 mumol of O2/min/kidney) and a slope of 2.3 mumol of O2/mL. Patients with moderate chronic renal failure have normal systemic oxygen consumption but reduced RBF and renal oxygen consumption. The latter parameters are even lower in patients with chronic renal failure and diabetes. Renal hypermetabolism is more likely to exist in nondiabetic than diabetic renal disease. Basic human renal physiology and pathophysiology are described by the relationships between renal oxygen consumption, blood flow, oxygen extraction, and creatinine clearance in patients with normal and abnormal renal function of varied cause.

To clarify the role of vitamin D in renal phosphate transport, weanling rats were fed a vitamin D-deficient diet containing 1.8% calcium and 1.2% phosphorus. After 5-6 wk, the rats were normocalcemic, normophosphatemic, and had normal levels of PTH. Assays of vitamin D metabolites revealed undetectable plasma levels of 25(OH)D, and 1,25(OH)2D levels of 92 +/- 16 pg/ml in partially vitamin D-depleted (PVDD) rats and 169 +/- 58 pg/ml in normal rats. PVDD rats had increased phosphate excretion, both absolute and fractional, and a decrease in Na+ gradient-dependent Pi transport in proximal tubular brush border membrane vesicles (BBMV) prepared from their kidneys. Vitamin D repletion of PVDD rats with 1,25(OH)2D3, 15 pmol/100 g body wt, decreased fractional excretion of Pi from 22.6 +/- 1.9 to 13.5 +/- 1.3%; the latter values were similar to normal rats. Repletion with 1,25(OH)2D3 also increased Na+-dependent phosphate transport in BBMV from 322 +/- 24 pmol X mg protein-1 X 15 s-1 in BBMV from PVDD rats to 698 +/- 70 pmol X mg protein-1 X 15 s-1. Repletion with larger doses of 1,25(OH)2D3 produced hypercalcemia and hyperphosphatemia from intestinal absorption, an increase in phosphate excretion, and a blunted response of Pi transport to 1,25(OH)2D3. Prevention of hyperphosphatemia by dietary adjustments allowed full expression of the stimulatory effects of 1,25(OH)2D3 on Pi transport. These later data may partially explain the inhibitory effects reported in prior studies in which plasma Pi was not controlled and the larger doses of 1,25(OH)2D3 administered.(ABSTRACT TRUNCATED AT 250 WORDS)

The phosphatidylcholine content of both the intestinal and renal brush-border membranes and ion transport are affected by 1,25-dihydroxycholecalciferol (1,25(OH)2D3). To investigate the mechanism of this effect, liposomes were prepared containing self-quenching concentrations of fluorescent phospholipid derivatives. When these liposomes were incubated with rat renal brush-border membrane vesicles, an immediate increase in the relative fluorescence of N-4-nitrobenz-2-oxa-1,3-diazole phosphatidylcholine (NBD-PC) was detected, indicating transfer of NBD-PC into a non-quenched membrane. Addition of 1,25(OH)2D3 to the liposomes produced a dose-dependent stimulation of NBD-PC transfer to the acceptor brush-border membrane vesicles. Peripheral fluorescence was visible when the brush-border membrane vesicles were viewed with a fluorescent microscope. Using brush-border membrane vesicles from kidneys of vitamin D-deficient animals, quantitation of lipid transfer revealed a 1,25(OH)2D3 (10(-7) M) stimulation of NBD-PC transfer from 1.38 +/- 0.27 to 2.07 +/- 0.26 micrograms/h, and of PC transfer, assessed by vesicle phosphatidylcholine content, from 49.7 +/- 12 to 57.3 +/- 12 micrograms/mg protein per h (P less than 0.05). There was no significant transfer of N-(lissamine rhodamine B sulfonyl)dioleoylphosphatidylethanolamine (N-Rh-PE). In the absence of hormone, the amount of NBD-PC transferred to brush-border membrane vesicles prepared from normal rats was significantly greater than that transferred to brush-border membrane vesicles prepared from vitamin D-deficient animals (2.12 +/- 0.02 vs. 1.39 +/- 0.27 micrograms of NBD-PC/h, P less than 0.05). Both physiologic and pharmacologic concentrations of 1,25(OH)2D3 stimulated NBD-PC transfer with maximum response at 10(-14) M (2.98 +/- 0.15 micrograms/h). 24,25-Dihydroxycholecalciferol and 25-hydroxycholecalciferol (25(OH)D3) also stimulated transfer, although dose-response curves were less effective than for 1,25(OH)2D3. Cortisol and vitamin D-3 did not stimulate transfer. 1,25(OH)2D3 did not stimulate NBD-PC transfer between liposome populations.

Vitamin D has been shown to stimulate renal phosphate transport and to alter membrane phospholipid composition. The present studies examine the possibility that the effects of 1,25(OH)2D3 on phosphate transport are related to its effects on membrane lipids. Arrhenius plots, which relate maximum rates of sodium dependent phosphate uptake into brush-border membrane vesicles to temperature were constructed. Phosphate transport was studied using brush-border membrane vesicles from normal, vitamin D-deficient, and physiologically replete (15 pmol/100 g body weight per 24 h) rats. These plots were triphasic with characteristic, lipid-dependent, slopes (M1,M2,M3) representing activation energies and transition temperatures (T1,T2). Physiologic 1,25(OH)2D3 repletion normalized these plots by stimulating phosphate transport at all temperatures, increasing T2 from 18 +/- 0.7 to 23.5 +/- 0.9 degrees C and decreasing M2 and M3 from -5.8 +/- 0.2 and -10.2 +/- 0.4 to -4.5 +/- 0.4 and -7.7 +/- 0.3, respectively. Pharmacologic (1.2 nmol/100 g per 3 h) 1,25(OH)2D3 treatment resulted in a change in the Arrhenius plot of phosphate transport to a biphasic one with a transition temperature of 30 degrees C. This effect was not blocked by cycloheximide. The Arrhenius plots of glucose transport were triphasic and unchanged with vitamin D repletion. These data support a liponomic mechanism of action for 1,25(OH)2D3 on phosphate transport.

Controversy exists regarding the mechanisms by which 1,25-dihydroxycholecalciferol (1,25(OH2)D3) alters membrane lipid composition and ion transport. Recent studies have demonstrated stimulation of the transfer of 1-acyl-2-(N-4-nitrobenz-2-oxa-1,3-diazole)aminocaproylphosphatidylcholine (NBD-PC) by 1,25(OH)2D3. In the present studies, brush border membrane vesicle phosphatidylcholine content was increased after incubation with liposomes composed of dioleoylphosphatidylcholine or beta-linoleyl, gamma-palmitoyl phosphatidylcholine and 1,25(OH)2D3 (10(-7) M). Vesicular phosphatidylcholine content was increased from control levels of 49.5 micrograms/mg protein to 56.9 and 58.5, respectively, after treatment with the liposomes containing 1,25(OH)2D3, P less than 0.05. When the vesicles were incubated with liposomes composed of beta-linoleyl, gamma-palmitoyl phosphatidylcholine, phosphate transport was stimulated from 231 +/- 20 to 431 +/- 41 pmol/mg protein per 15 s in the presence of 1,25(OH)2D3 if the vesicles were derived from vitamin D deficient rats and from 443 +/- 33 to 601 +/- 42 pmol/mg protein per 15 s if the vesicles were prepared from normal rats. Despite phosphatidylcholine transfer, incubation with liposomes composed of dioleoylphosphatidylcholine and 1,25(OH)2D3 did not stimulate phosphate transport. Furthermore, incubation of vesicles with liposomes and 1,25(OH)2D3 did not alter glucose transport.