Macroamylases: Differences in Activity against Various-Sized Substrates

Hyperamylasemia caused by macroamylases can lead to the overdiagnosis of acute pancreatitis. We examined whether interference from macroamylase is less in assays that use high-molecular-mass (high-M(r)) substrates rather than oligosaccharide substrates. We hypothesized that high-M(r) substrates would be sterically excluded from macroamylasemic complexes and thus would be hydrolyzed less efficiently. Eighteen macroamylasemic samples were assayed by using red-dyed amylopectin or blue-dyed starch as polysaccharide substrates or by using maltoheptaose or maltotetraose as oligosaccharide substrates. The oligosaccharide substrates gave comparable results (y = 0.81x + 83), but we observed consistently lower activities for amylopectin than for maltotetraose (y = 0.32x + 38). We observed no bias among methods when nonmacroamylasemic specimens were analyzed. The mechanism of this difference was examined by adding antihuman pancreatic amylase antibodies to hyperamylasemic serum samples from patients without macroamylasemia and to purified human pancreatic or salivary isoamylases. In each case, polyclonal and monoclonal antibodies lowered amylase activity more in assays with complex polysaccharides than in those with oligosaccharides. The use of high-M(r) substrates diminishes interference, and detection of suspected macroamylasemia may be possible through comparing activities determined from automated methods that use different substrates.

Analysis of 3-hydroxydodecanedioic acid for studies of fatty acid metabolic disorders: preparation of stable isotope standards

Current diagnostic tests to detect disorders of fatty acids metabolism, such as long-chain hydroxyacyl CoA dehydrogenase deficiency (LCHAD), are hampered by insensitivity or a long delay time required for results. Children with LCHAD deficiency are known to excrete 3-hydroxydicarboxylic acids with chain lengths of 10-16 carbons, but a quantitative method to measure excretion of these potentially diagnostically important compounds has not been reported. We report synthetic schemes for synthesis of 3-hydroxydodecanedioic acid and a di-deuterated analog, suitable for use in a stable-isotope dilution mass spectrometric analytical approach. Evaluation of several common derivatization protocols to produce a volatile derivative for gas chromatography determined that trimethylsyl derivatives produced the best efficiency and stability. Positive-ion chemical ionization mass spectrometry provided the greatest yield of characteristic ions. These results indicate the basic reagents needed to develop sensitive and accurate 3-hydroxydodecanedioic acid measurements for diagnosis of LCHAD deficiency and other fatty acid oxidation disorders.

Effect of short-term fasting on free and bound leptin concentrations in lean and obese women

Plasma leptin exists in protein-bound and free forms, which may affect its hormonal bioactivity. Therefore, the relationship between bound and free leptin may be particularly important during physiological conditions that cause rapid alterations in total plasma leptin concentration, such as fasting. The purpose of this study was to evaluate the effect of short-term fasting on bound and free plasma leptin concentrations and leptin binding capacity (a measure of plasma leptin-binding protein content) in lean and obese women. Six lean (body mass index, 21 +/- 1 kg/m2) and 6 abdominally obese (BMI, 36 +/- 1 kg/m2) women were studied after 14 h and 22 h of fasting. Although total plasma leptin concentration was more than 6-fold greater in obese (45.4 +/- 7.6 microg/liter) compared with lean (7.4 +/- 1.0 microg/liter) women at 14 h of fasting (P < 0.05), the percentage of leptin in the bound form was greater in lean than obese subjects (29 +/- 2% vs. 12 +/- 3%; P < 0.05). Arterial total, free, and bound plasma leptin concentrations all declined between 14 h and 22 h of fasting in both lean and obese groups, but the relative decline of these fractions was greater in lean (36 +/- 4%, 60 +/- 9%, and 51 +/- 13%, respectively) than in obese (19 +/- 5%, 21 +/- 8%, and 12 +/- 7%, respectively) subjects (all P < 0.05). In contrast, leptin binding capacity was unchanged. The percentage of total plasma leptin present in bound form was constant between 14 h and 22 h of fasting in lean subjects and increased slightly but significantly in obese subjects. These data demonstrate that both free and bound fractions of leptin in plasma decrease quickly in response to energy restriction, but the decline is blunted in abdominally obese compared with lean women. In addition, the equilibrium between bound and free leptin fractions is maintained during brief fasting and is not regulated by leptin binding capacity.

Acute exercise effect on postabsorptive serum leptin

We postulated that high circulating cortisol levels during intense exercise would lead to increased serum leptin concentrations. Young, lean men ate a small meal and then exercised on a cycle ergometer for 41 min or rested on a control day. Serum leptin concentration was 10% greater during exercise than in the control condition (P < 0.05). Directly after exercise, serum leptin dropped to approximately 10% less than the control level (P < 0.05) but had recovered to the nonexercised level after approximately 2 h of recovery. Rapid exercise effects on circulating leptin were related to changes in hemoconcentration rather than changes in leptin mass. When serum leptin was normalized to serum protein, leptin increased by 10% in the exercise condition compared with control by the end of recovery (P < 0.05). Although exercise increased serum cortisol concentration threefold, there was no relation between differences in cortisol and exercise vs. control differences in normalized leptin. The increased leptin mass after exercise may have been related to greater plasma glucose concentration during recovery after exercise compared with the control condition.

Postictal cerebrospinal fluid abnormalities in children

OBJECTIVES

To determine the frequency and characteristics of seizure-induced cerebrospinal fluid (CSF) abnormalities in children and to identify potential alternative causes of these findings.

METHODS

Consecutive patients (n = 80) who underwent lumbar puncture within 24 hours after a seizure were studied retrospectively. The presence of CSF abnormalities in total leukocytes, polymorphonuclear cells, and protein was determined by using age-specific reference values. Coexisting conditions that could affect CSF findings, such as traumatic lumbar puncture, concurrent neurologic disease, and undiagnosed meningitis, were identified.

RESULTS

Eighteen of the 80 patients were excluded from the final study group because of the presence of another condition that could alter the CSF. More than 50% of the excluded patients had an abnormal CSF leukocyte count or protein level, including 2 patients with initially undiagnosed meningitis, which was subsequently detected by post-hoc polymerase chain reaction testing. In the remaining 62 patients, postictal pleocytosis was detected in only 3 (5%), and increased protein was detected in only 6 (10%). The maximal postictal pleocytosis and protein level were 8 x 10(6) leukocytes/L (8 leukocytes/mm(3)) and 0.52 g/L (52 mg/dL), respectively.

CONCLUSIONS

Seizure-induced CSF abnormalities are rare in children, and alternative, often unidentified, disease processes may account for many observed postictal abnormalities. All patients with abnormal CSF after a seizure should be thoroughly evaluated for other causes of the abnormality.

Glyceraldehyde preserves glucose concentrations in whole blood specimens

BACKGROUND

Glucose concentrations decrease in blood specimens during transport/processing, primarily because of continuing metabolism (glycolysis) by erythrocytes. Several means to reduce the loss of glucose in blood specimens have been developed, but all have major drawbacks. Glyceraldehyde, which has antiglycolytic activity, was assessed for potential in preserving glucose in blood specimens.

METHODS

Heparinized blood from volunteers was treated with glyceraldehyde and other agents. After incubation for various times, plasma concentrations of glucose and other common analytes were determined with prevalent commercial analyzers.

RESULTS

The racemic mixture of glyceraldehyde (D,L-GA) preserved glucose concentrations for up to 8 h at room temperature. Half-maximal effect was attained with 0.9 mmol/L D,L-GA. Trials of the D and L stereoisomers individually indicated that the L isomer (L-GA) was responsible for all or most of the antiglycolytic activity of the racemic mixture. Other related compounds were ineffective. Measurements of most common clinical laboratory analytes were unaffected by the presence of D,L-GA or L-GA.

CONCLUSIONS

Glyceraldehyde (D,L-GA or L-GA) effectively preserves glucose concentrations in whole blood specimens for up to 8 h. Specimens collected with D,L-GA or L-GA are suitable for analysis of many analytes commonly comeasured with glucose.

Cerebrospinal fluid protein concentration in pediatric patients: defining clinically relevant reference values

OBJECTIVES

To define clinically relevant reference ("normal") values for cerebrospinal fluid (CSF) protein concentrations in pediatric patients who were evaluated for meningitis by traditional criteria and by enterovirus-polymerase chain reaction (EV-PCR).

DESIGN AND PATIENTS

A cohort of 906 consecutive pediatric patients to receive CSF analysis at St Louis Children's Hospital, St Louis, Mo, from June 1, 1998, to December 31,1998, was studied for clinical and laboratory data. Age-dependent CSF protein concentrations were then derived from a reference group of 225 patients in whom meningitis and other neurologic diseases were excluded by traditional clinical or laboratory criteria (excluding EV-PCR). Available CSF samples from 132 patients of the reference group were subsequently tested for EV-PCR.

RESULTS

In the reference group, the CSF protein concentration was highest and most variable in neonates, with a maximum of approximately 1.0 g/L. Cerebrospinal fluid protein concentration decreased rapidly to a nadir by 6 months and remained low throughout childhood, rarely exceeding 0.3 g/L and, finally, increasing in adolescence toward adult values. Enterovirus- polymerase chain reaction was positive in CSF of 11% of the reference group, with EV-PCR-positive patients having significantly higher CSF protein concentrations than EV-PCR-negative patients aged between 4 months and 14 years.

CONCLUSIONS

Reference values for CSF protein exhibit a characteristic age dependence in pediatric patients. Continued standard use of adult reference values in the pediatric population is inappropriate. The unexpected finding of a positive EV-PCR in patients not diagnosed with meningitis by traditional criteria further emphasizes the importance of selecting the most clinically relevant reference group for age and other variables when defining normal laboratory values. Arch Pediatr Adolesc Med. 2000;154:827-831

Leptin in cerebrospinal fluid from children: correlation with plasma leptin, sexual dimorphism, and lack of protein binding

BACKGROUND

Previous studies in adults have established that leptin is present at very low concentrations in cerebrospinal fluid (CSF), but few data exist concerning CSF leptin in children. Current evidence suggests that CSF leptin concentrations interact with hypothalamic centers controlling food intake. Serum leptin concentrations manifest a sexual dimorphism that arises during puberty.

METHODS

Leptin concentrations were determined in CSF from 42 pre- and postpubertal children who had been objectively classified into non-neurological disease or aseptic meningitis groups. Multivariate analysis of the dependence of CSF leptin on gender, pubertal state, body mass index (BMI), presence of aseptic meningitis, and CSF protein concentration was performed.

RESULTS

CSF leptin concentrations correlated with log-transformed plasma leptin concentrations in concomitantly collected samples (r = 0.582; P = 0.029). BMI and gender were significant determinants of CSF leptin in postpubertal children, but only BMI was significant in prepubertal children. Analysis with HPLC to separate protein-bound and free forms of leptin found only free leptin in CSF.

CONCLUSIONS

CSF leptin concentrations in children reflect plasma leptin concentrations, including the advent of sexual dimorphism at puberty. Only free leptin is detectable in CSF, suggesting that it is the biologically active form.

Leptin binding and binding capacity in serum

BACKGROUND

Leptin, a hormone produced primarily by adipose tissue, is known to be present in serum as both monomeric (free) and higher molecular mass (bound) forms, but little is known about the nature of the bound forms or physiological variation in binding capacity.

METHODS

A new method to quantify the free and bound forms was developed, based on HPLC separation and RIA quantification in chromatography fractions. Reanalysis of specimens after addition of exogenous leptin allowed direct determination of leptin-binding capacity and the degree of saturation of leptin-binding capacity.

RESULTS

HPLC chromatography fractionated serum leptin into both the free form and as a broad peak of 59-130 kDa. Several experiments were conducted to validate the new method. The concentrations of bound leptin in serum were 0.45-3.94 micro/L, and they increased as total leptin (reflecting adiposity) increased in 24 lean and obese volunteers. Leptin was readily dissociated from the bound fraction by competition from exogenous leptin. Rechromatography of the bound fraction led to dissociation of leptin, which was promoted by warming the sera before chromatography. Leptin-binding capacity was 1.8-5.3 microg/L; binding capacity was nearly constant over a range of total leptin concentrations of 2-10 microg/L, and slowly increased at higher total leptin concentrations. Saturation of binding capacity was low (15%) at very low total leptin concentrations (<5 microg/L), but rose quickly to a plateau near 80% at higher total leptin concentrations.

CONCLUSIONS

The new method facilitates measurement of free and bound fractions of serum leptin, and is the first method measuring leptin-binding capacity. These experiments demonstrate that the concentration of bound leptin and leptin-binding capacity vary physiologically, with binding/binding capacity increasing with adiposity. Except in very lean individuals, binding capacity is nearly completely saturated.

Leptin production during early starvation in lean and obese women

We evaluated abdominal adipose tissue leptin production during short-term fasting in nine lean [body mass index (BMI) 21 +/- 1 kg/m(2)] and nine upper body obese (BMI 36 +/- 1 kg/m(2)) women. Leptin kinetics were determined by arteriovenous balance across abdominal subcutaneous adipose tissue at 14 and 22 h of fasting. At 14 h of fasting, net leptin release from abdominal adipose tissue in obese subjects (10.9 +/- 1.9 ng x 100 g tissue x (-1) x min(-1)) was not significantly greater than the values observed in the lean group (7.6 +/- 2.1 ng x 100 g(-1) x min(-1)). Estimated whole body leptin production was approximately fivefold greater in obese (6.97 +/- 1.18 microg/min) than lean subjects (1.25 +/- 0.28 microg/min) (P < 0.005). At 22 h of fasting, leptin production rates decreased in both lean and obese groups (to 3.10 +/- 1.31 and 10.5 +/- 2.3 ng x 100 g adipose tissue(-1) x min(-1), respectively). However, the relative declines in both arterial leptin concentration and local leptin production in obese women (arterial concentration 13.8 +/- 4.4%, local production 10.0 +/- 12.3%) were less (P < 0.05 for both) than the relative decline in lean women (arterial concentration 39.0 +/- 5.5%, local production 56.9 +/- 13.0%). This study demonstrates that decreased leptin production accounts for the decline in plasma leptin concentration observed after fasting. However, compared with lean women, the fasting-induced decline in leptin production is blunted in women with upper body obesity. Differences in leptin production during fasting may be responsible for differences in the neuroendocrine response to fasting previously observed in lean and obese women.

Leptin concentrations in Prader-Willi syndrome before and after growth hormone replacement

OBJECTIVE

This study explored leptin concentrations in Prader-Willi syndrome (PWS), a genetic disorder characterized by significant obesity and presumed hypothalamic dysfunction. The potential interaction of leptin metabolism with the growth hormone (GH) axis was also studied.

STUDY DESIGN

Plasma leptin concentrations and percent body fat were determined by radioimmunoassay and dual energy x-ray absorptionmetry, respectively, in 23 children with Prader-Willi syndrome and 23 children with exogenous obesity.

RESULTS

Log plasma leptin concentrations were positively correlated with percentage body fat in PWS (r = 0.844) and exogenous obesity (r = 0.869). When the regression lines for the two groups were compared, there were no differences in their slopes (P = 0.737) or intercepts (P = 0.701). Administration of recombinant human growth hormone to PWS children for 12 months significantly reduced both percentage body fat and plasma leptin concentrations, but the relationship of log plasma leptin to percentage body fat was unchanged.

CONCLUSION

Prader-Willi syndrome is not accompanied by deranged leptin concentrations and there was no evidence of an interaction of the GH axis with leptin metabolism in these GH-deficient children.

Leptin response to insulin in humans is related to the lipolytic state of abdominal subcutaneous fat

Insulin-induced leptinemia in humans appears to be blunted by insulin resistance. We therefore examined the relationship between insulin action and plasma leptin by monitoring regional and whole body lipolysis and plasma leptin levels in 15 premenopausal women (body fat range, 14-59%) during a two-stage euglycemic clamp (insulin was infused 90 min each at 6-10 and 12-20 mU/m2 x min). Microdialysis probes were placed in abdominal and femoral sc adipose tissue. Subjects were given a primed, constant infusion of a stable isotope tracer (2H5-glycerol), and plasma glycerol isotope enrichments were analyzed by mass spectrometry to determine glycerol kinetics. Although there was no mean change in plasma leptin during the clamp (baseline, 16.6 +/- 4.5 ng/mL; final, 16.3 +/- 4.3 ng/mL), there was large interindividual variability in the changes in plasma leptin (range, -18% to +19%). Changes in plasma leptin during the clamp stages were correlated with abdominal dialysate glycerol concentrations (r = -0.44; P < 0.05), but not femoral dialysate glycerol concentrations (r = -0.15), the rate of appearance of glycerol in plasma (r = 0.005), or plasma insulin levels (r = 0.16). The results suggest that insulin-induced changes in plasma leptin are more related to the lipolytic state (i.e. low leptin response when lipolysis is high) of abdominal sc adipose tissue than that of other fat depots.

Site-specific phosphorylation of synapsin I by Ca2+/calmodulin-dependent protein kinase II in pancreatic betaTC3 cells: synapsin I is not associated with insulin secretory granules

Increasing evidence supports a physiological role of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) in the secretion of insulin from the pancreatic beta-cell, but the precise sites of action are not known. A role of this enzyme in neuroexocytosis is implicated by its phosphorylation of a vesicle-associated protein, synapsin I. Because of emerging similarities to the neuron with respect to exocytotic mechanisms, the expression and phosphorylation of synapsin I in the beta-cell have been studied. Synapsin I expression in clonal mouse beta-cells (betaTC3) and primary rat islet beta-cells was initially confirmed by immunoblot analysis. By immunoprecipitation, in situ phosphorylation of synapsin I was induced in permeabilized betaTC3 cells within a Ca2+ concentration range shown to activate endogenous CaM kinase II under identical conditions. Proteolytic digests of these immunoprecipitates revealed that calcium primarily induced the increased phosphorylation of sites identified as CaM kinase II-specific and distinct from protein kinase A-specific sites. Immunofluorescence and immunogold electron microscopy verified synapsin I expression in betaTC3 cells and pancreatic slices but demonstrated little if any colocalization of synapsin I with insulin-containing dense core granules. Thus, although this study establishes that synapsin I is a substrate for CaM kinase II in the pancreatic beta-cell, this event appears not to be important for the mobilization of insulin granules.

Leptin elimination in hyperleptinaemic peritoneal dialysis patients

BACKGROUND

Elevated plasma concentrations of leptin, a hormone thought to regulate body composition by influencing food intake/metabolic rate, are prevalent in renal failure patients. The mechanism for these increases is not known, but evidence suggests that simple accumulation due to decreased elimination is insufficient explanation.

METHODS

We studied the incidence of hyperleptinaemia in 28 end-stage renal disease patients treated with continuous ambulatory peritoneal dialysis (CAPD), compared with body-mass-index-and sex-matched controls. Results were separated by gender because women have higher leptin concentrations than men. Excretion of leptin and other substances in dialysis fluid was also studied.

RESULTS

Hyperleptinaemia was prevalent in women CAPD subjects, but not in men. Plasma leptin concentrations correlated strongly with the daily excretion of leptin in dialysis fluid. Clearance of leptin in dialysis fluid was greater in men than women CAPD subjects. Single regression analysis found that fasting insulin, glucose content of dialysis fluid, plasma albumin, C-reactive protein, erythropoietin dose, urinary creatinine clearance and plasma beta2-microglobulin were not determinants of plasma leptin concentrations. Stepwise forward multiple regression, examining the dependence of plasma leptin on body mass index, renal creatinine clearance, plasma albumin, daily dialysis fluid glucose load, daily leptin in dialysis fluid, erythropoietin dose and plasma C-reactive protein found only erythropoietin dose as a consistent negative predictor of plasma leptin concentrations.

CONCLUSIONS

The results suggest that hyperleptinaemia of CAPD was due to predisposing loss of renal elimination capacity combined with increased production due to obesity (more prevalent in women subjects of this study) and potentially female gender.

Regional leptin kinetics in humans

BACKGROUND

Leptin is known to be cleared by the kidney, a tissue with substantial leptin receptor mRNA expression; however, lung, liver, and muscle tissues also express leptin receptor messenger RNA and it is not known whether these tissues also clear leptin from the circulation.

OBJECTIVE

This study was conducted to determine whether net leptin clearance takes place in the pulmonary, splanchnic, and leg tissue beds to a similar extent as in the kidney.

DESIGN

Plasma leptin concentrations were measured in blood entering and exiting the renal bed, pulmonary bed, splanchnic bed, and leg in 4 groups of subjects. Regional plasma flow was measured in 3 of the 4 groups.

RESULTS

Renal leptin uptake was substantial, whereas no net uptake of leptin by the splanchnic or pulmonary vascular beds was detected; leg tissue was a net leptin producer. Net leptin release by leg tissue, relative to leg adipose tissue mass, was comparable with that reported previously for abdominal subcutaneous adipose tissue.

CONCLUSION

These results confirm that the kidney is a significant site of leptin clearance in humans, whereas pulmonary and splanchnic beds are not.

Measurement of plasma ibuprofen by gas chromatography-mass spectrometry

A gas-chromatography-mass spectrometry (GC-MS) method for the determination of plasma ibuprofen was developed. Plasma samples from cystic fibrosis (CF) patients receiving high-dose ibuprofen therapy were analyzed by GC-MS and the result compared to analysis by high-performance liquid chromatography (reference method). Analysis of ibuprofen was sensitive to at least 5 mg/L, and the method was linear to 200 mg/L. Within-run variations of plasma samples were 4.6% (131.7 +/- 6.0 mg/L) and 5.4% (44.4 +/- 2.4 mg/L), respectively. The between-run variation was 9.3% (45.4 +/- 4.2 mg/L) and 7.4% (88.0 +/- 6.5 mg/L). This method is suited for routine clinical use for the monitoring of plasma ibuprofen levels in treatment of CF. It may be particularly applicable in pediatric laboratories, which are likely to possess GC-MS capability.

High-flux dialysis lowers plasma leptin concentration in chronic dialysis patients

Leptin is a protein produced by fat cells and involved in body weight regulation. Plasma leptin is significantly higher in some hemodialysis (HD) patients than in normal controls. We examined the influence of dialyzer membrane biocompatibility and flux on elevated plasma leptin concentrations in hemodialysis patients. Employing a crossover design, leptin and tumor necrosis factor-alpha (TNF-alpha) levels were serially determined in eight chronic dialysis patients. Patients were dialyzed sequentially on low-flux cellulosic (TAF) dialyzers, low-flux (F8) polysulfone, high-flux (F80B) polysulfone, then low-flux polysulfone and cellulosic dialyzers again. Mean leptin concentrations were similar when low-flux polysulfone or cellulosic dialyzers were employed (141.9+/-24.2 microg/L versus 137.8+/-18.4 microg/L, respectively (P=NS). In contrast, leptin fell significantly on the high-flux polysulfone dialyzer (99.4+/-16.2 microg/L) compared with cellulosic (P < 0.005), and low-flux polysulfone dialyzers (P < 0.02). Leptin clearance by the high-flux polysulfone dialyzer was significantly higher than the low-flux dialyzers (50.4+/-21.5 v -9.6+/-10.3 mL/min; P=0.043), but did not account fully for the 30% decline in plasma leptin during the high-flux arm of the study. Concentrations of TNF-alpha were lower when high-flux polysulfone dialyzers were employed, but there was no correlation of individual TNF-alpha levels with leptin concentrations. High-flux dialysis lowers plasma leptin concentrations an average of 30%, but biocompatibility does not influence leptin levels. The decrease in plasma leptin on high-flux dialysis cannot be explained solely by enhanced clearance.

HPLC detection of fetal blood in meconium: improved sensitivity compared with qualitative methods

We describe an HPLC-based method for the detection and quantification of fetal hemoglobin in stools of newborns. The new procedure is an alternative to the classic qualitative test for adult hemoglobin in meconium based on the differential stability of hemoglobin species in dilute base (Apt test). The HPLC method, based on a commercial device for hemoglobin characterization (Bio-Rad Variant), readily separates fetal and adult hemoglobin from non-hemoglobin components of meconium. To validate the method, blood and meconium were mixed in various proportions and then prepared for analysis with extraction in saline. The HPLC method accurately identified hemoglobin species even when the blood constituted only 5 mL per 100 g of the meconium specimen, and nearly quantitative recovery of hemoglobin was obtained at a blood content of 20 mL per 100 g of the meconium. Analysis time was 6.5 min, and preparation of sample was simple. HPLC detection of fetal blood in stools or other specimens markedly improves detection/characterization of blood in meconium.

Plasma leptin concentrations are only transiently increased in nephrectomized rats

Leptin is an adipocyte-secreted hormone that has effects on appetite and energy expenditure. Several studies have shown that end-stage renal disease results in elevated plasma leptin concentrations and that the kidney is responsible for most of leptin elimination in rodents. Leptin metabolism was investigated in rats that underwent unilateral nephrectomy to experimentally limit renal elimination function. Within 4 h of nephrectomy, plasma leptin concentrations increased from 2.9 +/- 0.8 to 5.8 +/- 1.0 & microg/l but thereafter rapidly (<24 h) decreased to prenephrectomy concentrations, despite continued elevated plasma creatinine levels. Sham-operated rats maintained presurgical concentrations of leptin and creatinine throughout the experiment. Kinetic studies of 125I-labeled leptin elimination showed that fractional catabolic rates and half-lives of leptin in circulation were similar at 48 h in nephrectomized and sham-operated rats, suggesting that production of leptin was unchanged after nephrectomy. Excretion of 125I derived from leptin in urine of nephrectomized rats was similar to that of sham-operated rats, and residual radioactivity was increased in the remaining kidneys excised from nephrectomized rats. These results demonstrate that 1) leptin concentrations are quickly restored to presurgical levels in nephrectomized rats, and 2) it is leptin elimination, not leptin production, that compensates to maintain leptin concentrations. Rapid metabolic adaptation of remaining renal tissue may explain the restoration of normal leptin elimination in nephrectomized rats.

Hyperleptinaemia of end-stage renal disease is corrected by renal transplantation

BACKGROUND

Previous studies have reported that patients with end-stage renal disease (ESRD) have elevated plasma leptin concentrations, but the cause and significance of the elevations are unknown. We studied leptin concentrations in 29 adults undergoing renal transplantation, to determine if restoration of renal function reduced leptin concentrations in ESRD.

METHODS

Leptin concentrations were measured by radioimmunoassay in plasma specimens collected within 1 week before transplant, 6 days post-transplant, and 60 days post-transplant.

RESULTS

Mean plasma leptin concentrations were higher in both male and female ESRD patients compared with a control population of similar age and body mass index (BMI), but most of the disparity was due to a minority of patients with grossly elevated concentrations; the majority of ESRD patients had normal or near-normal leptin concentrations after accounting for their adiposity with BMI. Six days after successful renal transplantation, average plasma leptin concentrations decreased to control levels. The grossly elevated pretransplant concentrations in a minority of patients were greatly reduced in relation to BMI, and the reduction persisted to 60 days post-transplant. The decrease in creatinine with transplant did not correlate with the decrease in leptin.

CONCLUSIONS

These results demonstrate that restoration of renal function in ESRD patients reduces hyperleptinaemia, which provides further evidence of a cause/effect relationship between impaired renal function and abnormal leptin metabolism.

Patterns and potential value of cardiac troponin I elevations after pediatric cardiac operations

BACKGROUND

Perioperative myocardial injury is a major determinant of postoperative cardiac dysfunction for congenital heart disease, but its assessment during this period is difficult. The objective of this study was to determine the suitability of using postoperative serum concentrations of cardiac troponin I (cTnI) for this purpose.

METHODS

Cardiac troponin I levels were measured serially in the serum of patients undergoing uncomplicated repairs of atrial septal defect (n = 23), ventricular septal defect (n = 16) or tetralogy of Fallot (n = 16). The concentrations were correlated with intraoperative parameters (cardiopulmonary bypass time, aortic cross-clamp time, and cardiac bypass temperature), and postoperative parameters (magnitude of inotropic support, duration of intubation, and postoperative intensive care and hospital stay).

RESULTS

Postoperative absolute cTnI levels were lesion specific, with a pattern of increase and decrease similar for each lesion. For the total cohort, significant correlations between postoperative cTnI levels at all times (r = 0.43 to 0.83, p < 0.05) until 72 hours were noted for all parameters, except for cardiac bypass temperature. When evaluated as individual procedure groups, no significant relationships were noted in the atrial septal defect group, whereas postoperative cTnI levels were more strongly correlated with all intraoperative and postoperative parameters in the ventricular septal defect group than in the tetralogy of Fallot group.

CONCLUSIONS

This study suggests that cTnI values immediately after operation reflect the extent of myocardial damage from both incisional injury and intraoperative factors. Cardiac tropinin I levels in the first hours after operation for congenital heart disease are a potentially useful prognostic indicator for difficulty of recovery.

Plasma leptin concentrations in newborns of diabetic and nondiabetic mothers

Leptin plays an important role in regulating body composition through modulation of appetite and energy expenditure. We hypothesized that leptin levels in umbilical cord blood correlate with newborn body weight and habitus. We also hypothesized that infants of diabetic mothers would demonstrate altered leptin metabolism. Venous blood was sampled at birth from the umbilical cords of 105 infants (74 infants of nondiabetic mothers, and 31 infants of diabetic mothers). Thirty-nine mothers had plasma leptin concentrations measured. Analysis was done using Student's t-test, Pearson's correlation, and Spearman's correlation. Univariate/multivariate regression was used for analysis of factors associated with leptin concentration in umbilical cord plasma. Maternal and newborn characteristics were correlated with log leptin levels in umbilical venous plasma. Leptin concentration in umbilical cord plasma correlated best with birth weight for newborns of both nondiabetic and diabetic mothers (p < 0.01 for either). Umbilical cord plasma concentration of leptin was higher in infants of diabetic mothers than in infants of nondiabetic mothers (2.53 +/- 1.09 vs. 1.76 +/- 0.82; p < 0.001). Multiple regression analysis revealed a significant (p < 0.01) relationship between umbilical cord leptin level and newborn birth weight, as well as maternal DM, but not with gestational age. Similarly, there was no significant correlation with maternal plasma leptin concentration. The strong correlation of leptin concentration in umbilical cord plasma with newborn birth weight, and the lack of significant correlation with maternal leptin plasma levels, suggest that normal fetal leptin metabolism reflects fetal size and/or body habitus independent of maternal leptin metabolism. On the other hand, the higher umbilical plasma levels in infants of diabetic mothers may reflect an influence of altered fetal insulin homeostasis on fetal leptin metabolism, and suggests that maternal diabetes may influence fetal leptin metabolism.

Radioimmunoassay of rat leptin: sexual dimorphism reversed from humans

Adipose tissue secretes leptin, which interacts with receptors in the hypothalamus. In rodent models of obesity, leptin increases metabolism and decreases food intake, which helps to maintain normal body composition. Accurate and precise methods to quantitate circulating leptin concentrations are needed for physiological studies. We developed an RIA to measure leptin in rat plasma, serum, or adipocyte culture fluids. The working range of the assay, defined by the detection limit and the highest calibrator, was 0.5-50 micrograms/L. Recovery of 1.6-11.6 micrograms/L leptin added to serum was 92-103%. The rat leptin RIA correlated well with a previously developed mouse RIA when rat plasma was assayed with both methods (r = 0.94), but the mouse leptin assay underestimated rat leptin in plasma. Within- and between-run CVs were 2.4% to 5.7%. Plasma leptin concentrations correlated directly with percentage of body fat, and correlation improved when the results were separated by gender (r = 0.796, P < 0.001 for males; r = 0.710, P < 0.001 for females). Leptin concentrations were generally higher in male rats than in females; plasma leptin increased 0.60 microgram/L for each percentage of increase in body fat for males but only 0.22 microgram/L for females. We conclude that rat serum/plasma leptin concentrations are accurately and precisely measured with this new RIA.

Hyperleptinemia in patients with end-stage renal disease undergoing continuous ambulatory peritoneal dialysis

OBJECTIVE

To determine whether the increased plasma leptin levels reported in hemodialyzed patients is a feature of end-stage renal disease or an artifact of hemodialysis, we studied plasma levels in patients treated exclusively by continuous ambulatory peritoneal dialysis (CAPD).

DESIGN

Prospective comparison of end points in CAPD patients and matched healthy subjects.

SETTING

Tertiary care institutional dialysis center.

PARTICIPANTS

Fifty-six healthy subjects, age 50.8 +/- 2.3 years, body mass index (BMI) 27.7 +/- 1.3 kg/m2, recruited through public announcement, and 36 patients with end-stage renal disease, age 51.0 +/- 2.4 yr, BMI 28.2 +/- 1.3 kg/m2, enrolled in a CAPD treatment program.

INTERVENTION

Four exchanges of CAPD per day, using 2.0, 2.5, or 3.0 L of dialysate over a period of 1 - 96 months (median 22 mth).

MAIN OUTCOME MEASURES

The primary outcome measure was plasma leptin concentration. Secondary measures included plasma glucose, insulin, C-peptide, and cortisol concentrations; and residual renal function and dialysis adequacy (Kt/V).

RESULTS

Plasma leptin levels in CAPD patients were 27.1 - 490 ng/mL (women) and 1.3 - 355 ng/mL (men); the levels in healthy subjects were 2.0 - 84.7 ng/mL (women) and 1.8 - 55.4 ng/mL (men). The mean leptin levels were 5-fold higher among CAPD-treated men than control men (49.9 +/- 18.4 vs 9.8 +/- 2.5 ng/mL, p < 0.001) and 7.5-fold higher among CAPD-treated women than control women (220 +/- 28.1 vs 29.3 +/- 3.7 ng/mL, p < 0.0001). Female gender and BMI were the strongest predictors of hyperleptinemia in CAPD patients.

CONCLUSION

These results indicate that hyperleptinemia is a feature of terminal renal failure, not an artifact of hemodialysis.

Whole body leptin kinetics and renal metabolism in vivo

Leptin metabolism was investigated in male Sprague-Dawley rats by use of 125I-labeled leptin plasma kinetic and arteriovenous balance studies. When conscious rats received bolus venous injections of 125I-leptin, intact (precipitable) leptin quickly disappeared from circulation in a biexponential manner during the 2-h experimental period. After substantial delay, most of the injected radioactivity appeared in the urine. The data were described by a two-compartment model, which postulated that plasma leptin exchanged with a nonplasma pool and that all of the tracer cleared from plasma appeared in urine or in a degraded form in plasma. The half-life of leptin was 9.4 +/- 3.0 min, and the leptin production rate was 3.6 +/- 1.2 ng 100 g fat-1.min-1. The left kidney extracted 21 +/- 1.5% of intact arterial 125I-leptin 5 min after femoral venous injection. Endogenous arterial leptin was reduced 21 +/- 8 and 18 +/- 12%, respectively, in simultaneously sampled left and right renal veins. Renal elimination appears to be the major elimination mechanism for leptin in normal rats, and the kinetic studies suggest that uptake of leptin by renal tissue rather than glomerular filtration is the predominant elimination mechanism.

Calcium-stimulated phosphorylation of MAP-2 in pancreatic betaTC3-cells is mediated by Ca2+/calmodulin-dependent kinase II

An understanding of the role of CaM kinase II in the pancreatic beta-cell is dependent on the identification of its cellular targets. One of the best substrates of CaM kinase II in vitro that could function in secretory events is the microtubule-associated protein, MAP-2. By immunoblot analysis, a high molecular weight protein with electrophoretic properties characteristic of MAP-2, was identified in rat insulinoma betaTC3 cells and isolated rat islets. In immunoprecipitation experiments employing alpha-toxin-permeabilized betaTC3 cells, elevation of intracellular Ca2+ or addition of forskolin, an adenylate cyclase activator, induced significant phosphorylation of MAP-2 in situ. The effect of Ca2+ was rapid, concentration-dependent and closely correlated with activation of CaM kinase II under similar experimental conditions. H-89, a specific and potent inhibitor of cAMP-dependent protein kinase (PKA), prevented forskolin-induced MAP-2 phosphorylation but had little effect on MAP-2 phosphorylation stimulated by elevated Ca2+. Phosphopeptide mapping revealed that the phosphorylation pattern observed in situ upon incubation of the betaTC3 cells with increased free Ca2+, was strikingly similar to that generated in vitro by CaM kinase II, most notably with regard to the increased phosphate incorporated into one prominent site. These data provide evidence that MAP-2 is phosphorylated by CaM kinase II in the pancreatic beta-cell in situ, and that this event may provide an important link in the mediation of Ca2+-dependent insulin secretion.

Prolonged exercise decreases serum leptin concentrations

Serum leptin and free fatty acid concentrations were determined in two groups of subjects undergoing strenuous exercise: 12 men who fasted overnight and then pedaled a stationary ergometer for 2 hours, and 14 nonfasting ultramarathon runners. Blood samples were collected before exercise, immediately after cessation of exercise, and 6 to 24 hours after the end of the exercise period. Two hours of strenuous pedaling following an overnight fast significantly reduced mean leptin levels by 8.3%; free fatty acids were highly increased and correlated well with the decrease in serum leptin (r = .737, P = .01). After 6 hours of rest and refeeding, leptin concentrations recovered to preexercise levels and free fatty acid concentrations were decreased to less than preexercise levels. A similar decrease in serum leptin levels (12.3%) occurred in subjects who fasted overnight and then for a period corresponding to the cycle exercise period. The prolonged exercise of an ultramarathon significantly reduced leptin concentrations by 32% in comparison to prerace levels; free fatty acid concentrations were highly increased, but did not correlate with the change in serum leptin concentrations (r = .366, P = .20). Leptin and free fatty acid concentrations all trended toward prerace levels in blood samples collected 18 to 24 hours after cessation of racing. The results suggest that the negative energy balance of exercise can reduce serum leptin concentrations, but that the significant decrease occurs only at extremes of severity/duration of the exercise-induced negative balance. The possible physiological role of reduced leptin concentrations in response to energy balance and the role of free fatty acids in mediating the response are discussed.

Comparison of a new anti-glutamic acid decarboxylase (GAD) enzyme-linked immunosorbent assay (ELISA) with radioimmunoassay methods: a multicenter study

Several methods are available for the measurement of antibodies to glutamic acid decarboxylase (anti GAD). These antibodies are valuable tools for the immunodiagnosis of insulin-dependent (type 1) diabetes mellitus (IDDM) and for the assessment of risk for the future development of IDDM. We here describe a new enzyme-linked immunosorbent assay (ELISA) for the detection of anti-GAD which was tested in a multicenter study. The results of the new anti-GAD ELISA correlate well with those obtained by radioimmunoassays (RIA) and they have a higher sensitivity (69%) and specificity (98%) compared to other anti-GAD enzyme immunoassays as determined in the IDW Proficiency Test Program for the detection of GAD antibodies. The new ELISA is simple and easy to perform, with convenient handling of the reagents. Quantitative and reproducible test results are available within approximately four hours. The new anti-GAD ELISA can be used for large scale population screening to indicate a prediabetic state as well as to diagnose autoimmune diabetes in adults (LADA) and the risk for IDDM in pregnant women with gestational diabetes.

Extraction of glyceric and glycolic acids from urine with tetrahydrofuran: utility in detection of primary hyperoxaluria

Primary hyperoxaluria (PH) is an autosomal recessive metabolic abnormality characterized by excessive oxalate excretion leading to nephrocalcinosis and progressive renal dysfunction. Type I primary hyperoxaluria (PH I) results from a deficiency of alanine:glyoxylate aminotransferase, whereas type II disease has been traced to a deficiency of D-glycerate dehydrogenase. The two syndromes are often distinguished on the basis of organic acids that are coexcreted with oxalate: glycolate and L-glycerate in type I and type II disease, respectively. Routine organic acid analysis with diethyl ether extraction followed by gas chromatographic analysis failed to detect normal and increased concentrations of these diagnostic metabolites. Subsequent extraction of urine with tetrahydrofuran (THF), however, extracted 75% of added glycerate, 42% of added glycolate, and 75% of added ethylphosphonic acid (internal calibrator). THF extraction was analytically sensitive enough to allow determination of normal excretion of glycolate (14-72 micrograms/mg creatinine) and glycerate (0-5 years, 12-177 micrograms/mg creatinine and > 5 years, 19-115 micrograms/mg creatinine). Four of five patients with PH I and both patients with type II disease were correctly identified. Thus, THF extraction is a convenient adjunct to routine organic acid analysis and facilitates the detection of PH.

Interleukin-1 reduces the glycolytic utilization of glucose by pancreatic islets and reduces glucokinase mRNA content and protein synthesis by a nitric oxide-dependent mechanism

Culture of rat pancreatic islets with interleukin-1 (IL-1) results in up-regulation of the inducible isoform of nitric oxide synthase and overproduction of nitric oxide (NO). This is associated with reversible inhibition of both glucose-induced insulin secretion and islet glucose oxidation, and these effects are prevented by the inducible nitric oxide synthase inhibitor NG-monomethylarginine. IL-1 also induces accumulation of nonesterified arachidonic acid in islets by an NO-dependent mechanism, and one potential explanation for that effect would involve an IL-1-induced enhancement of islet glycolytic flux. We have therefore examined effects of IL-1 on islet glycolytic utilization of glucose and find that culture of islets with IL-1 in medium containing 5.5 mM glucose results in suppression of islet glucose utilization subsequently measured at glucose concentrations between 6 and 18 mM. The IL-1-induced suppression of islet glucose utilization is associated with a decline in islet glucokinase mRNA content, as determined by competitive reverse transcriptase-polymerase chain reaction, and in glucokinase protein synthesis, as determined by immuoprecipitation experiments, and all of these effects are prevented by NG-monomethylarginine. These findings suggest that IL-1 can down-regulate islet glucokinase, which is the primary component of the islet glucose-sensor apparatus, by an NO-dependent mechanism. Because reductions in islet glucokinase levels are known to cause a form of type II diabetes mellitus, these observations raise the possibility that factors which increase islet NO levels might contribute to development of glucose intolerance.

Response of serum leptin concentrations to 7 d of energy restriction in centrally obese African Americans with impaired or diabetic glucose tolerance

The aim of this study was to determine whether serum leptin concentrations are reduced in response to short-term energy restriction in centrally obese individuals with impaired glucose tolerance or non-insulin-dependent diabetes mellitus. Twenty African Americans [16 females and 4 males, 44 +/- 7 y (x +/- SD), 107.2 +/- 23.8 kg, 39 +/- 7% body fat] consumed a 7-d energy-restricted diet (4.03 +/- 0.72 MJ/d) of whole foods. Oral-glucose-tolerance tests (OGTTs) were performed before and immediately after the diet to assess changes in serum leptin, glucose, and insulin concentrations. Baseline leptin concentration correlated significantly with percentage body fat (r = 0.80), body mass index (r = 0.72), fat mass (4 = 0.64), waist-height ratio (r = 0.6), body weight (r = 0.59, all P < 0.01), waist circumference (r = 0.49), and basal insulin concentration (r = 0.48, both P < 0.05). Seven days of energy restriction resulted in significant reductions (P < 0.005) in leptin (-6.1 +/- 8.4 micrograms/L), basal glucose (-0.9 +/- 0.8 mmol/L), OGTT glucose area under the curve (-158 +/- 164 mmol/L), and basal insulin concentration (-34 +/- 69 pmol/L, P < 0.05). In addition, there was a trend for a reduction in OGTT insulin area under the curve (-15,567 +/- 3,658 pmol/L, P = 0.05), and a tendency for basal insulin and leptin to change together (r = 0.41, P = 0.07). Despite the weight loss of 3.1 +/- 1.3 kg (P < 0.0001), the loss of fat mass was calculated to be only -1.0 +/- 0.1 kg. These results suggest that negative energy balance or improved insulin action was responsible for the changes in leptin, glucose, and insulin concentrations. In summary, short-term energy restriction effectively reduced serum leptin concentrations and improved glucose tolerance and insulin action in obese individuals with impaired or diabetic glucose tolerance.

Leptin production during moderate-intensity aerobic exercise

Leptin, the protein product of the ob gene, may be involved in the regulation of energy balance. Although a clear relationship between energy intake and plasma leptin concentrations has been demonstrated in humans, little is known about the effect of exercise on leptin metabolism. In the present study, we evaluated abdominal adipose tissue leptin production in vivo by arteriovenous balance at rest and during 60 min of moderate-intensity cycle ergometer exercise (50% of maximal heart rate) in five sedentary male subjects (mean age 38.4 +/- 1.7 yr, body mass index (28.4 +/- 4.2 kg/m2). Blood samples were taken simultaneously from an abdominal vein, draining sc adipose tissue, and a radial artery, at rest and every 10 min during exercise. Adipose tissue blood flow was determined by the xenon washout technique. Plasma leptin concentrations did not change throughout exercise and were the same as the values obtained during resting conditions. Average net adipose tissue leptin production rates during exercise (3.07 +/- 0.89 ng/100 g-1.min-1) also were similar to resting values (3.86 +/- 0.95 ng/100 g-1.min-1). These results demonstrate that plasma leptin concentrations and leptin production do not change during an acute bout of moderate-intensity aerobic exercise.

Black children deficient in galactose 1-phosphate uridyltransferase: correlation of activity and immunoreactive protein in erythrocytes and leukocytes

A recent study found a high prevalence of a missense mutation (S135L) in the gene for galactose 1-phosphate uridyltransferase (GALT) in black children with galactosemia (J Pediatr 1996; 128:89-95). In the present study, GALT activity and GALT protein content were measured in erythrocytes and leukocytes of eight black and seven white galactosemic (GALT-deficient) children, for correlation with the presence of the S135L and Q188R (highly prevalent in white galactosemic children) missense mutations. The S135L mutation was found in 9 of 16 alleles of black children but not in white children; the Q188R mutation was found in 10 of 14 alleles examined in white galactosemic children and in 4 of 16 alleles in black galactosemic children. The GALT activity was near zero in the erythrocytes of white and black galactosemic children (0.26 +/- 0.28 vs 0.33 +/- 0.25 mumol/hr per gram of hemoglobin, respectively; p = 0.61) (normal 17 to 26 mumol/hr per gram), and no correlation of erythrocyte activity with genotype was observed. The GALT activity was higher in the leukocytes of black galactosemic children compared with white children (5 +/- 6 vs 1 +/- 2 mumol/hr per gram, respectively) (normal 172 to 374 mumol/hr per gram), but the difference was not statistically significant (p = 0.11). Analysis by genotype revealed that the two S135L homozygotes had much more leukocyte activity (9 and 17 mumol/hr per gram) than Q188R homozygotes or than all non-S135L allelic genotypes. Compound heterozygotes (S135L/G) had intermediate activity. The GALT protein was not detectable by Western blot in the erythrocytes of either white or black galactosemic children, as determined by antibodies specific for both C- and N-terminal sequences. The GALT protein was undetectable in the leukocytes of white galactosemic children, but leukocytes from black galactosemic children with the S135L mutation contained reduced but readily detectable GALT protein. Erythrocyte galactose 1-phosphate levels were significantly lower in galactosemic children with an S135L mutant allele (1.1 +/- 0.2 gm/dl) compared with children who had other mutations (3.1 +/- 0.9 mg/dl; p = 0.0001). The correlation of protein content data with activity levels in the blood cells suggests that the S135L missense mutation affects the stability of GALT protein to produce a deficiency state.

Cardiac troponin I in pediatrics: normal values and potential use in the assessment of cardiac injury

OBJECTIVE

To establish normal values and determine the impact of congenital or acquired heart disease on serum cardiac troponin I (cTnI).

METHODS

Concentrations of cTnI were measured in two groups of children. Group A represented ambulatory pediatric patients with no apparent cardiac disease (n = 120) and patients in stable condition with known congenital or acquired cardiac abnormalities (n = 96); group B was composed of patients admitted to intensive care units with normal echocardiograms (n = 16), with abnormal echocardiograms (n = 36), and those with blunt chest trauma who were thought to have cardiac contusions (n = 7).

RESULTS

The cTnI concentrations were generally less than 2.0 ng/ml in group A and frequently below the level of detection for the assay (1.5 ng/ml). There was no statistical difference between the two outpatient subgroups (p = 0.66). Nine intensive care patients had cTnI values greater than 2.0 ng/ml. Six of these patients, all with abnormal echocardiograms, had values less than 7.7 ng/ml. All improved and had subsequent normal cTnI concentrations. None of the three remaining patients (two with systemic illness (trauma and sepsis) and one with severe pulmonary hypertension), all with values greater than 8.0 ng/ml, survived. Three of the four patients with high likelihood of cardiac contusion had cTnI concentrations greater than 2.0 ng/ml (including one patient who died).

CONCLUSIONS

Cardiac troponin-I values are generally not elevated in children with stable cardiac disease or general pediatric conditions. In the context of severe acute illness, significant elevation of cTnI may be an indicator of poor outcome. Elevation of cTnI may also have diagnostic value in cases when cardiac contusion is suspected.

Correlation of the activation of Ca2+/calmodulin-dependent protein kinase II with the initiation of insulin secretion from perifused pancreatic islets

An experimental procedure has been designed to permit the simultaneous assessment of the activation status of the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) with insulin secretion in perifused islets. By this procedure, the activation of CaM kinase II by glucose correlated closely with the initial and sustained phases of insulin secretion within a 30-min test period. By contrast, islets (160-200/tube) in static incubations neither supported second-phase insulin secretion nor CaM kinase II activation beyond 10-15 min. This was not the result of the accumulation of insulin, because the introduction of insulin (40-160 ng/ml) into the perifusion medium failed to mimic the suppression of glucose-induced insulin secretion or CaM kinase II activation. A similar addition of SRIF (0.01-1 microM) or epinephrine (1 microM) profoundly suppressed insulin secretion although failing to significantly influence CaM kinase II activation. Finally, on withdrawal of glucose from perifused islets, insulin secretion rapidly returned to basal rates, but CaM kinase II deactivation was significantly delayed. The correlation of kinase activation with the initiation of insulin secretion suggests that CaM kinase II may be important in the regulation of glucose-induced insulin secretion. The observed dissociation of these parameters in the presence of inhibitory hormones or after the withdrawal of a glucose stimulus, however, suggests that the kinase is not directly involved in the final steps of insulin exocytosis.

Serum leptin concentrations in human immunodeficiency virus-infected men with low adiposity

The product of the obese gene (ob) is the protein leptin, which is synthesized in and secreted from adipocytes. Fasting serum leptin concentrations are closely related to body fat content and are higher in obese than in normal-weight individuals. Leptin may contribute to body weight regulation. Overproduction of leptin in certain pathologic conditions such as acquired immunodeficiency syndrome (AIDS) might in principle contribute to the low body fat content associated with body wasting. We measured fasting serum leptin levels by radioimmunoassay in individuals infected with the human immunodeficiency virus (HIV) and in a group of healthy lean men to determine whether HIV infection increases leptin levels. Thirteen HIV-infected men aged 26 to 50 years with a body mass index (BMI) of 15 to 26 kg/m2 and 4 to 24 kg body fat (7% to 29% body fat) had serum leptin levels (3.4 +/- 1.6 ng/mL) that were not elevated compared with the levels in 17 healthy men (4.0 +/- 1.4 ng/mL) matched for age (23 to 47 years), BMI (18 to 26 kg/m2), and body fat (5 to 21 kg; 9% to 28%). In both groups of men, serum leptin concentrations were correlated with percent body fat and body fat content (P < .001), and these relationships were not different between the two groups. In both groups, leptin concentrations were not correlated with lean body mass (P > or = .24). Energy intake in the HIV-infected men, assessed from 3-day intake records, was within the normal range. These findings extend the hypothesis that circulating leptin concentrations directly reflect adipose tissue mass, even in HIV-infected men with low body fat content. These findings do not support the hypothesis that HIV infection is associated with high circulating leptin concentrations, and suggest that low leptin levels do not stimulate food intake in HIV-infected individuals.

Increased plasma leptin concentration in end-stage renal disease

Leptin is a 16-kDa protein recently identified as the obese gene product involved in body weight regulation. Administration of recombinant leptin to ob/ob mice, which have a genetic defect in leptin production, reduces food intake and increases energy expenditure. Leptin is synthesized by fat cells, and in normal humans, plasma concentrations are proportional to adiposity. The physiological actions and the degradation pathways of leptin in humans are unknown. We investigated renal elimination of leptin by comparing plasma leptin concentrations in end-stage renal disease (ESRD) patients with normal controls. Our hypothesis was that if renal filtration is a significant route of elimination, the hormone would accumulate in ESRD patients. Mean plasma levels in 141 ESRD patients (26.8 +/- 5.7 and 38.3 +/- 5.6 micrograms/L for males and females, respectively) were significantly higher (P < 0.001) than mean values obtained in normal controls (11.9 +/- 3.1 and 21.2 +/- 3.0 micrograms/L for males and females, respectively). Leptin concentrations in ESRD patients correlated directly with body mass index (BMI; r = 0.77 for men and 0.78 for women). The rate of increase in leptin concentrations with BMI was significantly greater in ESRD patients (5.5 and 6.6 micrograms/L/U BMI for men and women, respectively) than in normal controls (1.4 and 2.6 micrograms/L/U for men and women, respectively). Pre- and postdialysis leptin levels in hemodialysis patients were similar. Western blot of plasma from ESRD patients with high leptin levels showed bands corresponding to the intact protein (16 kDa) with no lesser or greater molecular mass species observed. Leptin concentrations in patients with ESRD did not correlate with measures of residual renal function (serum creatinine, beta 2-microglobulin, PTH, or GH levels). Similarly, we found no correlation between leptin levels and the number of years patients had been on dialysis or with recent weight changes. We conclude that intact leptin is increased in ESRD patients, but does not appear to cause decreased weight. As leptin levels did not correlate with residual renal function, increased production may account for the high levels observed.

Serum leptin levels are reduced in response to exercise training, but not hormone replacement therapy, in older women

The aim of this study was to evaluate the effects of exercise training and hormone replacement therapy (HRT) on serum leptin levels in older women. Previously sedentary, healthy women, aged 60-72 yr, were assigned to control (n = 16), exercise (n = 17), HRT (n = 15), or exercise + HRT (n = 13) groups. Exercise training consisted of a 2-month flexibility-exercise program followed by a 9-month exercise program that included walking, jogging, and stair climbing. HRT consisted of 11 months of continuous conjugated estrogens (0.625 mg/day) and medroxyprogesterone acetate (5 mg/day) for 13 days every third month. Body composition was assessed by dual-energy x-ray absorptiometry, and serum insulin levels were measured in the fasted state and in response to a glucose challenge. Leptin levels were reduced by 23 +/- 25% and 22 +/- 27% (both P < 0.01) in response to exercise and exercise + HRT, respectively. There was no effect of HRT on leptin. Fat mass was the strongest predictor of serum leptin concentration, both before (r = 0.81; P < 0.001) and after (r = 0.85; P < 0.001) the study period, and the change in fat mass in the exercisers was significantly correlated with the change in leptin (r = 0.55; P < 0.01). There did not seem to be an effect of exercise, independent of the reduction in fat mass, on leptin. Insulin levels were significantly correlated with leptin levels, but this was not independent of the association with adiposity. The curvilinear relationship between leptin level and fat mass and the finding that the ratio of leptin mass to fat mass decreased after weight loss suggest that fat cell size is an important determinant of circulating leptin levels.

Serum leptin levels in women with anorexia nervosa

Leptin is a protein encoded by the ob gene that is expressed in adipocytes and regulates eating behavior via central neuroendocrine mechanisms. Serum leptin levels have been shown to correlate with weight and percent body fat in normal and obese individuals; however, it is not known whether the regulation of leptin is normal below a critical threshold of body fat in chronic undernutrition. We investigated serum leptin levels in 22 women, aged 23 +/- 4 yr, with anorexia nervosa. Duration of disease, weight, BMI, percent body fat, and serum leptin levels were determined for each patient. Nutritional status was assessed further by caloric intake and measurement of insulin and insulin-like growth factor I (IGF-I) levels. Twenty-three healthy women, aged 23 +/- 4 yr, taking no medications, with normal menstrual function and body mass index (BMI) between 20-26 kg/m2 (mean, 23.7 +/- 1.7 kg/m2), served as a control population for comparison of leptin levels. Subjects with anorexia nervosa were low weight (BMI, 16.3 +/- 1.6 kg/m2; normal, 20-26 kg/m2) and exhibited a striking reduction in percent body fat (7 +/- 2%; normal, 20-30%). The mean serum leptin level was significantly decreased in subjects with anorexia nervosa compared with that in age- and sex-matched controls of normal body weight (5.6 +/- 3.7 vs. 19.1 +/- 8.1 ng/mL; P < 0.0001). Serum leptin levels were correlated highly with weight, as expressed either BMI (r = 0.66; P = 0.002) or percent ideal body weight (r = 0.68; P = 0.0005), body fat (r = 0.70; P = 0.0003), and IGF-I (r = 0.64; P = 0.001), but not with caloric intake or serum levels of estradiol or insulin in subjects with anorexia nervosa. The correlation between leptin and body fat was linear, with progressively lower, but detectable, leptin levels measured even in patients with less than 5% body fat, but was not significant when the effects of weight were taken into account. In contrast, the correlation between leptin and IGF-I remained significant when the effects of weight, body fat, and caloric intake were taken into account. In normal controls, leptin correlated with BMI (r = 0.55; P = 0.007) and IGF-I (r = 0.44; P < 0.05), but not with fat mass. These data demonstrate that serum leptin levels are reduced in association with low weight and percent body fat in subjects with anorexia nervosa compared to normal controls. Leptin levels correlate highly with weight, percent body fat, and IGF-I in subjects with anorexia nervosa, suggesting that the physiological regulation of leptin is maintained in relation to nutritional status even at an extreme of low weight and body fat.

Adipose tissue leptin production and plasma leptin kinetics in humans

Abdominal adipose tissue leptin production was determined in vivo by arteriovenous balance in 14 lean and obese men (mean BMI 27.0 +/- 1.9, range 21.4-45.2). Blood samples were taken simultaneously from an abdominal vein that drains subcutaneous adipose tissue and from a radial artery. Adipose tissue blood flow was measured by xenon washout. Abdominal vein leptin concentrations (mean 8.9 +/- 2.4 ng/ml, range 2.1-36.5 ng/ml) were consistently greater than arterial values (mean 6.6 +/- 1.9 ng/ml, range 1.7-28.2 ng/ml) (P < 0.001). The net rate of abdominal adipose tissue leptin production (mean 3.2 +/- 0.5 ng x 100 g(-1) x min(-1)) correlated directly with percentage body fat (rs = 0.59, P = 0.016). Estimated whole-body leptin production rate (797 +/- 283 ng x person(-1) x min(-1)) correlated directly with percent body fat (rs = 0.93, P < 0.0001) and with regional leptin production (rs = 0.81, P < 0.001). In contrast, the rate of leptin clearance from plasma (mean 1.50 +/- 0.23 ml x kg(-1) x min(-1)) and plasma leptin half-life (mean 24.9 +/- 4.4 min) was unrelated to adiposity (rs = 0.06, P = 0.30; rs = 0.16, P = 0.30, respectively). These results provide direct evidence that leptin is produced by adipose tissue in humans and that the rate of production is directly related to adiposity. A combination of greater leptin production per unit of body fat and increased production from expanded total body fat mass, rather than alterations in leptin clearance, account for the increase in plasma leptin concentrations observed in obese humans.

Relationship between insulin sensitivity and plasma leptin concentration in lean and obese men

Alterations in the production of or the sensitivity to leptin, the protein encoded by the ob gene, cause obesity and diabetes in rodents. We evaluated the isolated relationship between leptin and insulin sensitivity in lean and obese humans. Three groups of subjects who were carefully matched for either insulin sensitivity (determined by the modified intravenous glucose tolerance test and minimal model analysis) or adiposity (determined by hydrodensitometry) were studied: 1) lean insulin-sensitive men (percentage body fat, 15 +/- 1%); 2) lean insulin-resistant men (percentage body fat, 16 +/- 1%), matched on percentage body fat and fat mass with the lean insulin-sensitive group; and 3) obese insulin-resistant men (percentage body fat, 31 +/- 3), matched on insulin sensitivity with the lean insulin-resistant group. Basal plasma leptin concentrations were significantly lower in the lean insulin-sensitive than in the lean insulin-resistant men (1.90 +/- 0.4 vs. 4.35 +/- 1.21 ng/ml, P < 0.05) despite identical body composition. Plasma leptin in the obese men (9.27 +/- 1.4 ng/ml) was significantly higher than values in the two lean groups (P < 0.01). Marked alterations in plasma glucose and insulin concentrations induced by glucose and tolbutamide injection did not cause any change in plasma leptin levels. These results demonstrate that insulin resistance is associated with elevated plasma leptin levels independent of body fat mass. However, plasma insulin itself does not acutely regulate leptin production.

Muscarinic activation of Ca2+/calmodulin-dependent protein kinase II in pancreatic islets. Temporal dissociation of kinase activation and insulin secretion

We have demonstrated previously that glucose activates the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) in isolated rat pancreatic islets in a manner consistent with a role of this enzyme in the regulation of insulin secretion [Wenham, Landt and Easom (1994) J. Biol. Chem. 269, 4947-4952]. In the current study, the muscarinic agonist, carbachol, has been shown to induce the conversion of CaM kinase II into a Ca(2+)-independent, autonomous form indicative of its activation. Maximal activation (2-fold) was achieved by 15 s, followed by a rapid return to basal levels by 1 min. This response was primarily the result of the mobilization of Ca2+ from intracellular stores since it was not affected by a concentration (20 microM) of verapamil that completely prevented the activation of CaM kinase II by glucose. Surprisingly, carbachol added prior to, or simultaneously with, glucose attenuated nutrient activation of CaM kinase II. This effect was mimicked by cholecystokinin-8 (CCK-8) and thapsigargin, suggesting its mediation by phospholipase C and the mobilization of intracellular Ca2+. In contrast, carbachol, CCK-8 and thapsigargin markedly potentiated glucose (12 mM)-induced insulin secretion. These results suggest that CaM kinase II activation can be temporally dissociated from insulin secretion but do not exclude the potential dependence of insulin exocytosis on CaM kinase II-mediated protein phosphorylation.

Radioimmunoassay of leptin in human plasma

Recent studies suggest that leptin, the ob gene product absent in ob/ob mice, is a negative regulator of adiposity. We developed an RIA to measure human leptin in plasma or serum. The minimum detectable concentration by the assay is 0.5 microg/L leptin and the limit of linearity is 100 microg/L. Recovery of leptin added to serum was 99-104% over by the linear range of the assay. The RIA agreed reasonably well with rough quantification by Western blot (RIA = 0.90 blot + 3.7 microg/L, Sy/x = 10.9 microg/L). CVs within- and between-run ranged from 3.4% to 8.3% and from 3.6% to 6.2%, respectively. Variation in plasma leptin concentrations in specimens collected on consecutive mornings was large (CVs of 10.9% and 22.5%). After an overnight fast, leptin concentrations were similar to those 1-2 h after 1-2 meals. Plasma leptin concentrations in specimens from 83 lean and obese adults correlated directly with body mass index (BMI; kg/m2): r = 0.72, P <0.001. Correlations were significantly improved by separating results by gender (men r = 0.84, women r = 0.87; p <0.001). The increase in leptin concentrations with increasing BMI was greater in women than in men (slope 2.53 vs 0.97 microg/L per unit BMI, respectively). Leptin concentrations determined in lean subjects (BMI between 18 and 25) were higher in women (7.36 +/- 3.73 microg/L) than in men (3.84 +/- 1.79 microg/L) (P <0.001). Plasma leptin varied little with age and no significant difference was observed between whites and blacks. We conclude that: (a) plasma leptin concentrations are accurately and precisely measured by this new RIA; (b) leptin concentrations vary little due to short-term fasting, age, or race; but (c) plasma leptin concentrations are gender specific.

Radioimmunoassay of leptin in human plasma

Recent studies suggest that leptin, the ob gene product absent in ob/ob mice, is a negative regulator of adiposity. We developed an RIA to measure human leptin in plasma or serum. The minimum detectable concentration by the assay is 0.5 microg/L leptin and the limit of linearity is 100 microg/L. Recovery of leptin added to serum was 99-104% over by the linear range of the assay. The RIA agreed reasonably well with rough quantification by Western blot (RIA = 0.90 blot + 3.7 microg/L, Sy/x = 10.9 microg/L). CVs within- and between-run ranged from 3.4% to 8.3% and from 3.6% to 6.2%, respectively. Variation in plasma leptin concentrations in specimens collected on consecutive mornings was large (CVs of 10.9% and 22.5%). After an overnight fast, leptin concentrations were similar to those 1-2 h after 1-2 meals. Plasma leptin concentrations in specimens from 83 lean and obese adults correlated directly with body mass index (BMI; kg/m2): r = 0.72, P &lt;0.001. Correlations were significantly improved by separating results by gender (men r = 0.84, women r = 0.87; p &lt;0.001). The increase in leptin concentrations with increasing BMI was greater in women than in men (slope 2.53 vs 0.97 microg/L per unit BMI, respectively). Leptin concentrations determined in lean subjects (BMI between 18 and 25) were higher in women (7.36 +/- 3.73 microg/L) than in men (3.84 +/- 1.79 microg/L) (P &lt;0.001). Plasma leptin varied little with age and no significant difference was observed between whites and blacks. We conclude that: (a) plasma leptin concentrations are accurately and precisely measured by this new RIA; (b) leptin concentrations vary little due to short-term fasting, age, or race; but (c) plasma leptin concentrations are gender specific.

Plasma leptin and insulin relationships in obese and nonobese humans

Hyperinsulinemia. is associated with an overexpression of mRNA for the ob protein leptin in rodent models of genetic obesity, and insulin has been reported to directly stimulate leptin mRNA in rat adipocytes. Human obesity is also associated with increased leptin mRNA as well as plasma levels, but there have been no reports of the effect of insulin on leptin secretion. We, therefore, tested the hypothesis that insulin stimulates leptin secretion in humans. Using a newly developed leptin assay, immunoreactive leptin was measured in fasting and postprandial plasma samples from 27 healthy adults and in samples before and during euglycemic-hyperinsulinemic then stepped hypoglycemic (hourly steps at 85, 75, 65, 55, and 45 mg/dl) clamps from 10 healthy subjects and 11 patients with IDDM. Plasma leptin was correlated (r = 0.84, P = 0.0005) with BMI in obese but not nonobese subjects and with fasting (r = 0.75, P = 0.008) but not postprandial plasma insulin levels. (Leptin levels did not change postprandially.) Euglycemic hyperinsulinemia did not alter leptin levels, nor did hyperinsulinemic hypoglycemia. Thus, because circulating leptin levels are not increased during postprandial hyperinsulinemia or during euglycemic (or hypoglycemic) hyperinsulinemia, we conclude that, at least in the short term, insulin does not increase leptin secretion in humans and that hyperleptinemia in obese individuals is not likely the result of hyperinsulinemia.