Urea Determination in Dialysis, based on a Differential pH Technique

The application of a new technique, based on differential measurements of pH, to determine urea concentration in patients of a dialysis center, is reported. Urea in plasma, whole blood or dialysis fluids is measured by an enzymatic reaction, with urease; the procedure, requiring 10 μL of sample, is simple, fast and correlates well with a reference spectrophotometric method, in the 0-300 mg/dL concentration range, according to the equation y = 1.0291 x - 0.0777; r = 0.9991; n = 73.

Amino acid- and lipid-induced insulin resistance in rat heart: molecular mechanisms

Lipids compete with glucose for utilization by the myocardium. Amino acids are an important energetic substrate in the heart but it is unknown whether they reduce glucose disposal. The molecular mechanisms by which lipids and amino acids impair insulin-mediated glucose disposal in the myocardium are unknown. We evaluated the effect of lipids and amino acids on the insulin stimulated glucose uptake in the isolated rat heart and explored the involved target proteins. The hearts were perfused with 16 mM glucose alone or with 6% lipid or 10% amino acid solutions at the rate of 15 ml/min. After 1 h of perfusion (basal period), insulin (240 nmol/l) was added and maintained for an additional hour. Both lipids and amino acids blocked the insulin effect on glucose uptake (P<0.01) and reduced the activity of the IRSs/PI 3-kinase/Akt/GSK3 axis leading to the activation of glucose transport and glycogen synthesis. Amino acids, but not lipids, increased the activity of the p70 S6 kinase leading to the stimulation of protein synthesis. Amino acids induce myocardial insulin resistance recruiting the same molecular mechanisms as lipids. Amino acids retain an insulin-like stimulatory effect on p70 S6 kinase, which is independent from the PI 3-Kinase downstream effectors.

The application of the differential pH method to the biochemical evaluation of seminal plasma

OBJECTIVE

To apply the differential pH method to the biochemical evaluation of seminal plasma.

DESIGN AND METHODS

Fructose, citric acid and free L-carnitine concentration in human seminal plasma were measured by the differential pH method. These are routinely taken as markers of seminal vesicle, prostate and epididymal function, respectively. The pH differential method was then compared with standard spectrophotometry.

RESULTS

The linearity, recovery and imprecision calculation for each assay were found to be reliable and the method correlated well with the reference spectrophotometric technique. In addition, the pH differential method showed certain advantages compared to spectrophotometry: a) speed of procedure (40 to 120 sec/assay); b) ability to measure the analytes even in small volumes of seminal plasma (10 microL to 50 microL) without deproteinization or dilution, thus eliminating manual operations.

CONCLUSIONS

The differential pH method is an innovative approach to the biochemical assessment of seminal plasma.

Reduction of insulin resistance by combined kidney-pancreas transplantation in type 1 (insulin-dependent) diabetic patients

To evaluate the effect of combined kidney and pancreas transplantation on insulin action and glucose metabolism, 15 Type 1 (insulin-dependent) diabetic patients who were undergoing combined kidney-pancreas transplantation were studied before transplantation by means of the euglycaemic hyperinsulinaemic clamp technique combined with 3-3H-glucose infusion and indirect calorimetry. Nine of the original 15 patients were studied again after four months and six after 12 months, successful combined kidney-pancreas transplantation with the same experimental protocol. Nine volunteers formed the group of normal subjects. Combined kidney-pancreas transplantation normalised hepatic glucose production and reduced peripheral insulin resistance in Type 1 diabetic uraemic patients, despite chronic immunosuppressive therapy. To further evaluate the hypothesis that residual insulin resistance was due to chronic steroid therapy. 11 additional subjects with chronic uveitis (six of whom were treated with only prednisone, and five treated only with cyclosporin) underwent the same protocol demonstrating a normal hepatic glucose production. The insulin-stimulated peripheral glucose uptake was reduced in the prednisone-treated group, but normal in cyclosporin-treated subjects. Four additional diabetic patients with a kidney transplant were also studied. They showed a peripheral insulin sensitivity intermediate between diabetic uraemic patients and patients after combined transplant. We conclude that short-term (one year) combined kidney-pancreas transplantation improves glucose metabolism by restoring normal rates of hepatic glucose production and reducing peripheral insulin resistance; chronic steroid therapy is the major determinant of residual reduced insulin action. Both kidney and pancreas substitution play a role in reducing peripheral insulin resistance.

Whole blood L-lactate assay by a new differential pH method: application to metabolic investigations

We propose a new quantitative method for L-lactate assay in whole blood, based on the measurement of pH variation caused by specific and irreversible oxidation of L-lactate to pyruvate in the presence of an electron acceptor (hexacyanoferrate) and of the enzyme cytochrome b2 (EC 1.1.2.3.). No sample pretreatment is needed; the method is simple and fast (1.5 min/analysis) and requires 10 microliters whole blood per assay. Linearity is confirmed up to 20 mmol/l L-lactate. Within-day and between-day variability was (as C.V.) 3.6% and 8.1% for blood lactate 1.3 and 1.0 mmol/l, respectively. The results by the present method correlate well with those from two reference methods (test method vs a lactate sensor based method: r = 0.996; test method vs a spectrophotometric method: r = 0.987). An application of the present method to the continuous monitoring of L-lactate in patients after combined kidney and pancreas transplantation, under conditions of euglycemic hyperinsulinemia and hyperglycemic hyperinsulinemia is reported. We conclude that the method is simple and reproducible and can be employed to measure whole blood lactate concentration continuously both in clinical protocols and in basic research.

Comparison of creatinine and inulin clearances in multiple trauma

Creatinine clearance (Ccr) is a good predictor of renal dysfunction. However, numerous analytical interferences have been observed with the classical measurement of creatinine by Jaffé's procedure. This prospective study was conducted to compare 4 methods for determining creatinine; and also endogenous creatinine clearance with inulin clearance (Cin) to estimate the glomerular filtration rate. The 4 different techniques for measuring creatinine were: 1) 2 techniques using Jaffé's colorimetric reaction: one with rapid and the other with slow kinetics: 2) 2 more selective methods: enzymatic procedure and high performance liquid chromatography (HPLC). Measurements were performed in 13 multiple trauma patients after stabilization and in 5 comatose patients (control group) over a 3-day period, with strict 24-h urine collection. On the second day, inulin clearance and para-aminohippuric acid clearance (Cpah) were measured. Measurement of creatinine by Jaffé's procedure yields significantly higher levels than those obtained by the other methods. Higher levels of both plasma and urinary creatinine were observed in the multiple trauma patients with all the methods used. There were no significant differences in Ccr, Cin, Cpah between the multiple trauma patients and the control patients. The best correlations between inulin clearance and creatinine clearance were observed for Jaffé's rapid kinetics (r = 0.90) in the control group and for the enzymatic procedure in the multiple trauma group (r = 0.55). Plasma creatinine is not a useful indicator in multiple trauma. The correlation between creatinine clearance and inulin clearance is not very strong in multiple trauma, indicating that the relative evolution (not the absolute values) of creatinine clearance is of interest.

Measurement of erythrocyte acetylcholinesterase and plasma cholinesterase activity by a differential pH technique

We describe a new electrochemical method for the determination of erythrocyte acetylcholinesterase activity (EC 3.1.1.7) and plasma cholinesterase (EC 3.1.1.8) activity, based on the measurements of pH variation due to release of acetic acid from acetylcholine. The major advantages of the differential pH procedure are simplicity, high reproducibility, no need for pre-treatment of samples, automatic correction of sample blanks, and speed and direct measurement of enzymatic reaction. The proposed methods are linear up to 7400 U/L at 30 degrees C and correlate well with the manual spectrophotometric method of Ellman for plasma cholinesterase and for washed erythrocytes. We adapted the same technique for the determination of erythrocyte cholinesterase using whole blood as sample and quinidine sulphate as inhibitor of pseudocholinesterase.

Urease sensors based on differential antimony electrodes

Urea-sensitive sensors, based on immobilized urease and the differential antimony electrode, have been constructed. The calibration curve of the sensor is linear up to 1.6-2.0 mmol litre-1 in the stationary or kinetic mode of operation. The sensors retain 50% of their sensitivity for 16-17 days. The analyser for urea, constructed on the basis of the sensor, ensures quick and precise urea determination in pure blood.