Evaluation of urea kinetics utilizing stable isotope urea and pharmacokinetic modeling

The determination of urea kinetics plays a central role in clinical dialysis prescription. There persist, however, significant limitations to current approaches, particularly as they pertain to rigorous explorations of urea metabolism, distribution, and removal. This report describes methodologies designed to address these limitations by coupling a stable nitrogen isotope method with strict compartmental pharmacokinetic modeling. The findings of the present study can be summarized as follows. First, the use of stable isotope labeled exogenous urea is a reliable clinically applicable method for determination of urea kinetics. Second, this method offers significant advantages in that it allows for an accurate measurement of urea distribution space, endogenous urea production, and non-renal clearance of urea. Third, this method is significantly more rigorous than urea kinetic models that utilize only endogenous urea and do not carefully fit data points. Finally, pharmacokinetic modeling suggests that a two-compartment model satisfies all aspects of urea distribution and removal, but these compartments should not be equated with specific physiologic spaces. The combination of stable isotope urea compartmental modeling is a rigorous methodology for the assessment and validation of urea kinetics.

Activity of ecdysone analogs in enhancing N-acetylglucosamine incorporation into the cultured integument of Chilo suppressalis

Ecdysone analogs with various side chains at the 17-position of the steroid structure enhanced the incorporation of N-acetylglucosamine as 20-hydroxyecdysone into the cultured integument prepared from Chilo suppressalis. Their activity in terms of the concentration required to give 50% of the maximum response varied with the structure. Piperonyl butoxide, an inhibitor of oxidation metabolism, did not enhance the in vitro effect of the compounds. The order of potency was ponasterone A > 20-hydroxyecdysone > cyasterone > inokosterone > makisterone A >> ecdysone.

Identification of cholesta-7,24-dien-3 beta-ol and desmosterol in hamster cauda epididymal spermatozoa

The sterol composition of hamster cauda epididymal spermatozoa was remarkably different from that of several other mammalian spermatozoa. Desmosterol and cholesta-7,24-dien-3 beta-ol account for as much as 90% of the total sterols. Cholesterol and desmosterol are the major components of mouse cauda epididymal spermatozoa, and rabbit, boar and bull ejaculated spermatozoa. Cholesta-7,24-dien-3 beta-ol was not detected. Furthermore, cholesterol was the main sterol in hamster caput epididymal spermatozoa, while only a trace amount of desmosterol was detected and cholesta-7,24-dien-3 beta-ol was hardly detected at all. The sterol content of cauda and caput epididymal spermatozoa was 0.17 +/- 0.05 mumol/10(8) spermatozoa. During maturation, the desmosterol and cholesta-7,24-dien-3 beta-ol levels increase and the cholesterol level decreases. Cholesta-7,24-dien-3 beta-ol appears as a sterol in mature spermatozoa and seems to be a characteristic sterol of hamster cauda epididymal spermatozoa.

Assay of stable isotope labelled urea in biological fluids by selected ion monitoring

A selected ion-monitoring method, incorporating an internal standard, was developed to allow the direct and simultaneous measurement of stable isotope-labelled urea and unlabelled urea concentrations in biological specimens. After cyclization of urea with malonaldehyde bis(dimethylacetal) to form 2-hydroxypyrimidine, the volatile heptafluorobutyryl derivative was made in a two-step process utilizing a trimethylsilyl intermediate. The internal standard, 4-methyl-2-hydroxypyrimidine, was found to be stable under the cyclization conditions. The structures of the internal standard and urea derivatives were confirmed by electron impact and chemical ionization mass spectrometry. This method was applied to the measurement of [15N2]urea concentrations in dog plasma.

Thin-layer chromatographic in situ analysis of insect ecdysones via fluorescence-quenching

The possibility of quantitating insect ecdysones in situ on thin-layer chromatographic plates was examined. Two approaches were evaluated: 1) the induction of ecdysone fluorescence by sulfuric acid treatments and 2) the fluorescence-quenching of fluorescent thin-layer chromatographic plates by ecdysones. The fluorescence-quenching method was found to be most suitable and had a linear response range from 0.5 to 3 microgram for alpha-ecdysone and 20-hydroxyecdysone. Fluorescence-quenching and high pressure liquid chromatographic analyses obtained from extracts of alpha-ecdysone 20-hydroxylase incubations gave similar results. New data concerning the acid-induced fluorescence of ecdysones showed alpha-ecdysone to be twice as fluorescent as 20-hydroxyecdysone.

Use of silylating agents for the identification of hydroxylated steroids by gas chromatography and gas chromatography-mass spectrometry. Discrimination between phenolic and alcoholic hydroxyl groups

A phenolic trimethylsilyl (TMS) group was selectively exchanged for a dimethylalkylsilyl (DMAS) group on a gas chromatographic column by use of sandwich injection technique with DMAS-imidazole, and a TMS ether derivative of a phenolic steroid was converted into a DMAS ether or a mixed TMS and DMAS ether derivative with over 95% recovery. The selective exchange reaction seemed to be caused by the difference in lability between the ethereal TMS linkages to phenolic and alcoholic hydroxyl groups. This selectivity was found to be useful for discriminating gas chromatographically between the phenolic and alcoholic hydroxyl groups in steroids.

The formation of trimethylsilyl ethers of ecdysones. A reappraisal

The hydroxyl groups of 20-hydroxyecdysone react with trimethylsilylimidazole with varying ease, in the positional order 2,3,22,25 greater than 20 greater than 14. The 14alpha-hydroxyl group can only be silylated under forcing conditions. Confusion in silylation procedures has been caused by failure to recognize incomplete reaction. The conclusions are supported by mass spectra. In the presence of a catalyst, and absence of a 14alpha-oxy substituent, enol ethers are readily formed, but the rate is considerably reduced with a C-14 substituent present.

Arthropod Molting Hormone: Radioimmune Assay

A radioimmune assay for the arthropod molting hormone, ecdysterone, has been developed. The sensitivity of the assay is 200 picograms or 25 times the maximum sensitivity of the bioassay. Closely related steroids also bind the anti-body, but with lower affinities.