Mechanism, temporal patterns, and magnitudes of the metabolic responses to the KATP channel agonist diazoxide

To assess the mechanism, temporal patterns, and magnitudes of the metabolic responses to the ATP-dependent potassium channel agonist diazoxide, neuroendocrine and metabolic responses to intravenous diazoxide (saline, 1.0 and 2.0 mg/kg) and oral diazoxide (placebo, 4.0 and 6.0 mg/kg) were assessed in healthy young adults. Intravenous diazoxide produced rapid, but transient, decrements (P = 0.0023) in plasma insulin (e.g., nadirs of 2.8 +/- 0.5 and 1.8 +/- 0.3 microU/ml compared with 7.0 +/- 1.0 microU/ml after saline at 4.0-7.5 min) and C-peptide (P = 0.0228) associated with dose-related increments in plasma glucose (P = 0.0044) and serum nonesterified fatty acids (P < 0.0001). After oral diazoxide, plasma insulin appeared to decline, as did C-peptide, again associated with dose-related increments in plasma glucose (P < 0.0001) and serum nonesterified fatty acids (P = 0.0141). Plasma glucagon, as well as cortisol and growth hormone, was not altered. Plasma epinephrine increased (P = 0.0215) slightly only after intravenous diazoxide. There were dose-related increments in plasma norepinephrine (P = 0.0038 and P = 0.0005, respectively), undoubtedly reflecting a compensatory sympathetic neural response to vasodilation produced by diazoxide, but these would not raise plasma glucose or serum nonesterified fatty acid levels. Thus selective suppression of insulin secretion, without stimulation of glucagon secretion, raised plasma glucose and serum nonesterified fatty acid concentrations. These findings define the temporal patterns and magnitudes of the metabolic responses to diazoxide and underscore the primacy of regulated insulin secretion in the physiological regulation of postabsorptive carbohydrate and lipid metabolism.     

The effect of storage on enzyme activities in tissues

The stability to storage at -20 degrees C of 29 enzymes of rat tissues was measured for periods of up to 100 days. The enzymes could be divided into four groups: (1) those that increased in activity with storage; (2) those that showed transient rises subsequently declining to the original activity or below; (3) those that showed no significant change; (4) those that declined in activity. Details of methods used and of results obtained have been deposited as Supplementary Publication SUP 50038 (19 pages) at the British Library (Lending Division) (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem.J. (1973), 131, 5.     

A comparison of the composition and catabolism in vitro of porcine very low density lipoprotein subfractions prepared by gel exclusion and heparin-affinity chromatography

A comparison of the subfractions prepared from porcine plasma very low density lipoproteins by gel exclusion and heparin-Sepharose affinity chromatography revealed that the smallest and largest particles had the highest affinity for the glycosaminoglycan and had the highest ratio of apolipoprotein E to apolipoprotein CII. When the rates of triglyceride hydrolysis catalysed by lipoprotein lipase were compared for the subfractions the results were consistent with the view that apolipoprotein E may play a role in facilitating the catabolism of very low density lipoprotein triglyceride in the presence of glycosaminoglycan.     

Maintenance of the postabsorptive plasma glucose concentration: insulin or insulin plus glucagon?

The prevalent view is that the postabsorptive plasma glucose concentration is maintained within the physiological range by the interplay of the glucose-lowering action of insulin and the glucose-raising action of glucagon. It is supported by a body of evidence derived from studies of suppression of glucagon (and insulin, among other effects) with somatostatin in animals and humans, immunoneutralization of glucagon, defective glucagon synthesis, diverse mutations, and absent or reduced glucagon receptors in animals and glucagon antagonists in cells, animals, and humans. Many of these studies are open to alternative interpretations, and some lead to seemingly contradictory conclusions. For example, immunoneutralization of glucagon lowered plasma glucose concentrations in rabbits, but administration of a glucagon antagonist did not lower plasma glucose concentrations in healthy humans. Evidence that the glycemic threshold for glucagon secretion, unlike that for insulin secretion, lies below the physiological range, and the finding that selective suppression of insulin secretion without stimulation of glucagon secretion raises fasting plasma glucose concentrations in humans underscore the primacy of insulin in the regulation of the postabsorptive plasma glucose concentration and challenge the prevalent view. The alternative view is that the postabsorptive plasma glucose concentration is maintained within the physiological range by insulin alone, specifically regulated increments and decrements in insulin, and the resulting decrements and increments in endogenous glucose production, respectively, and glucagon becomes relevant only when glucose levels drift below the physiological range. Although the balance of evidence suggests that glucagon is involved in the maintenance of euglycemia, more definitive evidence is needed, particularly in humans.     

Lipoprotein lipase, a key role in atherosclerosis?

Enzymes from hyperthermophiles can be efficiently purified after expression in mesophilic hosts and are well-suited for crystallisation attempts. Two enzymes of histidine biosynthesis from Thermotoga maritima, N'-((5'-phosphoribosyl)-formimino)-5-aminoimidazol-4-carb oxamid ribonucleotide isomerase and the cyclase moiety of imidazoleglycerol phosphate synthase, were overexpressed in Escherichia coli, both in their native and seleno-methionine-labelled forms, purified by heat precipitation of host proteins and crystallised. N'-((5'-phosphoribosyl)-formimino)-5-aminoimidazol-4-carb oxamid ribonucleotide isomerase crystallised in four different forms, all suitable for X-ray structure solution, and the cyclase moiety of imidazoleglycerol phosphate synthase yielded one crystal form that diffracted to atomic resolution. The obtained crystals will enable the determination of the first three-dimensional structures of enzymes from the histidine biosynthetic pathway.