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Defects in both insulin secretion and action have been documented in patients with noninsulin-dependent diabetes mellitus (NIDDM), leading to the suggestion that both fasting hyperglycemia and insulin resistance in NIDDM are secondary to insulin deficiency. In order to test this hypothesis, insulin secretion (plasma insulin response to oral glucose) and insulin action (insulin clamp) were determined in 25 patients with NIDDM. The results documented relationships between incremental plasma insulin response to glucose and degree of fasting hyperglycemia (r = -.045, P less than 0.05) and insulin-stimulated glucose utilization (r = 0.25, P = NS). These data indicate that differences in insulin secretory response accounted for only approximately 20% of the variance in fasting plasma glucose level and 6% of the variance in insulin resistance in NIDDM. Thus, differences in insulin-secretory response contribute modestly to magnitude of glycemia, and not at all to variations in insulin resistance in NIDDM, permitting rejection of the hypothesis that insulin resistance is secondary to insulin deficiency. 相似文献
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The two types of nodal roots of field-grown maize, sheathedand bare, were found to have such different water conductivitiesthat an investigation of the anatomy of their large metaxylemvessels was made. While the vessels of the bare roots were openfor scores of centimetres, those of the sheathed roots werefound to be not vessels but developing vessel elements, withcross walls at 1 mm intervals, and protoplasts. The cross wallsbetween the elements had several unique histochemical properties.Previous investigators have often failed to find the cross wallsbecause they are very easily dislodged during the usual methodsof tissue preparation. They are best identified by microdissectionof fresh xylem. The living elements persist in the late metaxylemup to 20 30 cm from the tip. As the roots become longerthan this both the cross walls and the soil sheaths disappearand there is a transition to a bare root with open vessels inthe proximal region. The soil sheath persists a little longerthan the cross walls. The two types are thus stages in a developmentalsequence through which all nodal roots pass. A fundamental differencebetween the two types is in their water status, since the estimatedconductive capacity of a bare root is about 100 times greaterthan that of a sheathed root. These observations point to theneed for a reassessment of the published work on transport ofions into the xylem of grass roots through a reinvestigationof the maturity of their xylem vessels. Grass roots, dimorphic roots, ion secretion to xylem, soil sheaths, xylem vessels, xylem differentiation, water conduction, Zea mays L 相似文献
5.
Peter J. Hollenbeck 《Journal of neurochemistry》1993,60(6):2265-2275
Abstract: The microtubule-based motor protein kinesin is thought to drive anterograde organelle transport in axons, but nothing is known about how its force-generating activity or organelle-binding properties are regulated. Studies in other motility systems suggest that protein phosphorylation is a reasonable candidate for this function. I report here that the kinesin heavy chain (HC) and light chain (LC), as well as the 160-kDa kinesin-associated protein kinectin, are phosphorylated in vivo in cultures of chick sympathetic neurons and PC12 cells labeled metabolically with 32 P. In neurons, both kinesin chains are phosphorylated exclusively on serine residues, and limiting tryptic digestion demonstrated that the phosphorylation sites are clustered in a region of ˜5 kDa for the HC and ˜14 kDa for the LC. Partial tryptic digestion of 32 P-labeled HC followed by immunoblotting with SUK4 monoclonal anti-HC and fluorography showed that the sites of HC phosphorylation are outside the globular N-terminal head region where kinesin's microtubulebinding and mechanochemical activities reside. Treatment of metabolically labeled neurons with forskolin, phorbol esters, or calcium ionophore did not alter the extent of phosphorylation, the phosphoamino acid composition, or the V8 protease phosphopeptide maps of the HC, LC, and 160-kDa protein, with one exception: treatment with calcium ionophore reduced the specific activity of the LC. In addition, when kinesin from PC12 cells was compared with that from PC12-derived cell lines lacking protein kinase A activity, neither the extent of phosphorylation nor the phosphopeptide maps were altered for either chain. Phosphopeptide mapping experiments also showed that postlysis kinase activity can phosphorylate both the neuronal HC and LC at sites not phosphorylated in vivo. 相似文献
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The rhodopsin system of the squid 总被引:6,自引:19,他引:6
Squid rhodopsin (λmax 493 mµ)—like vertebrate rhodopsins—contains a retinene chromophore linked to a protein, opsin. Light transforms rhodopsin to lumi- and metarhodopsin. However, whereas vertebrate metarhodopsin at physiological temperatures decomposes into retinene and opsin, squid metarhodopsin is stable. Light also converts squid metarhodopsin to rhodopsin. Rhodopsin is therefore regenerated from metarhodopsin in the light. Irradiation of rhodopsin or metarhodopsin produces a steady state by promoting the reactions, See PDF for Equation Squid rhodopsin contains neo-b (11-cis) retinene; metarhodopsin all-trans retinene. The interconversion of rhodopsin and metarhodopsin involves only the stereoisomerization of their chromophores. Squid metarhodopsin is a pH indicator, red (λmax 500 mµ) near neutrality, yellow (λmax 380 mµ) in alkaline solution. The two forms—acid and alkaline metarhodopsin—are interconverted according to the equation, Alkaline metarhodopsin + H+ acid metarhodopsin, with pK 7.7. In both forms, retinene is attached to opsin at the same site as in rhodopsin. However, metarhodopsin decomposes more readily than rhodopsin into retinene and opsin. The opsins apparently fit the shape of the neo-b chromophore. When light isomerizes the chromophore to the all-trans configuration, squid opsin accepts the all-trans chromophore, while vertebrate opsins do not and hence release all-trans retinene. Light triggers vision by affecting directly the shape of the retinene chromophore. This changes its relationship with opsin, so initiating a train of chemical reactions. 相似文献
7.
ALAN SHER MARK ST. J. CRANE LOUIS V. KIRCHHOFF 《The Journal of eukaryotic microbiology》1983,30(2):278-283
The differentiation of Trypanosoma cruzi epimastigotes into trypomastigotes was studied in diffusion chambers sub-cutaneously implanted in mice. Using epimastigotes of the Tulahuén strain, transformation was first evident at 16 h after implantation and reached its maximum (92% trypomastigotes) by 24 h. Shortly before their differentiation into trypomastigotes, epimastigotes were found to develop resistance to lysis by the alternative pathway of complement. Furthermore, implantation of stationary-phase (as opposed to log-phase) parasites resulted in the accumulation of large numbers of complement-resistant epimastigotes in the chambers. These observations suggest that epimastigotes pass through a complement-resistant transitional stage before differentiating into trypomastigotes and that transformation may require cell division. In a further series of experiments, epimastigotes recovered 7 h after implantation in mice were found to differentiate into trypomastigotes when cultured in vitro for an additional 17 h at 37°C. This observation indicates that the events which trigger the morphologic transformation of epimastigotes into trypomastigotes can be dissociated operationally from the differentiation process itself. 相似文献
8.
The present studies demonstrate that the removal rate of exogenously labelled 125I-VLDL-protein is prolonged when total serum from insulin-deficient rats combined with isolated 125I-VLDL is injected into normal recipient rats (6.8 +/- 0.7 vs 4.2 +/- 0.4 min; p < 0.01), but not when 125I-VLDL-protein is isolated and injected alone (4.2 +/- 0.8 vs 4.3 +/- 0.8 min). Furthermore, the present studies demonstrate that when isolated 125I-VLDL-protein is recombined with either VLDL-free (d > 1.006 g/ml), or lipoprotein-free serum (d > 1.215 g/ml) from insulin-deficient rats, the defect in removal rate of VLDL-protein observed in total serum is reestablished (125I-VLDL + VLDL-free serum from insulin-deficient rat vs that from normal rat: 7.6 +/- 1.2 vs 4.6 +/- 0.7 min, p < 0.05; and 125I-VLDL + lipoprotein-free serum from insulin-deficient rat vs that from normal rat: 6.4 +/- 0.7 vs 4.1 +/- 0.4 min, p < 0.01). These data suggest that a factor or factors exist in lipoprotein-free serum of insulin-deficient rats which interfere with the normal removal of 125I-VLDL. Since we have previously demonstrated a prolongation in the removal rate of endogenously labeled VLDL-3H-TG, the defect in removal of VLDL from the plasma of insulin-deficient rats appears to include both the lipid and protein moieties of the VLDL particles. 相似文献
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Rui Liu Xuguang Guo Yikyung Park Jian Wang Xuemei Huang Albert Hollenbeck Aaron Blair Honglei Chen 《PloS one》2013,8(6)