The role of glucose metabolism in sperm cell motility was examined in purified human spermatozoa from the perspective of elucidating its possible significance in spontaneous and experimental ...diabetes. After a 4-h incubation in the absence of D-glucose, the mean progressive velocity of human spermatozoa was 40% lower than that of control cells kept in the presence of D-glucose. The decline was rapidly overcome by the addition of D-glucose or D-fructose, the amplitude of this stimulatory effect being independent of the ambient hexose concentration. Between 1.4 and 16.7 mM glucose, spermatozoal glucose oxidation also proceeded independently of the extracellular glucose levels, whereas both insulin (100nM) and glucagon (100nM) failed to significantly affect the rate of glucose metabolism or cellular motility. It is speculated from these results that an alteration in seminal hexose concentrations or pancreatic hormone levels may be an unlikely cause for the reduced sperm motility that is characteristically observed in diabetic patients. Human spermatozoa rapidly incorporated D-glucose and 3-O-methyl-D-glucose but excluded the glucose-analogue alloxan, which may explain their resistance against the toxic effects of this diabetogenic drug, in spite of their intrinsic sensitivity to organic peroxides such as tert-butyl hydroperoxide.
In view of the well known species differences in the sensitivity of pancreatic B-cells to the toxic glucose analogue alloxan, it was tested whether spermatozoa from two species with a different ...diabetogenic effect of alloxan displayed a similar difference in their sensitivity to this drug. In canine spermatozoa, less than 2 mM alloxan profoundly reduced the rate of glucose oxidation and cellular motility whereas more than 5 mM was required to significantly alter these parameters in human spermatozoa. Such species difference was not observed in spermatozoal sensitivity towards the inhibitory effects of tert-butyl hydroperoxide. The phenomenon is not attributable to a different rate of alloxan uptake since the drug is not incorporated by dog or human spermatozoa. The alloxan toxicity was counteracted by D-glucose and its 3-O-methyl analogue in both species, and was potentiated by ascorbic acid; however, only in man. The protective effect of D-glucose was much less marked in tert-butyl hydroperoxide-cytotoxicity. It is concluded that the observed species difference in spermatozoal alloxan sensitivity is not related to differences in alloxan uptake or in sensitivity to organic peroxides; differences in cellular scavenging of superoxide anion radicals and/or ascorbic acid metabolism may explain the lower sensitivity of human spermatozoa for alloxan.
In this study on purified rat pancreatic beta-cells, we show that the second-generation sulfonylurea glyburide stimulates insulin release through a direct interaction with the beta-cells. During ...static incubations, 2 microM glyburide releases 0.16 pg insulin per beta-cell, which corresponds to a half-maximal glucose stimulation. This effect occurs independently from the glucose-recognition unit, being detectable at both nonstimulatory and stimulatory glucose concentrations and proceeding without alterations in the rate of glucose oxidation. The secretagogue action of glyburide appears not to be mediated through cAMP but is potentiated by cAMP-generating substances such as glucagon (10(-8) M; 0.31 pg insulin released per beta-cell). Its 10-fold higher potency in isolated islets is attributed to the markedly higher cAMP levels that are maintained in islet beta-cells under the influence of locally released glucagon. Perifused pancreatic beta-cells respond to glyburide with a biphasic insulin release. After removal of the drug, the cells continue to secrete insulin at the same rate for greater than or equal to 30 min. This prolonged secretory activity coincides with a cellular accumulation of the drug, primarily in association with membranes of secretory vesicles and mitochondria. Tolbutamide also stimulates insulin release from pure beta-cells, but it is less powerful on a molar basis and does not lead to a sustained hormone release after its removal from the extracellular medium. We conclude that the hypoglycemic action of glyburide is at least partly the result of a direct interaction with pancreatic beta-cells.
Glucose uptake and oxidation were markedly higher in cultured than in freshly isolated neural cells, prepared from murine fetal brain cortices. The hexose transport process--measured as ...3-O-methyl-D-glucose uptake--appeared comparable in both conditions, and proceeded proportionally to the extracellular sugar concentration up to 6 mM. In contrast, glucose oxidation occurred independently of the prevailing glucose concentration from 1.4 mM on. Acute or chronic exposure to insulin exerted no effect upon cellular glucose uptake or oxidation. These results suggest that glucose handling by maturing fetal cortical cells is mainly determined by the rate of cellular glucose breakdown rather than by the rate of glucose transport into the cell; the marked rise in cellular glucose metabolism during culture might result from the synthesis and/or activation of a key enzyme in glucose catabolism. Our observations also indicate that the previously described neurotrophic effects of insulin are not mediated via enhanced glucose handling.
