In conclusion, in these type 2 diabetic outpatients circulating levels of tHcy, folate, and the MTHFR C677T mutation are not associated with DPN, which was predicted by creatinine levels, CHD, and dyslipidemia. Metformin therapy is associated with a mild vitamin B12 level reduction, but not with DPN.

Pancreatic islets were cultured for 24 h in medium containing either low (1.4), normal (5.5), or high (16.7 mM) glucose, and then insulin secretion was measured at the end of 1 h incubation at 37 degrees C. Insulin release in the absence of glucose was 64 +/- 20, 152 +/- 11, and 284 +/- 30 pg.islet-1.h-1 (mean +/- SE, n = 6, G1.4 and G16.7 vs. G.5.5, P < 0.05) and the response to 22 mM glucose stimulation was 640 +/- 136, 2460 +/- 276, and 1890 +/- 172 pg.islet-1.h-1, respectively (n = 6, G1.4 vs. G5.5, P < 0.01, G16.7 vs. G5.5, P = 0.065). The 50% maximal response of insulin secretion (increment over baseline) was reached at an average glucose concentration of 9.9 +/- 0.7 mM in islets preexposed to G5.5, and at glucose 13.3 +/- 0.9 and 4.8 +/- 0.4 mM (P < 0.05 in respect to G5.5) in islets preexposed to G1.4 and G16.7, respectively. To investigate the molecular mechanism responsible for this altered glucose sensitivity, we measured, in parallel experiments, the kinetic characteristics of glucose transport, glucokinase, and glucose utilization. Glucose transport was measured by evaluating 3-O-methylglucose uptake. The apparent Km of the low-affinity transporter (GLUT2) was 16.6 +/- 2.4 mM in isolated pancreatic cells cultured at 5.5 mM glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin secretion was studied in rat pancreatic islets after 24-h exposure to various glyburide or tolbutamide concentrations. Glucose-induced insulin release was significantly (P < 0.05) reduced in islets cultured with 0.1 microM glyburide or 100 microM tolbutamide (2098 +/- 187, 832 +/- 93, and 989 +/- 88 pg/islet.h in control, glyburide-exposed, and tolbutamide-exposed islets, respectively). When glyburide-treated islets were stimulated with glyburide or tolbutamide, insulin release was also impaired compared to that in control islets (P < 0.05). In contrast, tolbutamide-exposed islets showed an impaired response to tolbutamide, but a normal response to glyburide. To investigate the mechanism of the sulfonylurea-induced impairment of insulin secretion, we measured insulin release and Rb+ efflux (a marker of the K+ channel activity) in a perifusion system and islet Ca2+ uptake under static conditions. Insulin release in response to 16.7 mM glucose increased in control islets from 9.4 +/- 1.1 to 131 +/- 19 pg/islet.min (first phase secretion peak). Simultaneously, the fractional 86Rb+ efflux declined from 0.015 +/- 0.002% to 0.006 +/- 0.001% (change in decrement, -63.5%). Glucose-induced insulin release in glyburide- and tolbutamide-treated islets was significantly reduced (first phase peak, 22.1 +/- 5 and 39.7 +/- 8 pg/islet.min, respectively; P < 0.05), and the fractional 86Rb+ efflux decrement was -21 +/- 6% for glyburide (P < 0.005 vs. control islets) and -65 +/- 4% (not different from control) for tolbutamide. When glyburide- or tolbutamide-exposed islets were stimulated with the corresponding sulfonylurea, insulin release was impaired compared to that in control islets (P < 0.05), but, again, 86Rb+ efflux was impaired (P < 0.05) only in glyburide-exposed islets. When 45Ca2+ uptake was studied, the increase in glucose concentration from 2.8 to 16.7 mM increased calcium uptake in control islets from 1.76 +/- 0.58 to 7.27 +/- 1.36 pmol/islet.2 min (n = 4). Preexposure to 0.1 microM glyburide did not change calcium uptake at a glucose concentration of 2.8 mM (1.44 +/- 0.45 pmol/islet.2 min) but significantly reduced calcium uptake stimulated by 16.7 mM glucose (3.21 +/- 0.35 pmol/islet.2 min; n = 4; P < 0.005 compared to control islets). In contrast, preexposure to 100 microM tolbutamide did not change either basal or glucose-stimulated calcium uptake (1.44 +/- 0.45 and 6.90 +/- 0.81 pmol/islet.2 min, respectively; n = 4). These data show that in vitro chronic exposure of pancreatic islets to the sulfonylureas glyburide and tolbutamide impairs their ability to respond to a subsequent glucose or sulfonylurea stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

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Combined estrogen/progestagen oral contraceptives (OC) have been reported to be associated with a deterioration of glucose tolerance and a decrease in insulin sensitivity; thus, since it has been suggested that steroids affect insulin receptor properties, the influence of OC on insulin receptors was investigated. The study groups were composed of nine normal menstruating women (controls), nine pill users, and two healthy women on OC for the first time. Insulin receptors on monocytes were evaluated at 7-day intervals during the 28 days between menses. Insulin receptor concentration and/or affinity did not show any variation in pill users during the test period and did not differ from values observed in controls in the luteal phase; consequently, the insulin receptor concentration in pill users is lower than that during the follicular phase or in men. The physiological variation of insulin receptor concentration and the increase of receptor affinity in the midfollicular phase, which characterize the normal menstrual cycle, are therefore abolished by OC. This effect occurs rapidly because it was also evident in the two women on OC for the first time. No difference was observed in fasting blood glucose and serum immunoreactive insulin concentrations between control subjects and pill users. The present data appear to confirm that sex steroids affect the insulin receptor and lend further support to the concept that caution must be used in clinical studies of insulin receptors when women are included. In addition, the results suggest that insulin receptors may play a role in the glucose intolerance and insulin insensitivity which have been described in pill users.