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The mechanism(s) by which the oral sulfonylurea, tolazamide, exerts its extrapancreatic hypoglycemic effects was studied using rat epididymal adipose tissue maintained 20-44 h in the presence or absence of the drug. Insulin binding, hexose transport and glucose metabolism were compared in adipocytes isolated from the cultured tissue. In contrast to earlier reports that suggested that sulfonylureas alter the binding of insulin, neither receptor number nor affinity were changed by tolazamide treatment. The uptake of the glucose analogs 2-deoxyglucose and 3-0-methylglucose in the absence of insulin (i.e., basal) was also unchanged. However, exposure to tolazamide resulted in a potentiation of the stimulatory effects of insulin by approximately 30% at each hormone concentration assayed (0.4-40 ng/ml). This potentiation was dependent on the tolazamide concentration (0.003-0.30 mg/ml), with a maximal effect observed at therapeutic levels. A tolazamide analog hypoglycemic activity in vivo was found not to enhance either basal or insulin-stimulated uptake in vitro. Conversion of 0.1-5.0 mM glucose to CO2 and total lipids in the presence of insulin was also potentiated by tolazamide treatment. The inability of the drug to directly stimulate basal glucose uptake was paralleled by its lack of effect on glucose metabolism. At 50 mM glucose, where transport is no longer rate-limiting, tolazamide did not potentiate metabolism in the absence or the presence of insulin. These studies demonstrate that tolazamide in vitro alters postreceptor insulin action without influencing the receptor, and suggests insulin-stimulated hexose transport as the cellular process responsible for the hypoglycemic effect of sulfonyureas in adipose tissue.

A B-cell-deficient model for Type 2 (non-insulin-dependent) diabetes mellitus has been investigated with regard to insulin action at the cellular level. Two-day-old male Sprague Dawley rats were injected with streptozotocin (90 mg/kg) or citrate buffer. At 6 weeks streptozotocin-treated animals were hyperglycaemic and exhibited glucose intolerance, e.g. at 45 min post-glucose (1.5 g/kg) the change in serum glucose level from baseline was 6 +/- 7 mg% in control rats vs. 212 +/- 18 mg% for the streptozotocin-treated rats. Basal activity and insulin action in isolated adipocytes, as estimated by 2-deoxyglucose uptake and glucose metabolism, were not influenced by streptozotocin treatment. For example, uptake of 0.1 mmol/1 2-deoxyglucose at 1000 microU insulin/ml was 58 +/- 8 pmol/10(5) cells min-1 vs 54 +/- 6 pmol for adipocytes isolated from experimental vs. control animals. Although serum insulin levels in streptozotocin-treated rats were significantly decreased (p less than 0.05), there was no difference in insulin receptor number or affinity. Glucose intolerance present in this model is similar to that in Type 2 diabetes. However, concomitant insulin intolerance was not observed. Taken together with our findings of unaltered insulin action at the cellular level, this suggests that the pathogenesis of insulin resistance is not dependent on glucose intolerance. Moreover, this hyperglycaemic model is responsive to oral hypoglycaemic agents and can be used to establish their direct effects on physiologic and cellular insulin action.

Pieces of rat epididymal fat tissue were maintained in a biochemically defined medium for 20 to 44 hours in either the absence or presence of a sulfonylurea at levels known to be effective in humans. Prolonged exposure of adipocytes to sulfonylureas did not influence the number of insulin receptors or their affinity to insulin or the ability of insulin to induce receptor loss (down-regulation). Also, the sulfonylureas did not influence the basal uptake of the D-glucose analogs 2-deoxyglucose and 3-O-methylglucose. However, exposure to these drugs resulted in a potentiation of the stimulatory effects of insulin on hexose transport at submaximal and maximally effective concentrations of insulin. The average potentiation was approximately 30%. In addition, sulfonylureas enhanced stimulation of hexose uptake by the insulin mimickers, hydrogen peroxide and vitamin K5. These oxidants are known to manifest insulin-like actions subsequent to insulin binding. Under conditions in which glucose transport was rate limiting, the conversion of glucose to carbon dioxide and the total lipids mirrored the findings of hexose uptake. However, at a glucose concentration of 50 mM, at which hexose transport is no longer rate limiting, sulfonylureas did not potentiate metabolism in th absence or presence of insulin. These results may help to explain the hypoglycemic action of the drug in view of the recent finding that a postreceptor deficit is present in noninsulin-dependent diabetes mellitus.

To define the cellular alterations responsible for insulin resistance during uremia, we studied insulin action in adipocytes isolated from rats 2 wk after 75% partial nephrectomy. Insulin binding to fat cells and purified liver plasma membranes prepared from uremic rats was unaltered. In contrast, hexose transport was significantly decreased, with and without insulin, in the fat cells from the uremic animals. The concentration of insulin that elicited half-maximal response was not altered. Glucose utilization was reduced in the absence or presence of insulin by partial nephrectomy. The stimulation of hexose transport and glucose metabolism by the insulin mimickers, hydrogen peroxide and vitamin K5, were also inhibited. Hexose transport activity in adipocytes obtained from uremic rats was no longer decreased when pieces of fat tissue were cultured for 20 h. Finally, hexose transport was reduced in the cells isolated from normal adipose tissue that was preincubated for 3 h with uremic serum, but insulin binding was not different than control. Thus, insulin resistance associated with uremia may be primarily accounted for by altered postreceptor events that appear to result from a circulating factor(s).

Nordihydroguaiaretic acid (NDGA) was investigated for its ability to interact with leukotriene B4 receptors on human polymorphonuclear leukocytes (hPMNs). 3H-LTB4 binding to specific receptors was reduced in a dose-dependent manner with maximal reduction at 100 microM NDGA and an IC50 of about 50 microM. Binding of another inflammatory stimulus. N-formyl-norleucyl-leucyl-phenylalanine (FNLP) was not affected by similar treatment. Chemotaxis and enzyme release stimulated by LTB4 and oligopeptide were inhibited by NDGA. In addition, LTB4-triggered inflammation in vivo in mice was inhibited by systemic administration of NDGA. These data suggest that LTB4 receptor antagonism may contribute to inhibition of inflammation by NDGA.