Insulin-like effect of pinitol.

D-pinitol (3-O-methyl-chiroinositol), an active principle of the traditional antidiabetic plant Bougainvillea spectabilis, is claimed to exert insulin-like effects. This study investigates the effect of D-pinitol on glucose homeostasis in animal models of diabetes, and on glucose transport by cultured muscle cells. Plasma glucose concentrations were measured in normal, obese-diabetic (ob/ob) and streptozotocin (STZ)-diabetic mice after oral (p.o.) and intraperitoneal (i.p.) administration of D-pinitol. Glucose transport was measured in L6 rat muscle cells by 2-deoxyglucose (2DG) uptake. In STZ-diabetic mice, 100 mg kg−1 p.o. D-pinitol acutely decreased the hyperglycaemia (by 22% at 6 h). A similar decrease in plasma glucose (by 21%) was observed after 100 mg kg−1 i.p. D-pinitol. Insulin concentrations and the rate of insulin-induced (1 unit kg−1 actrapid i.p.) glucose disappearance were not altered by 100 mg kg−1 p.o. D-pinitol. Chronic administration of D-pinitol (100 mg kg−1 i.p. twice daily for 11 days) to STZ-diabetic mice maintained a reduction in plasma glucose concentrations from about 14 to 10 mmol l−1. In normal non-diabetic and severely insulin resistant ob/ob mice, 100 mg kg−1 p.o. D-pinitol did not significantly affect plasma glucose or insulin during acute studies. Incubation of L6 muscle cells with D-pinitol (10−3 M) increased basal 2DG uptake by 41% after 10 min and by 34% after 4 h. The effect of D-pinitol was inhibited by the phosphatidylinositol 3-kinase inhibitor LY294002. D-pinitol did not increase insulin-stimulated 2DG uptake by L6 cells. The data support the view that D-pinitol can exert an insulin-like effect to improve glycaemic control in hypoinsulinaemic STZ-diabetic mice. D-pinitol may act via a post-receptor pathway of insulin action affecting glucose uptake. Keywords: D-pinitol, insulin action, glucose transport, muscle L6 cells, streptozotocin-diabetic mice, obese-diabetic ob/ob mice, antidiabetic agent Introduction D-chiroinositol is structurally related to the phosphatidylinositol phosphates which participate in the insulin signalling pathways that stimulate glucose transport (Holman & Kasuga, 1997; White, 1997). Reduced urinary excretion of D-chiroinositol has been observed in rhesus monkeys and human subjects with impaired glucose tolerance, insulin resistance and type 2 diabetes mellitus (Kennington et al., 1990; Ortmeyer et al., 1993a; Suzuki et al., 1994). Acute administration of D-chiroinositol reduced plasma glucose concentrations in streptozotocin (STZ)-diabetic rats, and increased glucose utilization in insulin-resistant monkeys (Ortmeyer et al., 1993b; Fonteles et al., 1996). D-chiroinositol also improved glucose tolerance in normal rats and increased glycogenesis in diaphragm (Ortmeyer et al., 1993b; Huang et al., 1993). The leaves of Bougainvillea spectabilis are used as a traditional treatment for diabetes in Asia and the West Indies (Narayanan et al., 1987). D-pinitol (1D-3-0-methyl-chiroinositol; Figure 1), a 3-methoxy analogue of D-chiroinositol, has been identified as an active principle, and is reported to reduce glucose concentrations in alloxan diabetic rats (Narayanan et al., 1987). Since this observation has not been confirmed or evaluated from a potential therapeutic perspective, the present study has investigated the effect of D-pinitol on blood glucose homeostasis in normal mice, hypoinsulinaemic STZ-diabetic mice and hyperinsulinaemic obese-diabetic ob/ob mice. The study has also examined the effect of D-pinitol on glucose transport by cultured L6 muscle cells. Figure 1 Structure of D-pinitol (1 D-3-O-methyl-chiroinositol). Methods The chemicals and their sources were: D-pinitol from Aldrich, Gillingham, U.K; human actrapid insulin from Novo Nordisk, Crawley, U.K.; 2-deoxy-D-[3H]-glucose (15.0 Ci mmol−1) from Amersham International, Amersham, U.K.; Hi-safe II scintillant from Fisons, Loughborough, U.K.; cell culture reagents from Gibco, Paisley, U.K.; other chemicals from Sigma, Poole, U.K. and BDH, Poole, U.K. Plastic ware was from Sarstedt, Leicester, U.K. Animals Obese-diabetic ob/ob mice and normal homozygous lean +/+ mice from the Aston colony were used at 12–16 weeks of age. The origin and characteristics of these mice have been described previously (Flatt & Bailey, 1981; Bailey et al., 1982). Mice were housed in an air-conditioned room at 22±2°C with a lighting schedule of 12 h light (0800–2000 h) and 12 h dark. A standard pellet diet (Economy Rodent Breeding Diet, SDS, Witham, Essex, U.K.) and tap water were provided ad libitum. Hypoinsulinaemic diabetes was induced in 5 h fasted lean mice by intraperitoneal (i.p.) injection of STZ 160 mg kg−1 in citrate buffer, pH 4.5. Food was returned 4 h after injection of STZ, and animals were accepted as diabetic if the basal plasma glucose concentration was >12 mmol l−1 after 9 days. For the 62 STZ-diabetic mice used in the present study the plasma glucose and insulin values (mean±s.e.mean) were 19.8±1.5 mmol l−1 and 160±51 pmol l−1. Values in the control non-diabetic state were 7.2±0.5 mmol l−1 and 283±33 pmol l−1.