fruit juice stimulates glucose and amino acid uptakes in L6 myotubes

The fruit of (family: Cucurbitacea) is used widely as a hypoglycaemic agent to treat diabetes mellitus (DM). The mechanism of the hypoglycaemic action of is not fully understood. This study investigated the effect of juice on either H-2-deoxyglucose or N-methyl-amino--isobutyric acid (C-Me-AIB) uptake in L6 rat muscle cells cultured to the myotube stage. The fresh juice was centrifuged at 5000 rpm and the supernatant lyophilised. L6 myotubes were incubated with either insulin (100 nM), different concentrations (1–10 μg ml) of the juice or its chloroform extract or wortmannin (100 nM) over a period of 1–6 h. The results were expressed as pmol min (mg cell protein), = 6–8 for each value. Basal H-deoxyglucose and C-Me-AIB uptakes by L6 myotubes after 1 h of incubation were (means ± S.E.M.) 32.14 ± 1.34 and 13.48 ± 1.86 pmol min (mg cell protein), respectively. Incubation of L6 myotubes with 100 nM insulin for 1 h resulted in significant (ANOVA, < 0.05) increases in H-deoxyglucose and C-Me-AIB uptakes. Typically, H-deoxyglucose and C-Me-AIB uptakes in the presence of insulin were 58.57 ± 4.49 and 29.52 ± 3.41 pmol min (mg cell protein), respectively. Incubation of L6 myotubes with three different concentrations (1, 5 and 10 μg ml) of either the lyophilised juice or its chloroform extract resulted in time-dependent increases in H-deoxy-D-glucose and C-Me-AIB uptakes, with maximal uptakes occurring at a concentration of 5 μg ml. Incubation of either insulin or the juice in the presence of wortmannin (a phosphatidylinositol 3-kinase inhibitor) resulted in a marked inhibition of H-deoxyglucose by L6 myotubes compared to the uptake obtained with either insulin or the juice alone. The results indicate that fruit juice acts like insulin to exert its hypoglycaemic effect and moreover, it can stimulate amino acid uptake into skeletal muscle cells just like insulin. (Mol Cell Biochem : 99–104, 2004)