Glycemic effect of stevioside and Stevia rebaudiana in streptozotocin-induced diabetic rats

The present experiment was conducted to study the glycemic effect of stevioside (SVS) and clarify whether SVS participates in glycemic action of aqueous extract of Stevia rebaudiana (SR) in diabetic rats. Male Wistar rats weighing between 150-180 g were used. Six groups of rats were established. The first three groups were normal rats daily fed by intragastric intubation of water or SVS (0.25 g / kg body weight) or SR at the dose corresponding to the dose of SVS administration (4.66 g / kg body weight) for 8 weeks. The other three groups were diabetic rats treated with the same procedure as normal rats. The plasma glucose level (PG) was determined once a week after overnight fasted. At the end of experiment, serum insulin and plasma glucagon levels were determined. Insulin-induced glucose uptake by isolated diaphragm was also evaluated using 2-deoxy-D-[3H]-glucose and [14C]-manitol. The results show that SVS slightly but significantly raised PG since the third week in normal rats whereas no significant change was noted in diabetic rats throughout experimental period. The PG was not altered in normal rats fed with SR whereas it significantly reduced since the second week until the end of experiment in diabetic rats fed with SR. Serum insulin and plasma glucagon levels were not significantly changed in normal rats fed with either SVS or SR. Serum insulin was raised to the same extent in diabetic rats fed with either SVS or SR. The high plasma glucagon level was suppressed in diabetic rats fed with SR to the level that was not significantly different from that of the normal rats. Insulin-induced glucose uptake by isolated diaphragm muscle were suppressed in both normal and diabetic rats fed with SVS whereas no change was apparent in both normal and diabetic rats fed with SR. A suppression of insulin-induced glucose uptake by muscle plays a predominant role to produce hyperglycemia in normal rats fed with SVS. Despite an improvement of insulin level, anti-hyperglycemic effect of SVS can not be displayed in diabetic rats since a reduction of insulin-induced glucose uptake was noted as well. An elevation of insulin and suppression of glucagon are the primary cause of anti-hyperglycemic effect of SR in diabetic rats. SVS containing in SR may indirectly contribute to anti-hyperglycemic action of SR in diabetic rats via its effect to potentiate insulin release.