Effects of Syzygium cordatum (Hochst.) [Myrtaceae] leaf extract on plasma glucose and hepatic glycogen in streptozotocin-induced diabetic rats

The present study investigates the hypoglycaemic effect of Syzygium cordatum (Hochst.) [Myrtaceae] leaf extract in non-diabetic and streptozotocin (STZ)-induced diabetic rats. Oral glucose tolerance tests (OGGT) were conducted in non-diabetic and STZ-diabetic rats using orally administered glucose (1.4 g 100 g−1 body weight) followed by either the leaf extract (6 mg 100−1 g body weight) or subcutaneous (sc) injection of metformin (50 mg 100 g−1). Weekly plasma glucose and terminal hepatic glycogen concentrations were recorded in control STZ-diabetic rats and diabetic rats orally treated with the leaf extract once every third day for 4 weeks. Administration of the leaf extract decreased plasma glucose from 7.7 ± 0.9 mmol l−1 to 3.7 ± 0.6 mmol l−1 (n = 6), and 21.1 ± 2.2 mmol l−1 to 12.5 ± 1.8 mmol l−1 (n = 7) by 212 h in non-diabetic and STZ-diabetic rats, respectively. OGTT data in metformin-treated rats were similar at the corresponding time in all groups, except for significant blood glucose reduction by the drug in non-diabetic rats between 1 and 112 h after treatment. Oral administration of the extract did not affect plasma glucose concentration in STZ-diabetic rats after 4 weeks, although it significantly increased hepatic glycogen content by comparison with untreated STZ-diabetic rats (28 ± 5 mg 100−1 g body weight, n = 7, versus 16 ± 3 mg 100−1 g body weight, n = 6). We conclude that Syzygium cordatum leaf extract contains compounds that could be effective in mild diabetes mellitus or in cases of glucose tolerance impairment. The possible mechanism(s) involved in the short-term hypoglycaemic effect of the extract could not be established by the current study.