Induction of insulin secretion by an aqueous extract of Tabernanhte iboga Baill. (Apocynaceae) in rat pancreatic islets of Langerhans.

ABSTRACT

The effect of an aqueous extract of Tabernanthe iboga (TBEt) was studied in the rat islets insulin secretion based on its use in traditional medicine for the treatment of diabetes. Rats islets were isolated by collagenase digestion. In insulin release experiments, the insulin content was determined by Enzyme-Link Immunosorbent Assay (ELISA). For experiments on 45Ca(2+) Uptake, the radioactive content was determined using a liquid scintillation analyzer. The extract (10⁻³ µg/ml-100 µg/ml) did not exert a significant increase of insulin secretion (p>0.05) in the presence of 2.8 mM of glucose (a none stimulatory concentration). Whereas, in the presence of 11.1 mM of glucose (stimulatory concentration), TBEt augmented glucose-stimulated insulin secretion in a dose-dependent manner. Interestingly, the secretory effect of the extract was glucose-dependent (5.6-16.7 mM). Furthermore, the insulinotropic effect of TBEt (1 µg/ml) was significantly potentiated (p<0.001) in K(+)-depolarised media as well as in the presence of 2.8 mM and 16.8 mM of glucose concentrations. In contrast, in the same conditions, TBEt failed to stimulate the high K(+) medium-induced insulin release. The extract significantly amplified (p<0.001 and p<0.05) the insulin secretion induced by either IBMX or tolbutamide. Diazoxide, cobalt or calcium removal inhibited the insulinotropic effect of the extract. TBEt increased glucose-induced 45Ca(2+) uptake in rat islets. Overall, our findings suggest that Tabernanthe iboga contains water soluble insulinotropic compounds. The insulin secretion of TBEt's active principles might involve the closure of K(+)-ATP and the intensification of calcium influx through voltage-sensitive Ca(2+) channels.