Role of sympathetic tone in the loss of syringin-induced plasma glucose lowering action in conscious Wistar rats

Syringin is an active principle purified from the rhizome and root parts of Eleutherococcus senticosus (Araliaceae). The present study is designed to clarify the role of sympathetic activation in the insulinotropic effect of syringin. Plasma glucose lowering effect accompanying with the increase of plasma insulin and C-peptide were obtained in pentobarbital anesthetized Wistar rats 60 min after an intravenous (i.v.) injection of syringin (100 μg/kg). However, neither the plasma glucose lowering action, nor the raised plasma levels of insulin and C-peptide can be obtained in conscious rats received same syringin treatment. Otherwise, the insulin-releasing and plasma glucose lowering actions of syringin (100 μg/kg, i.v.) were appeared in conscious rats under chemical sympathectomy using an intraperitoneal injection of guanethidine. In addition, plasma glucose lowering action of syringin (100 μg/kg, i.v.) was observed in conscious rats with α1-adrenoceptor blockade by prazosin. The stimulatory actions of syringin on the secretion of plasma insulin and C-peptide were also obtained in prazosin-treated conscious rats. The obtained results suggest that insulinotropic effect of syringin on the plasma glucose regulation is impaired in conscious rats with a regular sympathetic tone; decrease of sympathetic tone as observed in anesthetized animal might be helpful in the therapeutic benefit of syringin.