Effect of the water fraction isolated from Fructus Ligustri Lucidi extract on bone metabolism via antagonizing a calcium-sensing receptor in experimental type 1 diabetic rats

Our previous studies have demonstrated that the extract of Fructus Ligustri Lucidi (FLL) can maintain in vivo calcium homeostasis in aged and ovariectomized rats. This study was designed to elucidate the action of water fraction isolated from the FLL extract on bone metabolism and a calcium-sensing receptor (CaSR) in parathyroid glands and kidneys of diabetic rats. The streptozotocin-induced diabetic rats were treated with vehicle, FLL extract, and the water fraction (WF) isolated from the FLL extract for 4 weeks. Treatment with WF dramatically increased the serum levels of both calcium and parathyroid hormone and reduced urinary calcium excretion in diabetic rats as well as improved the pathological changes of trabecular bone as shown by the increased BA/TA, BMD/BV, and BV/TV. The mRNA expression of the calciumbinding protein 9k and protein expression of a vitamin D receptor (VDR) and plasma membrane CaATPase in duodenum were significantly increased in diabetic rats after treatment with WF, which reduced the expression of CaSR in parathyroid gland and kidney as well as inhibited the up-regulation of VDR and 25-hydroxyvitamin D-24 hydroxylase expressions in the kidney of diabetic rats. This study reveals that the water fraction may be an active component of the FLL extract that exerts beneficial effects on improving bone metabolism via regulating vitamin D metabolism in kidney and vitamin D-dependent calcium transporters in duodenum as well as modulating the expression of CaSR in the parathyroid gland and kidneys.