Sinusoidal electromagnetic field stimulates rat osteoblast differentiation and maturation via activation of NO–cGMP–PKG pathway

Abstract

Nitric oxide (NO) is an important intracellular and intercellular messenger, critically affecting bone metabolism. The purpose of this research is to investigate whether the effect of sinusoidal electromagnetic field (SEMF) on the differentiation and maturation of osteoblasts is mediated by the NO–cGMP–PKG signal pathway. We examined the impact of SEMF on nitric oxide synthase (NOS) activity, and found that l-NAME, nitric oxide synthase’s inhibitor, prevents SEMF-mediated increase in NOS activity and NO levels. We showed that an inhibitor of soluble guanylyl cyclase (ODQ) blocks the increase in cGMP levels triggered by exposure to SEMF. The inhibitor PDE5, which hydrolyzes 3′,5′-cyclic-GMP to 5′-GMP, prevents the SEMF’s stimulation of PKG activity. We also blocked the NO–cGMP–PKG pathway to determine whether the maturation and mineralization of osteoblasts, stimulated by SEMF, would be inhibited. This was evaluated by measuring alkaline phosphatase (ALP) activity, osterix gene expression and mineralized bone modulus. After treatment with SEMF, the NOS activity increases in comparison with the control group (P<0.01), reaching the highest level after 0.5h. Osterix gene expression, ALP activity and mineralized bone nodules in the SEMF experimental group also increase significantly. However, these effects are partially blocked in the l-NAME treated cultures. Surprisingly, all the osteogenic markers in the SEMF+l-NAME group were slightly higher than in the control culture, but lower than in the cells exposed to SEMF only. We conclude that the NO–cGMP–PKG signal pathway is activated by SEMF treatment, the stimulatory effect of SEMF on the differentiation and mineralization of osteoblasts is attenuated when the pathway is blocked.