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The Antiarrhythmic Peptide Analog Rotigaptide (ZP123) Stimulates Gap Junction Intercellular Communication in Human Osteoblasts and Prevents Decrease in Femoral Trabecular Bone Strength in Ovariectomized Rats

The Osteoporosis and Metabolic Bone Unit (N.R.J., S.C.T., Z.H., S.S.H., J.-E.B.J., O.H.S.), Department of Endocrinology and Clinical Biochemistry, Copenhagen University Hospital H:S, DK-2650 Hvidovre, Denmark; and Zealand Pharma A/S (E.M., J.S.P.), DK-2600 Glostrup, Denmark

Address all correspondence and requests for reprints to: Niklas Rye Jørgensen, M.D., Ph.D., Osteoporosis Research Clinic, Department 545, Copenhagen University Hospital Hvidovre, Kettegaard Allé 30, DK-2650 Hvidovre, Denmark. E-mail: niklas{at}dadlnet.dk.

Gap junctions play an important role in bone development and function, but the lack of pharmacological tools has hampered the gap junction research. The antiarrhythmic peptides stimulate gap junction communication between cardiomyocytes, but effects in noncardiac tissue are unknown. The purpose of this study was to examine whether antiarrhythmic peptides, which are small peptides increasing gap junctional conductivity, show specific binding to osteoblasts and investigate the effect of the stable analog rotigaptide (ZP123) on gap junctional intercellular communication in vitro and on bone mass and strength in vivo. Cell coupling and calcium signaling were assessed in vitro on human, primary, osteoblastic cells. In vivo effects of rotigaptide on bone strength and density were determined 4 wk after ovariectomy in rats treated with either vehicle, sc injection twice daily (300 nmol per kilogram body weight) or by continuous ip infusion (158 nmol per kilogram body weight per day). During metabolic stress, a high affinity-binding site (K_D = 0.1 nM) with low density (15 fmol/mg protein) for [¹²⁵I]di-I-AAP10 was demonstrated. During physiological conditions, specific binding sites for [¹²⁵I]AAP10 could not be shown. Studies of the effects of rotigaptide on propagation of intercellular calcium waves and cell-to-cell coupling demonstrated that 10 nM rotigaptide produced a small increase in intercellular communication during physiological conditions (+4.5 ± 1.6% vs. vehicle; P < 0.05). During conditions with metabolic stress, 10 nM rotigaptide produced an increase in coupling measured by both methods. Four weeks after ovariectomy, bone strength of the femoral head was reduced by 20% in vehicle-treated ovariectomized rats, which was completely prevented in both rotigaptide-treated groups. Rotigaptide also prevented decreases in bone mineral. We conclude that the stable analog rotigaptide increases gap junctional communication in osteoblasts in vitro and preferably during conditions with metabolic stress. Rotigaptide further prevents ovariectomy-induced bone loss in vivo. Thus, gap junction modulation may be a promising new target for osteoporosis therapy.

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