Hiroko Fujita

Decrease in serum leptin by troglitazone is associated with preventing bone loss in type 2 diabetic patients.

Abstract. The thiazolidinedione (TZD) class of antidiabetic drugs has been shown to inhibit the formation of bone-resorbing osteoclasts in vitro and to decrease bone resorption markers in vivo. These drugs also inhibit the expression of leptin in adipocytes. Less leptin can be associated with higher bone mass, based on analyses of mice deficient in leptin action. Effects of 1-year treatment with troglitazone, a member of the TZDs, on bone mineral density (BMD) and bone metabolism were examined in 25 Japanese type 2 diabetic patients. Glucose metabolism was improved, whereas body mass index and percent body fat did not change throughout the study. The percent change of BMD was negatively correlated with that of serum leptin, whereas it was not associated with changes of bone metabolic markers, type I collagen N-telopeptide (NTx), bone alkaline phosphatase (ALP), body mass index, or HbA1c. Serum leptin decreased in 68% of subjects (responders) after 1-month treatment and was consistently lower than the basal level throughout the treatment. Percent changes of BMD were significantly higher in the responders than in the nonresponders and in nondiabetic subjects at 6 and 12 months. NTx and bone ALP decreased at 1 month but increased thereafter in either group of patients. Thus, it is suggested that the decrease in serum leptin with no reduction in body fat mass by troglitazone is associated with preventing bone loss in type 2 diabetic patients. Hence, TZDs may have an advantage for diabetic patients who have risk factors for osteoporosis.

Activation of Cl− channels by extracellular Ca2+ in freshly isolated rabbit osteoclasts

Ionic channels regulated by extracellular Ca2+ concentration ([Ca2+]0) were examined in freshly isolated rabbit osteoclasts. K+ current was suppressed by intracellular and extracellular Cs+ ions. In this condition, high [Ca2+]0 evoked an outwardly rectifying current with a reversal potential of about −25 mV. When the concentration of extracellular Cl− ions was altered, the reversal potential of the outwardly rectifying current shifted as predicted by the Nernst equation. 4′,4‐diisothiocyanostilbene‐2′,2‐disulphonic acid (DIDS) inhibited the outwardly rectifying current. These results indicated that this current was carried through Cl− channels. Cd2+ or Ni2+ caused a transient activation of the Cl− current in contrast to the sustained activation elicited by Ca2+. Intracellular 20 mM ethylene glycol‐bis(β‐aminoethyl ether)‐N,N,N′,N′‐tetraacetic acid (EGTA) inhibited the divalent cation‐induced Cl− current. Either when the osmolarity of extracellular medium was increased, or when 100 μM cAMP was dissolved in the patch pipette solution, high [Ca2+]0 still elicited the Cl− current, indicating that the divalent cation‐induced Cl− current was carried through Ca2+‐activated Cl− channels. Under perforated whole cell clamp extracellular divalent cations evoked the Cl− current, indicating that the activation of Cl− current did not arise from possible leakage of divalent cations from the extracellular medium under the whole cell clamp condition. This experiment further excluded a possible activation of volume‐sensitive Cl− channels under whole cell clamp. Intracellular application of guanosine 5′‐O‐(3‐thiotriphosphate) (GTPγS) activated the Cl− current and it was inhibited by intracellular 20 mM EGTA, suggesting that the activation of Cl− current was mediated through a G protein, and that an increase in [Ca2+]i was critical for the activation of Cl− channels. A protein phosphatase inhibitor, okadaic acid (100 nM), caused an irreversible activation of the Cl− current, suggesting that protein phosphatase 1 or 2A was involved in the regulation of Ca2+‐activated Cl− channels.