Bushinsky D A, Riordon D R, Chan J S, Krieger N S
Department of Medicine, University of Rochester School of Medicine, New York 14642, USA.
Am J Physiol. 1997 Jun;272(6 Pt 2):F774-80. doi: 10.1152/ajprenal.1997.272.6.F774.
Metabolic acidosis induces net calcium efflux (JCa+) from cultured bone, in part, through an increase in osteoclastic resorption and a decrease in osteoblastic formation. In humans provision of base as potassium (K+) citrate, but not sodium (Na+) citrate, reduces urine Ca (UCa), and oral KHCO3 decreases bone resorption and UCa in postmenopausal women. Potassium deprivation alone leads to an increase in UCa. To determine whether decreased extracellular K+ concentration ([K+]) at a constant pH, PCO2, and [HCO-3] alters JCa+ and bone cell activity, we measured JCa+, osteoblastic collagen synthesis, and osteoclastic beta-glucuronidase release from neonatal mouse calvariae cultured for 48 h in medium of varying [K+]. Calvariae were cultured in control medium (approximately 4 mM [K+]) or medium with mildly low K+ (MLK, approximately 3 mM [K+]), very low K+ (VLK, approximately 2 mM [K+]), or extremely low K+ (ELK, approximately 1 mM [K+]) (n > or = 9 in each group). Compared with control, ELK, but not MLK or VLK, resulted in a marked increase in JCa+ and an increase in beta-glucuronidase release and a decrease in collagen synthesis. JCa+ was correlated directly with medium beta-glucuronidase activity and inversely with collagen synthesis. To determine whether the reduction in medium [K+] was associated with a decrease in intracellular pH (pHi), we measured pHi in MC3T3-E1 cells, a mouse osteoblastic cell line. Incubation in 1 mM [K+] led to a significant decrease in pHi compared with 3 mM [K+]. Thus incubation in a reduced [K+] medium stimulates JCa+ and osteoclastic enzyme release and inhibits osteoblastic collagen synthesis, which may be mediated by a reduction in bone cell pH.
代谢性酸中毒会导致培养的骨组织出现净钙外流（JCa+），部分原因是破骨细胞吸收增加和成骨细胞形成减少。在人类中，补充钾（K+）柠檬酸盐而非钠（Na+）柠檬酸盐可降低尿钙（UCa），口服碳酸氢钾（KHCO3）可减少绝经后女性的骨吸收和UCa。单独的钾缺乏会导致UCa增加。为了确定在恒定pH、PCO2和[HCO3-]条件下细胞外钾离子浓度（[K+]）降低是否会改变JCa+和骨细胞活性，我们在不同[K+]的培养基中培养新生小鼠颅骨48小时，测量了JCa+、成骨细胞胶原蛋白合成以及破骨细胞β-葡萄糖醛酸酶释放。颅骨在对照培养基（约4 mM [K+]）或低钾培养基（MLK，约3 mM [K+]）、极低钾培养基（VLK，约2 mM [K+]）或极低钾培养基（ELK，约1 mM [K+]）中培养（每组n≥9）。与对照相比，ELK而非MLK或VLK导致JCa+显著增加，β-葡萄糖醛酸酶释放增加，胶原蛋白合成减少。JCa+与培养基β-葡萄糖醛酸酶活性直接相关，与胶原蛋白合成呈负相关。为了确定培养基[K+]降低是否与细胞内pH（pHi）降低有关，我们在小鼠成骨细胞系MC3T3-E1细胞中测量了pHi。与3 mM [K+]相比，在1 mM [K+]中孵育导致pHi显著降低。因此，在低钾培养基中孵育会刺激JCa+和破骨细胞酶释放，并抑制成骨细胞胶原蛋白合成，这可能是由骨细胞pH降低介导的。
