Li W, Duncan R L, Karin N J, Farach-Carson M C
Department of Basic Science, University of Texas-Houston 77030, USA.
Am J Physiol. 1997 Sep;273(3 Pt 1):E599-605. doi: 10.1152/ajpendo.1997.273.3.E599.
We previously demonstrated electrophysiologically that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] shifts the activation threshold of L-type Ca2+ channels in osteoblasts toward the resting potential and prolongs mean open time. Presently, we used single-cell Ca2+ imaging to study the combined effects of 1,25(OH)2D3 and parathyroid hormone (PTH) during generation of Ca2+ transients in fura 2-loaded MC3T3-E1 cells. Pretreatment with 1,25(OH)2D3 concentrations, which alone did not produce Ca2+ transients, consistently enhanced Ca2+ responses to PTH. Enhancement was dose dependent over the range of 1 to 10 nM and was blocked by pretreatment with 5 microM nitrendipine during pretreatment. A 1,25(OH)2D3 analog that activates L-type channels and shifts their activation threshold also enhanced PTH responses. In contrast, an analog devoid of membrane Ca2+ effects did not enhance PTH-induced Ca2+ transients. The PTH-induced Ca2+ transient involved activation of a dihydropyridine-insensitive cation channel that was inhibited by Gd3+. Together, these data suggest that 1,25(OH)2D3 increases osteoblast responsiveness to PTH through rapid modification of L-type Ca2+ channel gating properties, whose activation enhances Ca2+ entry through other channels such as the PTH-responsive, Gd(3+)-sensitive cation channel.
我们之前通过电生理学方法证明，1,25 - 二羟维生素D3 [1,25(OH)2D3] 可使成骨细胞中L型Ca2+通道的激活阈值向静息电位移动，并延长平均开放时间。目前，我们使用单细胞Ca2+成像技术来研究1,25(OH)2D3和甲状旁腺激素（PTH）在fura 2负载的MC3T3 - E1细胞中Ca2+瞬变产生过程中的联合作用。用单独不会产生Ca2+瞬变的1,25(OH)2D3浓度进行预处理，持续增强了对PTH的Ca2+反应。在1至10 nM范围内，增强作用呈剂量依赖性，并且在预处理期间用5 microM尼群地平预处理可阻断这种增强作用。一种激活L型通道并改变其激活阈值的1,25(OH)2D3类似物也增强了PTH反应。相比之下，一种没有膜Ca2+效应的类似物并没有增强PTH诱导的Ca2+瞬变。PTH诱导的Ca2+瞬变涉及一种对二氢吡啶不敏感的阳离子通道的激活，该通道被Gd3+抑制。总之，这些数据表明，1,25(OH)2D3通过快速改变L型Ca2+通道的门控特性来增加成骨细胞对PTH的反应性，其激活增强了通过其他通道（如PTH反应性、Gd(3+)敏感阳离子通道）的Ca2+内流。
