Pastoriza-Munoz E, Harrington R M, Graber M L
Veterans Administration Medical Center, Northport, New York 11768.
J Clin Invest. 1992 May;89(5):1485-95. doi: 10.1172/JCI115739.
The mechanism of inhibition of HCO3 transport by parathyroid hormone (PTH) in the proximal tubule is not clearly defined. Previous studies in vitro have suggested that this effect is mediated via cAMP generation, which acts to inhibit Na/H exchange, resulting in cell acidification. To examine this question in vivo, intracellular pH (pHi) was measured in the superficial proximal tubule of the rat using the pH-sensitive fluoroprobes 4-methylumbelliferone (4MU) and 2',7'-bis(carboxyethyl)-(5, and 6)-carboxyfluorescein (BCECF). PTH was found to alkalinize the cell. This alkalinization suggested inhibition of basolateral base exit, which was confirmed by in situ microperfusion studies: lowering HCO3 in peritubular capillaries acidified the cell, an effect blunted by PTH. Removal of luminal Na promoted basolateral base entry, alkalinizing the cell. This response was also blunted by PTH. Readdition of luminal Na stimulated the luminal Na/H exchanger, causing an alkalinization overshoot that was partially inhibited by PTH. cAMP inhibited luminal H secretion but did not alkalinize the cell. Stimulation of phosphatidylinositol-bis-phosphate turnover by PTH was suggested by the effect to the hormone to increase cell Ca. Blocking the PTH-induced rise in cell Ca blunted the effect of the hormone to alkalinize the cell, as did inhibition of phosphatidylinositol breakdown. Furthermore, stimulation of protein kinase C by a phorbol ester and a diacylglycerol applied basolaterally alkalinized the cell and inhibited luminal H secretion. The findings indicate that both arms of the phosphatidylinositol-bis-phosphate cascade play a role in mediating the effect of PTH on the cell pH. The results are consistent with the view that PTH inhibits base exit in the proximal tubule by activation of the phosphatidylinositol cascade. The resulting alkalinization may contribute, with cAMP, to inhibit apical Na/H exchange and the PTH-induced depression of proximal HCO3 reabsorption.
甲状旁腺激素（PTH）对近端小管中HCO3转运的抑制机制尚未明确界定。以往的体外研究表明，这种效应是通过cAMP生成介导的，cAMP可抑制Na/H交换，导致细胞酸化。为了在体内研究这个问题，使用pH敏感荧光探针4-甲基伞形酮（4MU）和2',7'-双（羧乙基）-（5,6）-羧基荧光素（BCECF）测量大鼠浅表近端小管中的细胞内pH（pHi）。发现PTH可使细胞碱化。这种碱化表明基底外侧碱排出受到抑制，原位微灌注研究证实了这一点：降低肾小管周围毛细血管中的HCO3会使细胞酸化，而PTH可减弱这种效应。去除管腔中的Na会促进基底外侧碱进入，使细胞碱化。这种反应也会被PTH减弱。重新添加管腔中的Na会刺激管腔Na/H交换体，导致碱化超调，而PTH可部分抑制这种超调。cAMP可抑制管腔H分泌，但不会使细胞碱化。PTH对磷脂酰肌醇-双磷酸周转的刺激作用可通过该激素增加细胞Ca的作用来提示。阻断PTH诱导的细胞Ca升高会减弱该激素使细胞碱化的作用，抑制磷脂酰肌醇分解也会如此。此外，通过基底外侧施加佛波酯和二酰基甘油刺激蛋白激酶C会使细胞碱化并抑制管腔H分泌。这些发现表明磷脂酰肌醇-双磷酸级联反应的两个分支在介导PTH对细胞pH的作用中均发挥作用。结果与以下观点一致，即PTH通过激活磷脂酰肌醇级联反应抑制近端小管中的碱排出。由此产生的碱化可能与cAMP共同作用，抑制顶端Na/H交换以及PTH诱导的近端HCO3重吸收降低。
