Shahidullah Mohammad, To Chi-Ho, Pelis Ryan M, Delamere Nicholas A
Department of Physiology, University of Arizona, Tucson, Arizona 85724, USA.
Invest Ophthalmol Vis Sci. 2009 Apr;50(4):1791-800. doi: 10.1167/iovs.08-2487. Epub 2008 Nov 14.
Bicarbonate transport plays a role in aqueous humor (AH) secretion. The authors examined bicarbonate transport mechanisms and carbonic anhydrase (CA) in porcine nonpigmented ciliary epithelium (NPE).
Cytoplasmic pH (pH(i)) was measured in cultured porcine NPE loaded with BCECF. Anion exchanger (AE), sodium bicarbonate cotransporter (NBC), and CA were examined by RT-PCR and immunolocalization. AH secretion was measured in the intact porcine eye using a fluorescein dilution technique.
Anion exchanger AE2, CAII, and CAIV were abundant in the NPE layer. In cultured NPE superfused with a CO(2)/HCO(3)(-)-free HEPES buffer, exposure to a CO(2)/HCO(3)(-)-containing buffer caused rapid acidification followed by a gradual increase in pH(i). Subsequent removal of CO(2)/HCO(3)(-) with HEPES buffer caused rapid alkalinization followed by a gradual decrease in pH(i). The rate of gradual alkalinization after the addition of HCO(3)(-)/CO(2) was inhibited by sodium-free conditions, DIDS, and the CA inhibitors acetazolamide and methazolamide but not by the Na-H exchange inhibitor dimethylamiloride or low-chloride buffer. The phase of gradual acidification after removal of HCO(3)(-)/CO(2) was inhibited by DIDS, acetazolamide, methazolamide, and low-chloride buffer. DIDS reduced baseline pH(i). In the intact eye, DIDS and acetazolamide reduced AH secretion by 25% and 44%, respectively.
The results suggest the NPE uses a Na(+)-HCO(3)(-) cotransporter to import bicarbonate and a Cl(-)/HCO(3)(-) exchanger to export bicarbonate. CA influences the rate of bicarbonate transport. AE2, CAII, and CAIV are enriched in the NPE layer of the ciliary body, and their coordinated function may contribute to AH secretion by effecting bicarbonate transport into the eye.
碳酸氢盐转运在房水（AH）分泌中起作用。作者研究了猪非色素睫状上皮（NPE）中的碳酸氢盐转运机制和碳酸酐酶（CA）。
用BCECF加载培养的猪NPE，测量细胞质pH（pH(i)）。通过RT-PCR和免疫定位检测阴离子交换器（AE）、碳酸氢钠协同转运体（NBC）和CA。使用荧光素稀释技术在完整的猪眼中测量房水分泌。
阴离子交换器AE2、CAII和CAIV在NPE层中丰富。在用不含CO(2)/HCO(3)(-)的HEPES缓冲液灌注的培养NPE中，暴露于含CO(2)/HCO(3)(-)的缓冲液会导致快速酸化，随后pH(i)逐渐升高。随后用HEPES缓冲液去除CO(2)/HCO(3)(-)会导致快速碱化，随后pH(i)逐渐降低。添加HCO(3)(-)/CO(2)后逐渐碱化的速率受到无钠条件、DIDS以及CA抑制剂乙酰唑胺和甲醋唑胺的抑制，但不受Na-H交换抑制剂二甲基amiloride或低氯缓冲液的抑制。去除HCO(3)(-)/CO(2)后逐渐酸化的阶段受到DIDS、乙酰唑胺、甲醋唑胺和低氯缓冲液的抑制。DIDS降低了基线pH(i)。在完整的眼中，DIDS和乙酰唑胺分别使房水分泌减少25%和44%。
结果表明NPE使用Na(+)-HCO(3)(-)协同转运体来导入碳酸氢盐，并使用Cl(-)/HCO(3)(-)交换器来输出碳酸氢盐。CA影响碳酸氢盐转运的速率。AE2、CAII和CAIV在睫状体的NPE层中富集，它们的协同功能可能通过影响碳酸氢盐向眼内的转运而有助于房水分泌。
