Vidyasagar Sadasivan, Barmeyer Christian, Geibel John, Binder Henry J, Rajendran Vazhaikkurichi M
Dept. of Internal Medicine, Yale Univ., PO Box 208019, New Haven, CT 06520, USA.
Am J Physiol Gastrointest Liver Physiol. 2005 Jun;288(6):G1217-26. doi: 10.1152/ajpgi.00415.2004. Epub 2005 Jan 27.
Luminal isobutyrate, a relatively poor metabolized short-chain fatty acid (SCFA), induces HCO(3) secretion via a Cl-independent, DIDS-insensitive, carrier-mediated process as well as inhibiting both Cl-dependent and cAMP-induced HCO(3) secretion. The mechanism(s) responsible for these processes have not been well characterized. HCO(3) secretion was measured in isolated colonic mucosa mounted in Lucite chambers using pH stat technique and during microperfusion of isolated colonic crypts. (14)C-labeled butyrate, (14)C-labeled isobutyrate, and (36)Cl uptake were also determined by apical membrane vesicles (AMV) isolated from surface and/or crypt cells. Butyrate stimulation of Cl-independent, DIDS-insensitive 5-nitro-3-(3-phenylpropyl-amino)benzoic acid-insensitive HCO(3) secretion is greater than that by isobutyrate, suggesting that both SCFA transport and metabolism are critical for HCO(3) secretion. Both lumen and serosal 25 mM butyrate inhibit cAMP-induced HCO(3) secretion to a comparable degree (98 vs. 90%). In contrast, Cl-dependent HCO(3) secretion is downregulated by lumen 25 mM butyrate considerably more than by serosal butyrate (98 vs. 37%). Butyrate did not induce HCO(3) secretion in isolated microperfused crypts, whereas an outward-directed HCO(3) gradient-driven induced (14)C-butyrate uptake by surface but not crypt cell AMV. Both (36)Cl/HCO(3) exchange and potential-dependent (36)Cl movement in AMV were inhibited by 96-98% by 20 mM butyrate. We conclude that 1) SCFA-dependent HCO(3) secretion is the result of SCFA transport across the apical membrane via a SCFA/HCO(3) exchange more than intracellular SCFA metabolism; 2) SCFA-dependent HCO(3) secretion is most likely a result of an apical membrane SCFA/HCO(3) exchange in surface epithelial cells; 3) SCFA downregulates Cl-dependent and cAMP-induced HCO(3) secretion secondary to SCFA inhibition of apical membrane Cl/HCO(3) exchange and anion channel activity, respectively.
腔内异丁酸是一种代谢相对较差的短链脂肪酸（SCFA），它通过一种不依赖氯离子、对4,4'-二异硫氰基二苯乙烯-2,2'-二磺酸（DIDS）不敏感的载体介导过程诱导碳酸氢根（HCO₃）分泌，同时抑制依赖氯离子和环磷酸腺苷（cAMP）诱导的HCO₃分泌。负责这些过程的机制尚未得到很好的表征。使用pH计技术在安装于有机玻璃室中的分离结肠黏膜中以及在分离结肠隐窝的微灌注过程中测量HCO₃分泌。还通过从表面和/或隐窝细胞分离的顶膜囊泡（AMV）测定了¹⁴C标记的丁酸、¹⁴C标记的异丁酸和³⁶Cl摄取。丁酸对不依赖氯离子、对DIDS不敏感、对5-硝基-3-(3-苯丙基氨基)苯甲酸不敏感的HCO₃分泌的刺激作用大于异丁酸，这表明SCFA的转运和代谢对HCO₃分泌都至关重要。管腔和浆膜侧25 mM的丁酸对cAMP诱导的HCO₃分泌的抑制程度相当（分别为98%和90%）。相比之下，管腔侧25 mM的丁酸对依赖氯离子的HCO₃分泌的下调作用明显大于浆膜侧丁酸（分别为98%和37%）。丁酸在分离的微灌注隐窝中不诱导HCO₃分泌，而外向型HCO₃梯度驱动表面而非隐窝细胞AMV对¹⁴C-丁酸的摄取。20 mM的丁酸使AMV中³⁶Cl/HCO₃交换和电位依赖性³⁶Cl移动受到96 - 98%的抑制。我们得出结论：1）依赖SCFA的HCO₃分泌是SCFA通过SCFA/HCO₃交换跨顶膜转运的结果，而非细胞内SCFA代谢的结果；2）依赖SCFA的HCO₃分泌很可能是表面上皮细胞顶膜SCFA/HCO₃交换的结果；3）SCFA分别通过抑制顶膜Cl/HCO₃交换和阴离子通道活性，下调依赖氯离子和cAMP诱导的HCO₃分泌。
