Max-Planck-Institut für Molekulare Physiologie, Dortmund, Germany.
Mount Desert Island Biological Laboratory, Salsbury Cove, Maine.
Am J Physiol Regul Integr Comp Physiol. 2020 Jul 1;319(1):R96-R105. doi: 10.1152/ajpregu.00060.2020. Epub 2020 May 27.
The rectal gland of the spiny dogfish secretes a salt solution isosmotic with plasma that maintains the salt homeostasis of the fish. It secretes salt against an electrochemical gradient that requires the expenditure of energy. Isolated rectal glands perfused without glucose secrete salt, albeit at a rate about 30% of glands perfused with 5 mM glucose. Gradually reducing the glucose concentration is associated with a progressive decrease in the secretion of chloride. The apparent for the exogenous glucose-dependent chloride secretion is around 2 mM. Phloretin and cytochalasin B, agents that inhibit facilitated glucose carriers of the solute carrier 2 (Slc2) family such as glucose transporter 2 (GLUT2), do not inhibit the secretion of chloride by the perfused rectal glands. Phloridzin, which inhibits Slc5 family of glucose symporters, or α-methyl-d-glucoside, which competitively inhibits the uptake of glucose through Slc5 symporters, inhibit the secretion of chloride. Thus the movement of glucose into the rectal gland cells appears to be mediated by a sodium-glucose symporter. Sodium-glucose cotransporter 1 (SGLT1), the first member of the Slc5 family of sodium-linked glucose symporters, was cloned from the rectal gland. No evidence of GLUT2 was found. The persistence of secretion of chloride in the absence of glucose in the perfusate suggests that there is an additional source of energy within the cells. The use of 2-mercapto-acetate did not result in any change in the secretion of chloride, suggesting that the oxidation of fatty acids is not the source of energy for the secretion of chloride. Perfusion of isolated glands with KCN in the absence of glucose further reduces the secretion of chloride but does not abolish it, again suggesting that there is another source of energy within the cells. Glucose was measured in the rectal gland cells and found to be at concentrations in the range of that in the perfusate. Glycogen measurements indicated that there are significant stores of glucose in the rectal gland. Moreover, glycogen synthase was partially cloned from rectal gland cells. The open reading frame of glycogen phosphorylase was also cloned from rectal gland cells. Measurements of glycogen phosphorylase showed that the enzyme is mostly in its active form in the cells. The cells of the rectal gland of the spiny dogfish require exogenous glucose to fully support the active secretion of salt. They have the means to transport glucose into the cells in the form of SGLT1. The cells also have an endogenous supply of glucose as glycogen and have the necessary elements to synthesize, store, and hydrolyze it.
尖牙鱼的直肠腺分泌一种与血浆等渗的盐溶液，以维持鱼类的盐稳态。它通过消耗能量来分泌盐分，逆着电化学梯度进行。在没有葡萄糖的情况下灌注分离的直肠腺会分泌盐分，尽管速度约为用 5mM 葡萄糖灌注的直肠腺的 30%。逐渐降低葡萄糖浓度会导致氯离子分泌逐渐减少。外源性葡萄糖依赖的氯离子分泌的表观 Km 值约为 2mM。根皮苷和细胞松弛素 B，这两种抑制溶质载体 2（Slc2）家族易化葡萄糖载体（如葡萄糖转运蛋白 2(GLUT2)）的试剂，不抑制灌注直肠腺的氯离子分泌。抑制 Slc5 家族葡萄糖同向转运体的根皮苷，或竞争性抑制 Slc5 同向转运体摄取葡萄糖的α-甲基-d-葡萄糖苷，抑制氯离子分泌。因此，葡萄糖进入直肠腺细胞的运动似乎是由钠-葡萄糖同向转运体介导的。从直肠腺中克隆出 Slc5 家族的钠连接葡萄糖同向转运体的第一个成员——钠-葡萄糖同向转运体 1（SGLT1）。没有发现 GLUT2 的证据。在灌注液中没有葡萄糖的情况下，氯离子分泌仍然持续存在，这表明细胞内存在另一种能量来源。使用 2-巯基乙酸并没有导致氯离子分泌发生任何变化，这表明脂肪酸的氧化不是氯离子分泌的能量来源。在没有葡萄糖的情况下，用 KCN 灌注分离的腺体进一步减少了氯离子的分泌，但并没有完全消除它，这再次表明细胞内存在另一种能量来源。在直肠腺细胞中测量到葡萄糖，发现其浓度在灌注液的范围内。糖原测量表明直肠腺中有大量的葡萄糖储存。此外，从直肠腺细胞中部分克隆出糖原合酶。也从直肠腺细胞中克隆出糖原磷酸化酶的开放阅读框。糖原磷酸化酶的测量结果表明，该酶在细胞中主要处于活性形式。尖牙鱼的直肠腺需要外源性葡萄糖来完全支持主动分泌盐。它们有通过 SGLT1 将葡萄糖转运到细胞内的方法。细胞内也有作为糖原的内源性葡萄糖供应，并具有合成、储存和水解它所需的元素。
