Scott Brett N, Yu Ming-Jiun, Lee Lenora W, Beyenbach Klaus W
Department of Biomedical Sciences, Cornell University, Ithaca, NY 14853, USA.
J Exp Biol. 2004 Apr;207(Pt 10):1655-63. doi: 10.1242/jeb.00932.
The mechanisms of K(+) entry from the hemolymph into principal cells of Malpighian tubules were investigated in the yellow fever mosquito, Aedes aegypti. The K(+) channel blocker Ba(2+) (5 mmol l(-1)) significantly decreased transepithelial (TEP) fluid secretion (V(s)) from 0.84 nl min(-1) to 0.37 nl min(-1) and decreased the K(+) concentration in secreted fluid from 119.0 mmol l(-1) to 54.3 mmol l(-1) with no change in the Cl(-) concentration. Even though the Na(+) concentration increased significantly from 116.8 mmol l(-1) to 144.6 mmol l(-1), rates of TEP ion secretion significantly decreased for all three ions. In addition, Ba(2+) had the following significant electrophysiological effects: it depolarized the TEP voltage (V(t)) from 19.4 mV to 17.2 mV, increased the TEP resistance (R(t)) from 6.4 kOhmscm to 6.9 kOhmscm, hyperpolarized the basolateral membrane voltage of principal cells (V(bl)) from -75.2 mV to -88.2 mV and increased the cell input resistance from 363.7 kOhms to 516.3 kOhms. These effects of Ba(2+) reflect the block of K(+) channels that, apparently, are also permeable to Na(+). Bumetanide (100 micro mol l(-1)) had no effect on TEP fluid secretion and electrical resistance but significantly decreased TEP K(+) secretion, consistent with the inhibition of electroneutral Na(+)/K(+)/2Cl(-) cotransport. TEP Na(+) secretion significantly increased because other Na(+) entry pathways remained active. Bumetanide plus Ba(2+) completely inhibited TEP electrolyte and fluid secretion, with fast and slow kinetics reflecting the Ba(2+) block of basolateral membrane K(+) channels and the inhibition of Na(+)/K(+)/2Cl(-) cotransport, respectively. The single and combined effects of Ba(2+) and bumetanide suggest that (1) K(+) channels and Na(+)/K(+)/2Cl(-) cotransport are the primary mechanisms for bringing K(+) into cells, (2) K(+) channels mediate a significant Na(+) influx, (3) Na(+) has as many as four entry pathways and (4) the mechanisms of TEP K(+) and Na(+) secretion are coupled such that complete block of TEP K(+) renders the epithelium unable to secrete Na(+).
在黄热病蚊子埃及伊蚊中，研究了钾离子从血淋巴进入马氏管主细胞的机制。钾离子通道阻滞剂钡离子（5 mmol l⁻¹）显著降低了跨上皮（TEP）液体分泌（Vₛ），从0.84 nl min⁻¹降至0.37 nl min⁻¹，并使分泌液中的钾离子浓度从119.0 mmol l⁻¹降至54.3 mmol l⁻¹，而氯离子浓度没有变化。尽管钠离子浓度从116.8 mmol l⁻¹显著增加到144.6 mmol l⁻¹，但所有三种离子的TEP离子分泌速率均显著降低。此外，钡离子具有以下显著的电生理效应：它使TEP电压（Vₜ）从19.4 mV去极化至17.2 mV，使TEP电阻（Rₜ）从6.4 kΩ·cm增加到6.9 kΩ·cm，使主细胞的基底外侧膜电压（Vₙₗ）从 -75.2 mV超极化至 -88.2 mV，并使细胞输入电阻从363.7 kΩ增加到516.3 kΩ。钡离子的这些效应反映了钾离子通道的阻断，显然这些通道对钠离子也有通透性。布美他尼（100 μmol l⁻¹）对TEP液体分泌和电阻没有影响，但显著降低了TEP钾离子分泌，这与电中性的钠离子/钾离子/2氯离子共转运的抑制一致。TEP钠离子分泌显著增加，因为其他钠离子进入途径仍然活跃。布美他尼加钡离子完全抑制了TEP电解质和液体分泌，快速和慢速动力学分别反映了基底外侧膜钾离子通道的钡离子阻断和钠离子/钾离子/2氯离子共转运的抑制。钡离子和布美他尼的单一及联合效应表明：（1）钾离子通道和钠离子/钾离子/2氯离子共转运是使钾离子进入细胞的主要机制；（2）钾离子通道介导显著的钠离子内流；（3）钠离子有多达四种进入途径；（4）TEP钾离子和钠离子分泌机制相互耦合，以至于TEP钾离子的完全阻断使上皮细胞无法分泌钠离子。
