Schirmanns K, Zeiske W
J Exp Biol. 1994 Dec;197(1):179-200. doi: 10.1242/jeb.197.1.179.
The K+-secreting larval midgut of Manduca sexta in vitro was voltage- or current-clamped. In contrast to Tl+, NH4+ and Na+, both Rb+ and K+ generated a short-circuit current, although with different saturation kinetics. The dependence of the short-circuit current on Rb+/K+ mole fraction gave no evidence for multi-ion occupation of the basolateral K+ channels. After 'functionally' eliminating the apical membranes using the ionophore amphotericin B and the 'apical K+ pump' blockers trimethyltin chloride or Tl+, the K+ channels could be more closely investigated. By measuring zero-current potentials, permeability ratios PX/PK were estimated using an adapted version of the GoldmanHodgkinKatz voltage equation. Their sequence was K+ (1) = Tl+ > Rb+ (0.38) > NH4+ (0.3) > Cs+ (0.03) > Na+ (0). The K+ channels could not be blocked by basally applied Cs+, Na+ or tetraethylammonium. Blockade of K+ current by Ba2+ was typically voltage-dependent, but only at moderate transbasal voltages. The relative electrical distance delta of the Ba2+ binding site from the basal channel opening was determined to be 0.2. At zero transbasal voltage, the apparent inhibition constant for barium KBa* was 1.7 mmol l-1.
对烟草天蛾幼虫的分泌钾的中肠进行体外电压钳制或电流钳制。与铊离子（Tl⁺）、铵离子（NH₄⁺）和钠离子（Na⁺）不同，铷离子（Rb⁺）和钾离子（K⁺）均能产生短路电流，尽管其饱和动力学不同。短路电流对Rb⁺/K⁺摩尔分数的依赖性并未表明基底外侧钾通道存在多离子占据情况。在用离子载体两性霉素B以及“顶端钾泵”阻滞剂氯化三甲基锡或Tl⁺“功能性地”去除顶端膜后，可对钾通道进行更深入的研究。通过测量零电流电位，利用修正版的戈德曼-霍奇金- Katz电压方程估算通透率PX/PK。其顺序为K⁺（1） = Tl⁺ > Rb⁺（0.38） > NH₄⁺（约0.3） > Cs⁺（0.03） > Na⁺（约0）。基底施加的Cs⁺、Na⁺或四乙铵不能阻断钾通道。Ba²⁺对钾电流的阻断通常依赖电压，但仅在适度的跨基底电压下如此。确定Ba²⁺结合位点距基底通道开口的相对电距离δ为0.2。在零跨基底电压下，钡的表观抑制常数KBa*为1.7 mmol·l⁻¹。
