Gomez M D, Nasi E
Department of Physiology, Boston University School of Medicine, Massachusetts 02118.
J Gen Physiol. 1994 Sep;104(3):487-505. doi: 10.1085/jgp.104.3.487.
The tight-seal whole-cell recording technique was used to examine the effect of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on the photocurrent of hyperpolarizing ciliary photoreceptors isolated from the distal retina of the bay scallop (Pecten irradians). In these cells, light causes an increase in a conductance that is highly selective to potassium ions. Extracellular application of TEA at a concentration of 50 mM produced a modest, reversible block (approximately 35% at -20 mV holding potential). The blockage was weakly voltage dependent, increasing by approximately 20% for a 20-mV hyperpolarization, suggestive of a site of interaction superficially located within the electric field of the membrane. Treatment with TEA produced no significant changes either in the light sensitivity of the photocurrent or in its kinetics. The effects of superfusion with 4-AP were more dramatic: the light-evoked current was nearly abolished (> 95%) at submillimolar concentrations, with a half-maximal dose of approximately 0.6 microns. The blockage had a rapid onset and was slowly reversible. No significant use or voltage dependency were observed. A number of control experiments indicated that the phototransduction cascade remained functional during treatment with 4-AP: the early receptor current, the prolonged after current and its suppression, the photoresponse kinetics and the light sensitivity of the cell were little affected by 4-AP, suggesting that the suppression of the photocurrent is due to blockage of the light-sensitive channels, rather than impairment of some of the activation steps. The results are discussed in the light of a possible kinship between the light-activated potassium channels of invertebrate hyperpolarizing photoreceptors and the family of rapidly-inactivating voltage-dependent potassium channels, which typically exhibit high susceptibility to blockage by this drug.
采用紧密封接全细胞记录技术，研究了四乙铵（TEA）和4-氨基吡啶（4-AP）对从海湾扇贝（Pecten irradians）远端视网膜分离的超极化纤毛光感受器光电流的影响。在这些细胞中，光会导致对钾离子具有高度选择性的电导增加。细胞外施加浓度为50 mM的TEA会产生适度的、可逆的阻断（在-20 mV的钳制电位下约为35%）。这种阻断具有较弱的电压依赖性，超极化20 mV时增加约20%，提示相互作用位点位于膜电场表面。用TEA处理后，光电流的光敏感性及其动力学均无显著变化。用4-AP灌流的影响更为显著：在亚毫摩尔浓度下，光诱发电流几乎被消除（>95%），半数最大剂量约为0.6微摩尔。阻断起效迅速且可逆缓慢。未观察到明显的使用或电压依赖性。多项对照实验表明，在4-AP处理期间光转导级联仍保持功能：早期受体电流、延长的后电流及其抑制、光反应动力学和细胞的光敏感性受4-AP影响很小，这表明光电流的抑制是由于光敏感通道的阻断，而不是某些激活步骤的受损。根据无脊椎动物超极化光感受器的光激活钾通道与快速失活的电压依赖性钾通道家族之间可能存在的亲缘关系，对结果进行了讨论，后者通常对该药物的阻断表现出高度敏感性。
