Gomez M D, Nasi E
Department of Physiology, Boston University School of Medicine, Massachusetts 02118, USA.
J Gen Physiol. 1996 Jun;107(6):715-30. doi: 10.1085/jgp.107.6.715.
The receptor potential of rhabdomeric photoreceptors is mediated primarily by a Na influx, but other ions must also permeate through light-dependent channels to account for some properties of the photoresponse. We examined ion conduction in macroscopic and single-channel light-induced currents of slug and scallop photoreceptors. In the absence of Na, a fivefold change in extracellular K shifted the reversal voltage of the photocurrent (Vrev) by approximately 27 mV. Because the dependency of Vrev on [K]o was sub-Nernstian, and Vrev in each condition was more positive than Ek, some other ion(s) with a positive equilibrium potential must be implicated, in addition to K. We assessed the participation of calcium, an important candidate because of its involvement in light adaptation. Three strategies were adopted to minimize the impairments to cytosolic Ca homeostasis and loss of responsiveness that normally result from the required ionic manipulations: (a) Internal dialysis with Na-free solutions, to prevent reverse operation of the Na/Ca exchanger. (b) Rapid solution changes, temporally limiting exposure to potentially detrimental ionic conditions. (c) Single-channel recording, exposing only the cell-attached patch of membrane to the test solutions. An inward whole-cell photocurrent could be measured with Ca as the only extracellular charge carrier. Decreasing the [Ca]o to 0.5 mM reduced the response by 43% and displaced the reversal potential by -4.3 mV; the shift was larger (delta Vrev = -44 mV) when intracellular permeant cations were also removed. In all cases, however, the current carried by Ca was < 5% of that measured with normal [Na]o. Unitary light-activated currents were reduced in a similar way when the pipette contained only divalent cations, indicating a substantial selectivity for Na over Ca. The fall kinetics of the photoresponse was slower when external Ca was replaced by Ba, or when the membrane was depolarized; however, dialysis with 10 mM BAPTA failed to antagonize this effect, suggesting that mechanisms other than the Ca influx participate in the modulation of the time course of the photocurrent.
横纹肌光感受器的感受器电位主要由钠离子内流介导，但其他离子也必须通过光依赖通道渗透，以解释光反应的某些特性。我们研究了蛞蝓和扇贝光感受器宏观和单通道光诱导电流中的离子传导。在没有钠离子的情况下，细胞外钾离子浓度五倍的变化使光电流的反转电压（Vrev）偏移了约27毫伏。由于Vrev对[K]o的依赖性低于能斯特方程，并且在每种情况下Vrev都比Ek更正，除了钾离子之外，一定还涉及其他具有正平衡电位的离子。我们评估了钙的参与情况，钙是一个重要的候选离子，因为它参与光适应过程。我们采用了三种策略来尽量减少通常由所需离子操作导致的对细胞质钙稳态的损害和反应性的丧失：（a）用无钠溶液进行内部透析，以防止钠/钙交换器的反向运作。（b）快速更换溶液，在时间上限制对潜在有害离子条件的暴露。（c）单通道记录，仅将膜的细胞贴附片暴露于测试溶液中。以钙作为唯一的细胞外电荷载体时，可以测量到内向的全细胞光电流。将[Ca]o降低到0.5毫摩尔/升会使反应降低43%，并使反转电位偏移-4.3毫伏；当细胞内渗透性阳离子也被去除时，偏移更大（ΔVrev = -44毫伏）。然而，在所有情况下，由钙携带的电流都小于用正常[Na]o测量的电流的5%。当移液管中仅含有二价阳离子时，单位光激活电流以类似的方式降低，表明对钠的选择性远高于钙。当外部钙被钡取代或膜去极化时，光反应的下降动力学较慢；然而，用10毫摩尔/升的BAPTA进行透析未能拮抗这种效应，这表明除了钙内流之外的机制参与了光电流时间进程的调节。
