Cell Calcium (2002) 32(4), 209–216 0143-4160/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0143-4160(02)00156-2, available online at http://www.idealibrary.com on 2-Aminoethoxydiphenyl borate inhibits phototransduction and blocks voltage-gated potassium channels in Limulus ventral photoreceptors Y. Wang, M. Deshpande, R. Payne Department of Biology and Program in Neuroscience and Cognitive Science, University of Maryland, College Park, MD 20742, USA Summary 2-Aminoethoxydiphenyl borate (2-APB) is a membrane-permeable modulator that inhibits the activation of inositol (1,4,5) trisphosphate (InsP 3 ) receptors, store operated channels (SOCs) and TRP channels in cells that utilize the phosphoinositide cascade for cellular signaling. In Limulus ventral photoreceptors, light-induced calcium release via the phosphoinositide cascade is thought to activate the photocurrent. Injection of either exogenous InsP 3 or calcium ions can therefore mimic excitation by light. One hundred micromolar 2-APB reversibly inhibited the photocurrent of ventral photoreceptors in a concentration-dependent manner, acting on at least two processes thought to mediate the visual cascade. 2-APB reversibly inhibited both light and InsP 3 -induced calcium release, consistent with its role as an inhibitor of the InsP 3 receptor. In addition, 2-APB reversibly inhibited the activation of depolarizing current flow through the plasma membrane caused by pulsed pressure injection of calcium ions into the light-sensitive lobe of the photoreceptor. We also found that 100 M 2-APB reversibly inhibited both transient and sustained voltage-activated potassium current during depolarizing steps. 2-APB has previously been shown to block phototransduction in Drosophila photoreceptors. The lack of specificity of the action of 2-APB in Limulus indicates that this blockade need not necessarily arise from inhibition of InsP 3 -induced calcium release. © 2002 Elsevier Science Ltd. All rights reserved. INTRODUCTION The depolarizing light response of invertebrate microvil- lar photoreceptors is mediated by the phosphoinositide lipid-signaling pathway [1,2]. The PI pathway has two initial products, inositol (1,4,5) trisphosphate (InsP 3 ) and diacylglycerol (DAG). Current research centers on the role played by each of these products in activating the cation channels in the plasma membrane that carry the pho- tocurrent. Use of a specific InsP 3 receptor inhibitor could help resolve the role of InsP 3 . 2-Aminoethoxydiphenyl borate (2-APB) is a cell-perme- able inhibitor of InsP 3 -induced calcium release [3]. 2-APB blocks phototransduction in Drosophila photoreceptors Received 23 April 2002 Revised 18 July 2002 Accepted 19 July 2002 Correspondence to: Dr Richard Payne, Department of Biology, University of Maryland, College Park, MD 20742, USA. Tel.: +1-301-405-6955; fax: +1-301-314-9358; e-mail: rp12@umail.umd.edu [4,5] but since genetic deletion of the InsP 3 receptor protein does not affect phototransduction [6,7], 2-APB presumably acts at some other site. Interpretation of the effect of 2-APB is dependent upon its specificity, which is difficult to establish in Drosophila photoreceptors. A physiological response to InsP 3 has not been observed in Drosophila photoreceptors and although 2-APB does not block the activation of trp family phototransduction channels by metabolic inhibitors [4],itisunclearhowthis mechanism of activation relates to that of the phototrans- duction cascade. Incontrastto Drosophila photoreceptors,alargebodyof evidence from Limulus ventral photoreceptors implicates InsP 3 -induced calcium release in the activation of plasma membrane cationic channels during phototransduction (reviews: [8,9]). Pressure injection of InsP 3 into ventral photoreceptors mimics excitation by light, in so far as it activates a conductance in the plasma membrane whose permeabilitytosodiumionsandrectificationareverysim- ilartothatunderlyingthephotocurrent [10,11].Activation 209