Inositol 1,4,5-trisphosphate–induced Calcium Release Is Necessary for Generating the Entire Light Response of Limulus Ventral Photoreceptors

The experiments reported here were designed to answer the question of whether inositol 1,4,5-trisphosphate (IP₃)-induced calcium release is necessary for generating the entire light response of Limulus ventral photoreceptors. For this purpose the membrane-permeable IP₃ receptor antagonist 2-aminoethoxydiphenyl borate (2APB) (Maruyama, T., T. Kanaji, S. Nakade, T. Kanno, and K. Mikoshiba. 1997. J. Biochem. (Tokyo). 122:498–505) was used. Previously, 2APB was found to inhibit the light activated current of Limulus ventral photoreceptors and reversibly inhibit both light and IP₃ induced calcium release as well as the current activated by pressure injection of calcium into the light sensitive lobe of the photoreceptor (Wang, Y., M. Deshpande, and R. Payne. 2002. Cell Calcium. 32:209). In this study 2APB was found to inhibit the response to a flash of light at all light intensities and to inhibit the entire light response to a step of light, that is, both the initial transient and the steady-state components of the response to a step of light were inhibited. The light response in cells injected with the calcium buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) was reversibly inhibited by 2APB, indicating that these light responses result from IP₃-mediated calcium release giving rise to an increase in Ca_i. The light response obtained from cells after treatment with 100 μM cyclopiazonic acid (CPA), which acts to empty intracellular calcium stores, was reversibly inhibited by 2APB, indicating that the light response after CPA treatment results from IP₃-mediated calcium release and a consequent rise in Ca_i. Together these findings imply that IP₃-induced calcium release is necessary for generating the entire light response of Limulus ventral photoreceptors.