Limulus ventral photoreceptors contain two functionally dissimilar inositol triphosphate-induced calcium release mechanisms

Abstract

Injections of inositol 1,4,5-triphosphate (InsP 3) into Limulus ventral photoreceptors give rise to a rapid depolarization and an elevation of intracellular calcium concentration (Ca i). This response to InsP 3 is followed by a period of desensitization that persists as long as Ca i remains elevated (feedback inhibition). Limulus ventral photoreceptors have two types of lobe: a light-sensitive rhabdomeric lobe (R lobe), and a light-insensitive arhabdomeric lobe (A lobe). Evidence showing the presence of feedback inhibition has been so far demonstrated only in the R lobe. In this study, simultaneous measurements of Ca i were made using aequorin and double-barreled calcium-sensitive electrodes in each type of lobe. We carefully checked the location of the R lobe and A lobe by scanning a microspot of light across the whole photoreceptor. We then inserted a double-barreled calcium-sensitive microelectrode with InsP 3 in either type of lobe. In the R lobe, injections of InsP 3 led to a large Ca i increase, a rapid depolarization and feedback inhibition; a brief flash of light induced a rapid depolarization and a Ca i increase measured by both aequorin and the calcium-sensitive electrode. In the A lobe, injection of InsP 3 led to an increase in Ca i measured by the calcium-sensitive electrode but to no depolarization or aequorin luminescence. Further there was no evidence of feedback inhibition in the A lobe; the elevation of Ca i caused by the first injection did not desensitize the photoreceptor to a second injection of InsP 3 3 s later. To verify that the aequorin and the cell membrane respond to an increase in Ca i, we presented a brief flash of light. Following a uniform illumination, there is indeed a typical large luminescence increase and a receptor potential. The calcium-sensitive electrode measures a small and slow Ca i increase because its tip is located in the A lobe and it is measuring Ca 2+ diffusing from the R lobe. Our observation that the InsP 3-induced Ca i increase in the A lobe is not apparently accompanied by a subsequent desensitization to InsP 3 may suggest that there are more than one type of InsP 3 receptor in the same cell. Alternatively, the InsP 3 receptor could be the same but some additional factor, which confers feedback inhibition, could be missing in the A lobe.