Multiple Sources of Light-Evoked Intracellular Calcium Increases in Hermissenda Type B Photoreceptors

Abstract

Previous research suggests that learning-produced changes in excitability and K+ currents of Hermissenda Type B photoreceptors are Ca2+-dependent phenomena. Little information is available concerning the sources and dynamics of Ca2+ in these cells. We have used Fura-2 dual-wavelength (340/380 nm) photometry to measure somatic [Ca2+]i in B cells. Thirty sec light steps (LS) produce a large increase in [Ca2+]i (∼ 246%). To determine the contribution of Ca2+-influx vs Ca2+-release, we measured [Ca2+]i throughout 5 consecutive LSs in either normal or Ca2+-free ASW (0 mM Ca2+, 30 mM EGTA). Cells exposed to Ca2+-free ASW had a basal [Ca2+]i much lower than when external Ca2+ was present, often below detection limits. Ca2+-free ASW abolished light-induced [Ca2+]i increases in all 7 cells tested. We next explored the role of voltage-gated Ca2+ channels (VGCCs) to Ca2+ influx with the use of cobalt (5mM), a VGCC blocker in B cells. Co2+ did not affect either basal [Ca2+]i or light-induced [Ca2+]i increases (n = 5). To assess the contributions of the ER to light-induced [Ca2+]i changes, B cells were incubated in the ryanodine receptor (RyR) blocker dantrolene (50 μM). Dantrolene reduced the [Ca2+]i response by ∼33% (n =5), and also produced a progressive reduction in basal [Ca2+]i (∼ 60%). Exposure of Ca2+-free ASW cells (n =3) to thapsigargin (TH; 100μM - 1mM, a blocker of the ER Ca2+-ATPase pump) increased basal [Ca2+]i , consistent with store depletion. Collectively, our results indicate that [Ca2+]o is necessary for normal basal [Ca2+]i and critical for light-induced [Ca2+]i increases; but little Ca2+ enters through VDCCs. This suggests that [Ca2+]o enters the cytosol via other routes (e.g. TRP channels) or that the contribution of Ca2+ through VGCCs is slight but serves to trigger Ca2+-induced Ca2+-release (CICR) from ER stores.