Depletion of intracellular Ca2+ stores, mediated by Mg2+-stimulated InsP3 liberation or thapsigargin, induces a capacitative Ca2+ influx in prawn oocytes

Abstract

By voltage clamp technique and intracellular calcium measurements, we recorded in prawn oocytes simultaneous [Ca2+](i) and ionic current changes stimulated by external Mg2+. The [Ca2+](i) response consists of an oscillation period followed by a second state of sustained [Ca2+](i) level. The oscillation period successively comprises a first [Ca2+](i) peak, a series of [Ca2+](i) transients, and a [Ca2+](i) oscillatory plateau respectively concurrent with an initial transient outward K(Ca)/+ current, an inward Na(Ca)/+ current, and a final K(Ca)/+ outward current. By using inhibitor (heparin) or sensitizers (thimerosal or caffeine) of calcium release ER channels, and caged InsP3, we established that InsP3 is the sole second messenger releasing Ca2+ from intracellular stores. By sequential substitutions and reapplications of external Ca2+, and using econazole (50 μM), a Ca2+ influx inhibitor, we documented Ca2+ influx during the [Ca2+](i) oscillatory plateau. The intracellular Ca2+ store was depleted with thapsigargin (75-350 nM) in Ca2+-free ASW. Reapplication of external Ca2+ evoked a rise in [Ca2+](i), indicating a store-dependent capacitative Ca2+ influx, correlated with a K(Ca)/+ outward current increase. No measurable Ca2+ release-activated Ca2+ current (I(crac)) could be detected, but was indirectly demonstrated using the sensitivity of the K(Ca)/+ channels to [Ca2+](i). We propose that the involvement of external Ca2+, in the physiological [Ca2+](i) response of prawn oocytes to external Mg2+, consists of a store-dependent capacitative Ca2+ influx.