DEPLETION OF INTRACELLULAR CA2-STIMULATED INSP(3) LIBERATION OR THAPSIGARGIN, INDUCES A CAPACITATIVE CA2+ INFLUX IN PRAWN OOCYTES( STORES, MEDIATED BY MG2+)

By voltage clamp technique and intracellular calcium measurements, we recorded in prawn oocytes simultaneous [Ca2+](i) and ionic current cha nges stimulated by external Mg2+. The [Ca2+](i) response consists of a n 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 (divided by) current, an inward Na-Ca(divided by), current, and a fina l K-Ca(+) outward current. By using inhibitor (heparin) or sensitizers (thimerosal or caffeine) of calcium release ER channels, and caged In sP(3), we established that InsP(3) is the sole second messenger releas ing Ca2+ from intracellular stores. By sequential substitutions and re applications of external Ca2+, and using econazole (50 mu M), a Ca2+ i nflux inhibitor, we documented Ca2+ influx during the [Ca2+](i) oscill atory plateau. The intracellular Ca2+ store was depleted with thapsiga rgin (75-350 nM) in Ca2+-free ASW. Reapplication of external Ca-2 divi ded by evoked a rise in [Ca2+](i), indicating a store-dependent capaci tative 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 ext ernal Ca2+, in the physiological [Ca2+](i) response of prawn oocytes t o external Mg2+, consists of a store-dependent capacitative Ca2+ influ x. (C) 1998 Academic Press.