EXTERNAL MG2+ TRIGGERS OSCILLATIONS AND A SUBSEQUENT SUSTAINED LEVEL OF INTRACELLULAR FREE CA2+, CORRELATED WITH CHANGES IN MEMBRANE CONDUCTANCE IN THE OOCYTE OF THE PRAWN PALAEMON SERRATUS

We have provided evidence that external Mg2+ induces correlated change s in [Ca2+](i) and membrane current or potential in prawn oocytes with out any requirement of fertilization, using the fluorescent Ca2+ indic ator Ca green dextran and voltage and current clamp methods. Replaceme nt of Mg2+-free ASW with standard (40 mM Mg2+)ASW triggered a diphasic [Ca2+](i) response consisting of an oscillation period followed by a second state of sustained [Ca2+](i) level devoid of oscillation, lasti ng about 70 min and up to 3 hr, respectively. In contrast, oocytes mai ntained for a long time in Mg2+-free ASW showed no changes in [Ca2+](i ) which remained at a basal concentration of about 0.2 mu M. The simul taneous records of [Ca2+](i) and membrane or current changes showed th at the oscillation period started with a first [Ca2+](i) peak (about 1 .7 mu M) and was correlated with an initial transient membrane hyperpo larization or a transient initial outward current peak. The first [Ca2 +](i) peak was followed by a slow decrease in [Ca2+](i) followed by a series of [Ca2+](i) transients, concurrent with a slow depolarization of the membrane or a related inwardly directed current. The oscillatio n period ended with an oscillatory plateau of [Ca2+](i) (about 0.6 mu M) lasting 49.6 +/- 4.5 min, the onset of which was concomitant with a final membrane hyperpolarization or a related final outward current. A continuous contact between external Mg2+ and the oocyte membrane was necessary to maintain the program of [Ca2+](i) and membrane conductan ce changes. The sources of Ca2+ mobilization are of internal origin fo r both the first peak and the subsequent series of [Ca2+](i) oscillati ons, and are mainly of external origin for the oscillation plateau and for the second state of sustained [Ca2+](i) level. The first and seco nd step of the cortical reaction occurred during the oscillatory plate au and the second state of sustained [Ca2+](i) level, respectively. (C ) 1996 Academic Press, Inc.