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

Abstract

We have provided evidence that external Mg2+ induces correlated changes in [Ca2+](i) and membrane current or potential in prawn oocytes without any requirement of fertilization, using the fluorescent Ca2+ indicator Ca green dextran and voltage and current clamp methods. Replacement 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, lasting about 70 min and up to 3 hr, respectively. In contrast, oocytes maintained for a long time in Mg2+-free ASW showed no changes in [Ca2+](i) which remained at a basal concentration of about 0.2 μM. The simultaneous records of [Ca2+](i) and membrane or current changes showed that the oscillation period started with a first [Ca2+](i) peak (about 1.7 μM) and was correlated with an initial transient membrane hyperpolarization 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 oscillation period ended with an oscillatory plateau of [Ca2+](i) (about 0.6 μM) 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 conductance changes. The sources of Ca2+ mobilization are of internal origin for both the first peak and the subsequent series of [Ca2+](i) oscillations, and are mainly of external origin for the oscillation plateau and for the second state of sustained [Ca2+](i) level. The first and second step of the cortical reaction occurred during the oscillatory plateau and the second state of sustained [Ca2+](i) level, respectively.