Electrical maturation of the murine oocyte: an increase in calcium current coincides with acquisition of meiotic competence.
We have used the whole-cell recording technique to compare three stages of primary and secondary oocytes from F1 hybrid mice (C57BL/6J x SJL/J): neonatal germinal vesicle (NGV) stage primary oocytes from 10- to 20-day-old, prepubescent mice; mature germinal vesicle (MGV) stage primary oocytes from 12-week-old, post-pubescent, superovulated mice; first polar body (FPB) stage secondary oocytes from 12-week-old, post-pubescent mice during the normal oestrus cycle or following superovulation. NGV, MGV and FPB oocytes all exhibit two voltage-dependent currents: an inward, rapidly activating/inactivating current, and an outward, slowly activating/non-inactivating current. In 1.5 mmol/l external Ca the average peak inward current is -2.9, -12.4 and -13.8 microA/cm2 in NGV, MGV and FPB oocytes, respectively. In 20 mmol/l Ca these currents increase and the reversal potential shifts to the right. The outward current decreases slightly with growth and development: at 40 mV test potentials, NGV oocytes have average outward currents of 8.9 microA/cm2, and MGV and FPB oocytes have currents of 5.0 and 5.5 microA/cm2, respectively. Thus, MGV oocytes express FPB current patterns. The reversal potentials, kinetics and pharmacology of the currents indicate that Ca channels carry the inward current and K channels carry the outward current. During growth in vivo a gradual depolarisation accompanies maturation. Resting potentials ranged from -45 to -30 mV in NGV oocytes to -35 to -17 mV in MGV oocytes to -20 mV to -3 mV in FPB oocytes. These data suggest that a selective increase occurs in the number of Ca channels during oocyte growth. This increase precedes nuclear maturation and coincides with the acquisition of meiotic competence.
Murnane
J M
JM
Emory University School of Medicine, Atlanta, Georgia 30322.
DeFelice
L J
LJ
D000200:Action Potentials; D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008540:Meiosis / Q000502:physiology; D008564:Membrane Potentials; D051379:Mice; D008810:Mice, Inbred C57BL; D009865:Oocytes / Q000254:growth & development* / Q000378:metabolism*
D015220:Calcium Channels; D002118:Calcium