Calcium action potentials in unfertilized eggs of mice and hamsters

Measurements of membrane potential and resistance have been made in zona-free eggs of mice and hamsters. The mean +/- S.D. values for membrane potential were -91 +/- 28 mV (mouse) and -97 +/- 29 mV (hamster) and for input resistance were 430 +/- 230 M omega (mouse) and 410 +/- 150 M omega (hamster) respectively. Large fluctuations (20 mV) of membrane potential occurred apparently at random and these were accompanied by changes of membrane resistance. Depolarizing current pulses passed through the recording micro-electrode evoked action potentials in eggs of both species. The threshold for excitation was about -50 mV, the maximum rate of rise of the action potential was about 3 V.s-1 and its peak value was about +13 mV. Action potentials could be evoked in eggs bathed in sodium-free solution or in normal solution containing tetrodotoxin (3 microM). The presence of cobalt (5-20 mM), lanthanum (1 mM) or verapamil (200-400 microM) in the bathing solution suppressed the action potential. Raising the extracellular calcium concentration from 4 to 40 mM increased the peak value of the action potential by 25 mV. It is concluded that the plasma membranes of mouse and hamster eggs have voltage-dependent calcium channels.