CALCIUM ACTION POTENTIALS IN UNFERTILIZED EGGS OF MICE AND HAMSTERS

¹ Department of Veterinary Physiology, University of Edinburgh, Summerhall, Edinburgh EH9 1QH

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 (mouse) and 410 ± 150 M (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 3V.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 µM). The presence of cobalt (5-20 mM), lanthanum (1 mM) or verapamil (200-400 µM) 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.

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