CALCIUM ACTION POTENTIALS IN UNFERTILIZED EGGS OF MICE AND HAMSTERS

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Abstract
Measurements of membrane potential and resistance were made in zona-free eggs of mice and hamsters. The mean .+-. SD values for membrane potential were -91 .+-. 28 mV (mouse) and -97 .+-. 29 mV(hamster), and, for input resistance, they 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 .apprx. 3 V s-1, and its peak value was about +13 mV. Action potentials could be evoked in eggs bathed in Na-free solution or in normal solution containing tetrodotoxin (3 .mu.M). The presence of Co (5-20 mM), La (1 mM) or verapamil (200-400 .mu.M) in the bathing solution suppressed the action potential. Raising the extracellular Ca concentration from 4 to 40 mM increased the peak value of the action potential by 25 mV. Plasma membranes of mouse and hamster eggs evidently have voltage-dependent Ca channels.