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Quantal endplate currents (qEPCs) were recorded extracellularly by a macropatch electrode from excised diaphragms of mice. During the first 3 days after birth, the mean rise time tr was 0.5 ms (0.1-0.9 peak, 20 °C). Double-exponential, amplitude-weighted fits of the decay discerned almost equally abundant components [tau]1'[approximate]6 ms and [tau]2'[approximate]9 ms. Beginning on day 3, on days 4 and 5 after birth both tr and the [tau]' dropped. Further decreasing slowly, adult values were reached at day 8, with tr[approximate]0.3 ms, [tau]1'[approximate]2 ms and a very weak [tau]2'[approximate]6 ms component. When compared to the kinetics of fetal channels, the [tau]1' and [tau]2' of up to 3 day qEPCs could correspond to the short and long splice variants of the fetal channel type. The [tau]1' of adult muscles of 2 ms agrees well with the burst durations of adult channels while a weak longer [tau]2' component may represent [`]extrasynaptic' channels. The long tr of very young mice may correspond to the relatively slow rise of channel currents elicited by ACh pulses in mouse myoballs.
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