Contribution of L-type Ca2+ channels to evoked transmitter release in cultured Xenopus nerve-muscle synapses

Simultaneous pre- and postsynaptic patch recordings were obtained from the varicosity synapses formed by Xenopus motoneurons on muscle cells in embryonic cultures, in order to elucidate the contribution of N- and L-type Ca 2+ channels to the varicosity Ca 2+ current ( I Ca ) and evoked transmitter release. Although N-type channels are predominant in the varicosities and generally thought to be responsible for all evoked release, in most synapses a fraction of I Ca and release could be reversibly blocked by the L-type channel antagonist nifedipine, and enhanced by the agonist Bay K8644. Up to 50 % (mean, 21 %) of the I Ca evoked by a voltage clamp waveform mimicking a normal presynaptic action potential (APWF) is composed of L-type current. Surprisingly, the nifedipine-sensitive (L) channels activated more rapidly (time-constant, 0.46 ms at +30 mV) than the nifedipine-insensitive (N) channels (time constant, 1.42 ms). Thus the L-type current would play a disproportionate role in the I Ca linked to a normal action potential. The relationship between I Ca and release was the same for nifedipine-sensitive and -resistant components. The N- and L-components of I Ca are thus equally potent in evoking release. This may represent an immature stage before N-type channels become predominant. Replacing Ca 2+ in the medium with Ba 2+ strongly enhanced the L-type component, suggesting that L-type channels may be inactivated at Ca 2+ levels close to those at rest. We speculate that populations of L-type channels in different parts of the neuron may be recruited or inactivated by fluctuations of the cytosolic Ca 2+ concentration within the physiological range.