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

1. Simultaneous pre- and postsynaptic patch recordings were obtained from t he varicosity synapses formed by Xenopus motoneurons on muscle cells in emb ryonic cultures, in order to elucidate the contribution of N- and L-type Ca 2+ channels to the varicosity Ca2+ current, (I-Ca) and evoked transmitter r elease. potential. 2. Although N-type channels are predominant in the varicosities and general ly thought to be responsible for all evoked release, in most synapses a fra ction 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% (m ean, 21%) of the I-Ca evoked by a voltage clamp waveform mimicking a normal presynaptic action potential (APWF) is composed of L-type current. 3. Surprisingly, the nifedipine-sensitive (L) channels activated more rapid ly (time-constant, 0.46 ms at +30 mV) than the nifedipine-insensitive (N) c hannels, (time constant, 1.42 ms). Thus the L-type current, would play a di sproportionate role in the I-Ca linked to a normal action 4. The relationship between I-Ca and release was the same for nifedipine-se nsitive and -resistant components. The N- and L-components of I-Ca are thus equally potent in evoking release. This may represent an immature stage be fore N-type channels become predominant. 5. Replacing Ca2+ in the medium Nvith Ba2+ enhanced the L-type component,su ggesting that L-type channels may be inactivated at Ca2+ levels close to th ose at rest. 6. We speculate that populations of L-type channels in different parts of t he neuron may be recruited or inactivated by fluctuations, of the cytosolic Ca2+ concentration within the physiological range.