1. 1.

    Two recently identified channel types in Lymnaea stagnalis heart muscle cells were shown to conduct Na+ in the absence of extracellular Ca2+. They did not appear to be ‘voltage-gated’ as they were not activated by voltage. Also, they remained active over a wide range of membrane potentials. However, they were weakly ‘voltage-sensitive’ as their activity usually tended to increase with depolarization. The weak voltage-sensitivity and similarity to other non-voltagegated Ca2+ channels suggested that one or both of these channels may be receptor-operated Ca2+ channels. 2. One of the two channels had a slope conductance of 15 pS. The other appeared to have at least two subconductance states with slope conductances of 50 and 72pS. Both these conductance states had very similar open dwell-time constants and identical reversal potentials. The open dwell-time constants of both conductance states were not affected by voltage, suggesting that the channels' weak voltage-sensitivity was mediated by one of the closed states. 3. With divalent cations in the patch pipette, non-voltage-gated Ba2+ and Ca2+ currents were also detected. The Ba2+ conductance (12pS) was similar to the Ca2+ conductance (11pS).

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