Evolution towards higher unitary conductance in mammals makes BK channels more efficient and precise

The large conductance calcium-activated potassium channel, known as the BK channel, play an essential role in neuronal firing and is characterized by a very large [~]250pS unitary conductance in mammals. However, this high unitary conductance is not consistent across all species with invertebrates demonstrating a much lower unitary conductance. We explored the calcium activation properties of BK channels of different unitary conductance in computational models and found that mammalian channels are more efficiently activated by calcium and produce a stronger potassium current compared to the channels of lower unitary conductance found in invertebrates. The lower unitary conductance channels display fierce competition for the available calcium, which results in low activation and weaker current. Due to these properties, mammalian BK channels are more suitable to repolarize sodium action potentials, which enables more precise spike timing, and may explain why evolution appears to have favored a trend towards very high unitary conductance in mammalian BK channels. This may be an essential component of the more advanced brain functions achieved by these species compared to invertebrates.