Co-assembly of N-type Ca ²⁺ and BK channels underlies functional coupling in rat brain

Activation of large conductance Ca ²⁺ -activated potassium (BK) channels hastens action potential repolarisation and generates the fast afterhyperpolarisation in hippocampal pyramidal neurons. A rapid coupling of Ca ²⁺ entry with BK channel activation is necessary for this to occur, which might result from an identified coupling of Ca ²⁺ entry through N-type Ca ²⁺ channels to BK channel activation. This selective coupling was extremely rapid and resistant to intracellular BAPTA, suggesting that the two channel types are close. Using reciprocal co-immunoprecipitation, we found that N-type channels were more abundantly associated with BK channels than L-type channels (Ca _v 1.2) in rat brain. Expression of only the pore-forming α-subunits of the N-type (Ca _v 2.2) and BK (Slo ₂₇ ) channels in a non-neuronal cell-line gave robust macroscopic currents and reproduced the interaction. Co-expression of Ca _v 2.2/Ca _v β3 subunits with Slo ₂₇ channels revealed rapid functional coupling. By contrast, extremely rare examples of rapid functional coupling were observed with coexpression of Ca _v 1.2/Ca _v β ₃ and Slo ₂₇ channels. Action potential repolarisation in hippocampal pyramidal neurons was slowed by the N-type channel blocker ω-conotoxin GVIA, but not by the L-type channel blocker isradipine. These data showed that selective functional coupling between N-type Ca ²⁺ and BK channels provided rapid activation of BK channels in central neurons.