Functional characteristics of two BKCa channel variants differentially expressed in rat brain tissues

cDNAs encoding large-conductance Ca2+-activated K+ channel alpha-subunit (r Slo) were obtained from. rat brain. From the DNA sequence of multiple rslo clones, we identified a specific sequence variation of 81 nucleotides, whic h is either absent from or present at the N-terminal region of a putative C a2+-sensing domain of the channel, Transcripts containing such variations w ere detected in different ratios from several brain regions, and their func tional significance was further examined, When heterologously expressed in Xenopus oocytes, both rSlo variants, named rSlo(0) and rSlo(27), generated Ca2+-activated and voltage-activated K+ currents characteristic of neuronal large-conductance Ca2+-activated K+ (BKCa) channels. Single-channel record ings of the two channels showed almost identical permeation characteristics and steady-state gating behavior. Noticeable differences between rSlo(0) a nd rSlo(27) were revealed when the macroscopic currents were measured at va rious voltages and intracellular Ca2+ concentrations. rSlo(27) activated wa s more rapidly than rSlo(0) in the presence of the same voltage stimulus, a nd the differences in these activation kinetics were dependent on the conce ntration of intracellular Ca2+. Despite their similar apparent affinities f or Ca2+, rSlo(0) and rSlo(27) showed significant differences in their co-op erative gating behavior. The Hill coefficient for intracellular Ca2+ was es timated to be about 3.7 for rSlo(27) regardless of the membrane voltage, an d that for rSlo(0) was reduced from about 5 to 2 as the membrane voltage ch anged from 30 to 140 mV. As activation of BKCa channels is involved in rapi d hyperpolarization of action potentials, the differential processing of rs lo, transcripts, and the generation of channels with different activation k inetics and Ca2+ cooperativity may be a mechanism for tuning the excitabili ty of neurons in different brain regions.