Mechanism of inhibition of mouse Slo3 (KCa5.1) potassium channels by quinine, quinidine, and barium.

Background and Purpose: Slo3 is a major component of the membrane potassium conductance, KSper, of mammalian spermatozoa and inhibition by quinine and barium alters their motility. The aim of this investigation was to determine the mechanism by which these drugs inhibit Slo3. Experimental approach: We studied block of mouse Slo3 (mSlo3) potassium channels expressed in Xenopus oocytes by quinine, quinidine, and barium. Gain-of-function mSlo3 mutations were generated in order to explore state-dependence of inhibition. The interaction between quinidine and mSlo3 channels was modelled by in silico docking. Key results: We found that inhibition by several drugs known to block KSper also affected Slo3 with similar levels of inhibition. Inhibition by extracellular barium was prevented by increasing the extracellular potassium concentration. R196Q and F304Y mutations in the mSlo3 voltage-sensor and pore, respectively, both increased channel activity. The F304Y mutation did not alter the effects of barium, but increased the potency of inhibition by both quinine and quinidine approximately tenfold. This effect, however, was not observed with the R196Q mutation. Conclusions and Implications: Block of mSlo3 by quinine, quinidine, and barium is not state dependent. External barium inhibits mSlo3 by interacting with the selectivity filter, whilst quinine and quinidine block via the intracellular side by binding in a hydrophobic pocket formed by the S6 segment of each subunit. Furthermore, we propose that the Slo3 channel activation gate lies deep within the pore between F304 in the S6 segment and the selectivity filter.