A putative mechanism for the enhanced open state of the Ca²⁺-activated BK channel by intracellular Mg²⁺ in cultured human renal proximal tubule cells

Abstract

The Ca²⁺-activated BK channel in cultured human proximal tubule cells (RPTECs) is sensitive to voltage, ATP, and pH_i. Furthermore, we recently found out that this channel was regulated also by intracellular Mg²⁺ ([Mg²⁺]_i). One of the significant effects of [Mg²⁺]_i on channel was to extend the open time at a low intracellular Ca²⁺ ([Ca²⁺]_i). However, [Mg²⁺]_i effect on channel under the relatively high [Ca²⁺]_i is still unknown. In this study, we examined effects of [Mg²⁺]_i (1 - 10 mM) on open probability (P_o) and gating mechanisms of the BK channel in the presence of 0.1 - 1.0 mM [Ca²⁺]_i, and compared with the gating of high [Ca²⁺]_i-induced channel activation, using the patch-clamp technique. When the channel was activated by 0.1 mM [Ca²⁺]_i, raising [Mg²⁺]_i up to 10 mM elevated P_o and induced parallel shifts of P_o - V_m relationships. These effects of [Mg²⁺]_i on channel P_o and the P_o - V_m relationships were mimicked by either total or partial substitution of equimolar [Mg²⁺]_i to [Ca²⁺]_i. In the presence of 1 mM [Ca²⁺]_i, addition of 10 mM [Mg²⁺]_i further enhanced channel P_o, which was also similar to channel activation by raising [Ca²⁺]_i to 10 mM. The dwell-time histograms obtained from 10 mM [Mg²⁺]_i in the presence of 0.1 mM [Ca²⁺]_iwere similar to that obtained from 10 mM [Ca²⁺]_i. These results suggest that, in addition to high affinity Ca²⁺-binding sites, the BK channel in RPTECs possesses the binding site for both Ca²⁺ and Mg²⁺, which is involved in channel activation, and its affinity ratio of Ca²⁺/ Mg²⁺ is close to 1. [Jpn J Physiol 54 Suppl:S128 (2004)]