Role of intracellular Mg²⁺ in the activation of muscarinic K⁺ channel in cardiac atrial cell membrane

Abstract

Effects of intracellular Mg²⁺ in the activation of a muscarinic K⁺ channel were examined in single atrial cells, using patch-recording techniques. In “cell-attached” patch recordings, acetylcholine (ACh) or adenosine (Ado), present in the pipette, activated a specific population of K⁺ channels. In “inside-out” patches, openings of the K⁺ channel by ACh or Ado diminished and did not resume until Mg²⁺ was added to the perfusate which contained GTP or GTP-γS, a non-hydrolyzable GTP analogue. Channel openings caused by GTP faded by removing Mg²⁺, while GTP-γS-induced openings persisted steadily even when both Mg²⁺ and GTP-γS were removed. In contrast to the case of GTP-induced channel openings, the GTP-γS-induced openings were not inhibited by the A protomer of pertussi toxin with NAD. From these observations, we concluded: 1) Intracellular Mg²⁺ is essential for GTP to activate the GTP-binding protein. 2) Deactivation of the N protein may be caused by hydrolysis of GTP to GDP. This process may not require Mg²⁺. 3) During the activation by GTP analogues, the N protein may be dissociated into its subunits.