(A) Three levels of channel activity are promoted by the A- and L-sites. H, high F_o state; L, low F_o state; C, closed channels. Ca²⁺ binding to the A-site lead to high activity, whereas Mg²⁺ binding causes channel closure regardless of the state of the L-site. In the absence of ion binding to the A-site, Ca²⁺ binding to the L-site generates low activity, whereas Mg²⁺ binding leaves the channel closed. (B) The binding of either Ca²⁺ or Mg²⁺ to the open channel (O) is considered to promote channel inactivation (I). The activity of the channel depends on the number of subunits in each state (see Tables III and IV).