RyR1 channels show partial coupling. Reconstituted microsomes from rabbit skeletal muscle, passing Ca²⁺ current, as in Figs. 1 and 2, from a trans compartment with ∼50 mM free [Ca²⁺]. Unlike the previous examples, these authors report a requirement for Mg²⁺ and ATP at physiological concentrations in the cis compartment for the occurrence of coupled gating. (A) As interpreted by the authors, one channel gates independently and three others join in synchrony. (B) Two independently gating channels are intermittently joined by two pairs of coupled RyRs independent of each other, contributing individual currents of ∼5 pA. (C) Authors identify 10 levels of current, from “fully coupled” channels, which in their terminology means that no channel is gating independently, but coupling is not complete or obligatory, as revealed by a CR measure (Box 1) different from 0. Fig. 3 of Porta et al. (2012) reproduced by courtesy of Julio Copello. Fig. 3 is reprinted with permission from American Physiological Society.