Modulation of Ca(2+)-gated cardiac muscle Ca(2+)-release channel (ryanodine receptor) by mono- and divalent ions

The effects of mono- and divalent ions on Ca(2+)-gated cardiac muscle Ca(2+)-release channel (ryanodine receptor) activity were examined in [3H]ryanodine-binding measurements. Ca2+ bound with the highest apparent affinity to Ca2+ activation sites in choline chloride medium, followed by KCl, CsCl, NaCl, and LiCl media. The apparent Ca2+ binding affinities of Ca2+ inactivation sites were lower in choline chloride and CsCl media than in LiCl, NaCl, and KCl media. Sr2+ activated the ryanodine receptor with a lower efficacy than Ca2+. Competition studies indicated that Li+, K+, Mg2+, and Ba2+ compete with Ca2+ for Ca2+ activation sites. In 0.125 M KCl medium, the Ca2+ dependence of [3H]ryanodine binding was modified by 5 mM Mg2+ and 5 mM beta,gamma-methyleneadenosine 5'-triphosphate (a nonhydrolyzable ATP analog). The addition of 5 mM glutathione was without appreciable effect. Substitution of Cl- by 2-(N-morpholino)ethanesulfonic acid ion caused an increase in the apparent Ca2+ affinity of the Ca2+ inactivation sites, whereas an increase in KCl concentration had the opposite effect. These results suggest that cardiac muscle ryanodine receptor activity may be regulated by 1) competitive binding of mono- and divalent cations to Ca2+ activation sites, 2) binding of monovalent cations to Ca2+ inactivation sites, and 3) binding of anions to anion regulatory sites.