MODULATION OF CA2-GATED CARDIAC-MUSCLE CA2+-RELEASE CHANNEL (RYANODINE RECEPTOR) BY MONOVALENT AND DIVALENT IONS()

The effects of mono- and divalent ions on Ca2+-gated cardiac muscle Ca 2+-release channel (ryanodine receptor) activity were examined in [H-3 ]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 affinit ies of Ca2+ inactivation sites were lower in choline chloride and CsCl media than in LiCl, NaCl, and KCI media. Sr2+ activated the ryanodine receptor with a lower efficacy than Ca2+. Competition studies indicat ed that Li+, K+, Mg2+, and Ba2+ compete with Ca2+ for Ca2+ activation sites. In 0.125 M KC1 medium, the Ca2+ dependence of [H-3]ryanodine bi nding 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 apparen t 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 I) com petitive 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.