Regulation of skeletal muscle Ca2+ release channel (ryanodine receptor) by Ca2+ and monovalent cations and anions

Abstract

The effects of ionic composition and strength on rabbit skeletal muscle Ca2+ release channel (ryanodine receptor) activity were investigated in vesicle-45Ca2+ flux, single channel and [3H[ryanodine binding measurements. In <0.01 μM Ca2+ media, the highest 45Ca2+ efflux rate was measured in 0.25 M choline-Cl medium followed by 0.25 M KCl, choline 4- morpholineethanesulfonic acid (Mesh potassium 1,4-piperazinediethanesulfonic acid (Pipes), and K-Mes medium. In all five media, the 45Ca2+ efflux rates were increased when the free [Ca2+] was raised from <0.01 μM to 20 μM and decreased as the free [Ca2+] was further increased to 1 mM. An increase in [KCl] augmented Ca2+-gated single channel activity and [3H]ryanodine binding. In [3H]ryanodine binding measurements, bell-shaped Ca2+ activation/inactivation curves were obtained in media containing different monovalent cations (Li+, Na+, K+, Cs+, and choline+) and anions (Cl-, Mes-, and Pipes-). In choline-Cl medium, substantial levels of [3H]ryanodine binding were observed at [Ca2+] <0.01 μM. Replacement of Cl- by Mes- or Pipes- reduced [3H]ryanodine binding levels at all [Ca2+]. In all media, the Ca2+-dependence of [3H]ryanodine binding could be well described assuming that the skeletal muscle ryanodine receptor possesses cooperatively interacting high-affinity Ca2+ activation and low- affinity Ca2+ inactivation sites. AMP primarily affected [3H]ryanodine binding by decreasing the apparent affinity of the Ca2+ inactivation site(s) for Ca2+, while caffeine increased the apparent affinity of the Ca2+ activation site for Ca2+. Competition studies indicated that ionic composition affected Ca2+-dependent receptor activity by at least three different mechanisms: (i) competitive binding of Mg2+ and monovalent cations to the Ca2+ activation sites, (ii) binding of divalent cations to the Ca2+ inactivation sites, and (iii) binding of anions to specific anion regulatory sites.