STIMULATION BY POLYOLS OF THE 2 RYANODINE RECEPTOR ISOFORMS OF FROG SKELETAL-MUSCLE

While the stimulating effect of concentrated salts on ryanodine recept or (RyR) is widely accepted in Ca2+-induced Ca2+ release (CICR) and [H -3]ryanodine binding, the effect of non-ionic solutes on RyR is contro versial. We investigated the effects of polyols on [H-3]ryanodine bind ing to alpha- and beta-RyR purified from bullfrog skeletal muscle, and on CICR from sarcoplasmic reticulum (SR) in a skinned frog skeletal m uscle fibre. Addition of polyols (glucose, sucrose, sorbitol, glycerol and ethylene glycol) in submolar to molar concentrations to an isoton ic salt medium increased dose-dependently Ca2+-activated [H-3]ryanodin e binding to alpha- and beta-RyR of a similar magnitude. The increase is due to the rise in both apparent affinity (1/K-D) and maximal numbe rs of binding sites (B-max) for ryanodine. In addition to this stimula ting effect, glucose sensitized both isoforms to Ca2+ in the Ca2+-acti vated reaction, which is distinct in mechanism(s) from caffeine. These stimulating effects of polyols were not observed unless some NaCl was present, which might explain the discrepancy among reported results. Consistent with these findings, polyols reversibly enhanced the rate o f CICR from SR in skinned fibres with an increase in the Ca2+ sensitiv ity. The enhanced CICR was still sensitive to well-known modulators fo r CICR (Ca2+, Mg2+, adenine nucleotides and procaine), as with [H-3]ry anodine binding. The results of this study reveal that polyols stimula te alpha- and beta-RyR in frog skeletal muscle, bringing about increas ed CICR activity. The finding that the specific activity of polyols in stimulation of [H-3]ryanodine binding was approximately proportional to their molecular weights leads us to discuss the possible modificati on of protein surface-water molecule interaction as an underlying mech anism.