Ryanodine receptors (RyRs) of skeletal muscle, as calcium release channels, have been found to form semicrystalline arrays in the membrane of sarcoplasmic reticulum. Recently, both experimental observations and theoretical simulations suggested cooperative coupling within interlocking RyRs. To better understand the interactions between RyRs and their modulation, the aggregation and dissociation of isolated RyRs in aqueous medium containing various Na[sup +] and K[sup +] concentrations were investigated using photon correlation spectroscopy (PCS) and atomic force microscopy (AFM). RyRs aggregated readily at low salt concentrations. However, a different behavior was observed in the presence of Na[sup +] or K[sup +]. Detectable aggregates were formed in 5 µg/mL RyR sample when the concentration of Na[sup +] and K[sup +] was reduced from 1 M to below 0.28 and 0.23 M, respectively. The dissociation of RyR aggregates was also examined when raising the salt concentration. While aggregates formed in 0.15 M NaCl medium could reverse almost completely, those formed in 0.15 M KCl medium only dissolved partly. When keeping the total salt concentration at 0.15 M, the aggregation and dissociation of RyRs were seen to evidently depend on the relative concentration of Na[sup +] and K[sup +]. The interaction between RyRs was strengthened with increasing Na[sup +]/K[sup +] ratios in the mixed medium. Accompanying this, a decrease of [³H]ryanodine binding occurred. The results obtained with PCS and AFM provide further evidence for the interaction between RyRs and suggest the importance of Na[sup +], K[sup +], and their relative composition in modulating the interaction and cooperation between RyRs in vivo. [ABSTRACT FROM AUTHOR]