Abstract

The present study demonstrates the following characteristic suramin actions on the purified skeletal muscle calcium release channel in single-channel current recordings and [³H]ryanodine binding to HSR: 1) Suramin (0.3–0.9 mM) induced a concentration-dependent increase in the open probability (P _o ≅ 0.9) at 20 to 100 μM Ca²⁺ and an almost fully open channel at 1 mM Ca²⁺ (P _o = 0.95) with a marked shift to longer open states (τ_o3/τ_o4). Suramin increased the apparent calcium affinity to the activating high-affinity calcium binding sites and reduced the apparent magnesium affinity to the inhibitory low affinity Ca²⁺/Mg²⁺ binding sites. 2) Channel activation by suramin and sulfhydryl oxidation was additive. 3) Suramin (0.9 mM) reversed the Ca-calmodulin–induced channel inhibition at 0.1 or 1 to 5 μM Ca-calmodulin. 4) The open probability of the suramin activated channel was almost completely inhibited by 10 mM Mg²⁺ or Ca²⁺ on short suramin exposure. Prolonged suramin exposure (30–60 min) resulted in a time-dependent, slow increase in P _o, with long open states of low frequency in the presence of 10 to 20 mM Mg²⁺ or Ca²⁺. 5) Magnesium induced inhibition ofP _o (IC₅₀ = 0.38 mM) and equilibrium [³H]ryanodine binding (IC₅₀= 0.30 mM) agreed well in control channels, but were dissociated in the presence of 0.9 to 1.0 mM suramin (IC₅₀ = 0.82 mM versus 83 mM). [³H]ryanodine binding seemed to monitor predominantly the long-term alteration in channel function. 6) The multiple effects of suramin on channel function suggest an allosteric mechanism and no direct effects on binding of endogenous ligands involved in channel gating.