Short- and long-term functional alterations of the skeletal muscle calciumrelease channel (ryanodine receptor) by suramin: Apparent dissociation of single channel current recording and [H-3]ryanodine binding

The present study demonstrates the following characteristic suramin actions on the purified skeletal muscle calcium release channel in single-channel current recordings and [H-3] ryanodine binding to HSR: 1) Suramin (0.3-0.9 mM) induced a concentration-dependent increase in the open probability (P-o congruent to 0.9) at 20 to 100 muM Ca2+ and an almost fully open channel a t 1 mM Ca2+ (P-o = 0.95) with a marked shift to longer open states (tau (o) 3/tau (o)4). Suramin increased the apparent calcium affinity to the activat ing 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 muM Ca-calmodulin. 4) The open probability of the suramin activated chan nel was almost completely inhibited by 10 mM Mg2+ or Ca2+ on short suramin exposure. Prolonged suramin exposure (30-60 min) resulted in a time-depende nt, slow increase in P-o, with long open states of low frequency in the pre sence of 10 to 20 mM Mg2+ or Ca2+. 5) Magnesium induced inhibition of P-o ( IC50 = 0.38 mM) and equilibrium [H-3] ryanodine binding (IC50 = 0.30 mM) ag reed 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). [H-3] ryanodine binding se emed to monitor predominantly the long-term alteration in channel function. 6) The multiple effects of suramin on channel function suggest an alloster ic mechanism and no direct effects on binding of endogenous ligands involve d in channel gating.