Ca2 release from skeletal muscle sarcoplasmic reticulum is activated by adenine nucleotides and suramin. Because suramin is known to interact with ATP-binding enzymes and ATP receptors(P2-purinergic receptors), the stimulation by suramin has been postulated to occur via the adenine nudeotide-binding site of the ryanodine receptor/Ca2-release channel. We tested this hypothesis using suramin and the following suramin analogs: NF037, NFO18, NF023, and NFOO7. The suramin analogs stimulate the binding of [H] ryanodine binding to sarcoplasmic reticulum membranes with the follow-ing rank order of potency: suramin(EC50=-60 tM)> NF037 (EC50=.-1 50 pM)> NFO1 8> NF023> NFOO7. The suramininduced stimulation occurs via a myoplasmic binding site on the ryanodine receptor as confirmed by binding experiments and single-channel recordings with the purified protein. This binding site is different than that for ATP, a conclusion that is supported by the following observations:(i) Suramin stimulates the association rate and inhibits the dissociation rate of [3H] ry-anodine, whereas ATP analogs increase only the on-rate.(ii) In the presence of suramin but not of ATP analogs,[3H] ryanodine binding is resistant to the inhibitory effect of millimolar Mg2 and Ca2.(iii) ATP analogs and suramin have an additive effect on [H] ryanodine binding.(iv) Affinity labeling of the purified ryanodine receptor with 2’, 3’-dialdehyde[a-32P] ATP or after in situ oxidation of [y-32P] ATP is not affected by suramin. Thus, our results show that suramin acts as a direct and potent stimulator of the ryanodine receptor but that this action is mediated via a binding site different from that for adenine nucleotides.

Cytoplasmic Ca2 levels are tightly regulated by uptake into and release from intracellular stores as well as by fluxes across the plasma membrane. Signals generated at the plasma membrane are linked to the rapid efflux ofCa2 from the intracellular stores by two types of release channels: the inositol trisphosphate receptors and the ryanodine receptors. The latter exist in three distinct isoforms, termed the skeletal muscle isoform (type 1), cardiac muscle isoform (type 2), and a ubiquitous isoform (type 3)(1). The regulation of the skeletal muscle ryanodine receptor/Ca2trelease channel is understood in great detail; the receptor resides on the termi-nal cisternae of the sarcoplasmic reticulum and is intricately linked to the voltage-sensitive L-type Ca2 channel of the sarcolemmal T-tubular system (2). Although this specialized topology seems to include functional linkage, little is known