Caffeine modulates Ca²⁺ receptor activation in isolated rat osteoclasts and induces intracellular Ca²⁺ release

A ryanodine-sensitive pathway is involved in intracellular Ca²⁺ release in response to activation of the osteoclast cell surface Ca²⁺ receptor. We now report that the ryanodine-receptor modulator, caffeine itself released intracellularly stored Ca²⁺ and, strongly inhibited Ca²⁺ release triggered in response to Ca²⁺-receptor activation by Ni²⁺, a surrogate cation agonist. Caffeine yielded a bell-shaped concentration-response curve (0.005-2 mM) and displayed use-dependent inactivation. Furthermore, responses to caffeine were abolished on prior application of Ni²⁺ (5 mM). Subthreshold (0.005 mM) caffeine concentrations abolished Ni²⁺-induced elevations in the cytosolic Ca²⁺ concentration ([Ca²⁺]). However, in a Ca²⁺-free, ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid-containing solution (extracellular [Ca²⁺] < 10 nM), caffeine (0.5 mM) neither elevated [Ca²⁺] nor inhibited the response to Ni²⁺. Finally, when caffeine was applied to intercept the plateau phase of the cytosolic Ca²⁺ signal triggered by extracellular Ca²⁺ elevation (10 mM), a rapid but reversible inactivation followed. These studies strongly indicate the existence of a caffeine-sensitive mechanism for the release of intracellularly stored Ca²⁺ in the osteoclast.