CAFFEINE MODULATES CA2+ RECEPTOR ACTIVATION IN ISOLATED RAT OSTEOCLASTS AND INDUCES INTRACELLULAR CA2+ RELEASE

A ryanodine-sensitive pathway is involved in intracellular Ca2+ releas e in response to activation of the osteoclast cell surface Ca2+ recept or. We now report that the ryanodine-receptor modulator, caffeine itse lf released intracellularly stored Ca2+ and, strongly inhibited Ca2+ r elease triggered in response to Ca2+-receptor activation by Ni2+, a su rrogate cation agonist. Caffeine yielded a bell-shaped concentration-r esponse curve (0.005-2 mM) and displayed use-dependent inactivation. F urthermore, responses to caffeine were abolished on prior application of Ni2+ (5 mM). Subthreshold (0.005 mM) caffeine concentrations abolis hed Ni2+-induced elevations in the cytosolic Ca2+ concentration ([Ca2]). However, in a Ca2+-free, ethylene glycol-bis(beta-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 osteo clast.