THE OPENING OF THE INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE CA2+ CHANNEL IN RAT CEREBELLUM IS INHIBITED BY CAFFEINE

Ins(1,4,5)P3(InsP3)-induced Ca2+ release and [H-3]InsP3 binding were measured in rat cerebellar microsomes in the presence or absence of caffeine. The quantal Ca2+ release was shown to occur in an apparently co-operative fashion with a Hill coefficient (h) of 2.2. Half-maximal Ca2+ release was observed at 900 nM-InsP3. Addition of caffeine caused changes both to the concentration of InsP3 required to cause half-maximal Ca2+ release (3.9-mu-M at 50 mM-caffeine) and to the apparent co-operativity (h = 1.0 at 50 mM-caffeine). Under standard conditions for [H-3]InsP3 binding, caffeine had no effect, and it had no effect on InsP3 metabolism. Cyclic AMP also had no effect on the quantal release induced by InsP3. These results are consistent with the view that caffeine affects the opening (Ca2+ release) events rather than the ligand-binding events in the operation of the InsP3-sensitive Ca2+ channel.