Bell-shaped calcium-response curves of lns(l,4,5)P₃- and calcium-gated channels from endoplasmic reticulum of cerebellum

RELEASE of calcium from intracellular stores occurs by two pathways, an inositol 1,4,5-trisphosphate (InsP₃)-gated channel^1-3 and a calcium-gated channel (ryanodine receptor)^4-6. Using specific antibodies, both receptors were found in Purkinje cells of cerebellum^7,8. We have now compared the functional properties of the channels corresponding to the two receptors by incorporating endoplasmic reticulum vesicles from canine cerebellum into planar bilayers. InsP₃-gated channels were observed most frequently. Another channel type was activated by adenine nucleotides or caffeine, inhibited by ruthenium red, and modified by ryanodine, characteristics of the ryanodine receptor/channel⁶. The open probability of both channel types displayed a bell-shaped curve for dependence on calcium. For the InsP₃-gated channel, the maximum probability of opening occurred at 0.2 μM free calcium, with sharp decreases on either side of the maximum. Maximum activity for the ryanodine receptor/channel was maintained between 1 and 100 μM calcium. Thus, within the physiological range of cytoplasmic calcium, the InsP₃-gated channel itself allows positive feed-back and then negative feedback for calcium release, whereas the ryanodine receptor/channel behaves solely as a calcium-activated channel. The existence in the same cell of two channels with different responses to calcium and different ligand sensitivities provides a basis for complex patterns of intracellular calcium regulation.