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. 1999 Jun 1;113(6):837–849. doi: 10.1085/jgp.113.6.837

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Micromolar Ca inhibits InsP₃ binding to cerebellar membranes by increasing the K _d for InsP₃. (A) InsP₃ binding decreased 66% when the Ca concentration was increased from 0 to 100 μM (solid bars), and this inhibition was fully reversible upon chelation of Ca by EGTA (hatched bars). The InsP₃ concentration was 10 nM. One of three similar experiments. (B) Scatchard analysis indicated that Ca decreased InsP₃ binding affinity without a change in maximal binding. Concentrations of Ca tested were 0.0004 (•), 0.1 (▵), 0.5 (▪), and 10 (▿) μM. Note that complete inhibition of InsP₃ binding was not observed. (C) The K _d values from B plotted as a function of the Ca concentration for two experiments (• and ○). The Ca-dependent increase in the K _d for InsP₃ binding increased 3.5-fold over the Ca concentration range tested with half maximal increase at 0.3 μM Ca (data were fit using a Hill equation with a coefficient n = 1.5).