A memory for extracellular Ca2+ by speeding recovery of P2X receptors from desensitization - PubMed (original) (raw)

Increased rate of recovery from desensitization creates the Ca2+ memory. Facilitation is undiminished for 1 min (A) or 4 min (B) after removal of Ca2+. Data points (boxes ± SEM) were obtained using a protocol similar to that of the middle traces in Figure3_A_. ATP was applied at either 2 min (A) or 6 min (B) intervals. Ca2+ was increased from 1 to 10 m

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at either the beginning of the interval (▪) or the end (■) for either 1 min (A) or 2 min (B). Early and late Ca2+ treatments gave identical currents in each case. Solid lines are the recovery curves from Figure 4_C_ (see below) for 1 m

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Ca2+ (τ = 7.1 min) or 10 m

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Ca2+ (τ = 3.5 min). _Dashed curves_are the theoretical predictions (see Materials and Methods for calculation), assuming that the recovery rate changes immediately after changes in Ca2+ concentration (heavy arrows). This mechanism predicts the perfect persistence of facilitation between the Ca2+ applications. Data from each cell were normalized to the value obtained with 1 m

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Ca2+ (asterisks); vertical scales are identical to that in Figure 1_C. C_, Ca2+ and Gd3+ speed recovery from desensitization. ATP (30 μ

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) was applied at the indicated times after current was fully desensitized at time 0. During the interval between ATP applications, extracellular solution contained either 1 m

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Ca2+ (○,n = 8), 10 m

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Ca2+(•, n = 8), or 10 μ

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Gd3+ and 1 m

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Ca2+(▪, n = 6). In all cases, the solution was changed to 1 m

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Ca2+ 10 sec before the ATP application. Peak currents from each cell were normalized relative to that recorded with a 4 min interval in 10 m

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Ca2+ (asterisk). Error is expressed as SEM. The non-normalized currents ranged from −1.5 to −9.8 nA; mean = −4.6 ± 0.6 nA (4 min interval and 10 m

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extracellular Ca2+). Curves representa fit to the equation _I_t =I_max × [1 − exp(−_t/τ)]2. Recovery time constant τ = 7.1, 3.5, and 1.7 min for 1 m

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Ca2+, 10 m

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Ca2+, and 10 μ

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Gd3+, respectively. _Dashed line_represents predicted 100% recovery (_I_max = 1.96 corresponding to a mean maximum current = −9.0 nA).