Llach Anna, Huang Jingbo, Sederat Franklin, Tort Lluis, Tibbits Glen, Hove-Madsen Leif
Unitat de Fisiologia Animal, Departamento de Biologia Celular, Fisiologia i Immunología, Facultat de Ciencies, Universitat Autònoma de Barcelona, 08193, Cerdanyola, Barcelona, España.
J Exp Biol. 2004 Mar;207(Pt 8):1369-77. doi: 10.1242/jeb.00884.
Long depolarizations cause a steady tonic contraction and induce sarcoplasmic reticulum (SR) Ca(2+)-uptake in trout atrial myocytes. Simultaneous measurements of cytosolic [Ca2+] ([Ca2+]i) and whole membrane current showed an elevated [Ca2+]i throughout the depolarization. Rapid caffeine (Caf) applications at -80 mV before and after a long depolarization were used to determine SR Ca2+ loading and its dependency on membrane potential and [Ca2+]i during depolarization. Following a 10 s depolarization, the maximal SR Ca2+ load was 597 micromol l(-1) and loading was half-maximal at -12 mV. The beta-adrenergic agonist isoproterenol (ISO) did not affect the maximal SR Ca2+ loading but shifted the potential for half-maximal loading by -26 mV. Following a 3 s depolarization, the maximal SR Ca2+ uptake rate (Vmax) was 418 micromol l(-1) s(-1) in control conditions. ISO did not affect Vmax, but significantly lowered the average free Ca2+ transient during the depolarization and shifted the K(0.5) for the relationship between SR Ca2+ uptake and [Ca2+]i from 1.27 in control to 0.8 micromol l(-1) with ISO. Following repetitive 200 ms depolarizations, ISO increased the L-type Ca2+ current (ICa) amplitude by 91+/-29% and the peak Ca2+ transient by 41+/-10%, and decreased the half life of the Ca2+ transient from 151+/-12 to 111+/-6 ms. Using the relationship between [Ca2+]i and SR Ca2+ uptake to calculate the total SR Ca2+ uptake during a Ca2+ transient elicited by a 200 ms depolarization, a significant increase in the SR Ca2+ uptake from 37+/-6 micromol l(-1) in control to 68+/-4 micromol l(-1) with ISO was seen. When normalized to the total Ca2+ transport the contribution of the SR was not significantly different in the absence (35+/-6%) or presence of ISO (41+/-4%). Exposure of cells to ISO and low extracellular [Ca2+] increased ICa by 67+/-40% (N=5) but significantly reduced SR Ca2+ uptake at membrane potentials above -30 mV. Together, these results suggest that (i) ISO has a stimulatory effect on the SR Ca2+ pump that may contribute to the faster decay of the Ca2+ transient, and (ii) the relative contribution of the SR to the Ca2+ removal during relaxation is not altered by ISO in trout atrial myocytes.
长时间去极化会导致虹鳟心房肌细胞出现持续性强直收缩，并诱导肌浆网（SR）摄取Ca(2+)。同时测量胞质[Ca2+]（[Ca2+]i）和全细胞膜电流发现，在整个去极化过程中[Ca2+]i升高。在长时间去极化前后于-80 mV快速施加咖啡因（Caf），以确定SR Ca2+负载及其在去极化过程中对膜电位和[Ca2+]i的依赖性。在10 s去极化后，最大SR Ca2+负载为597 μmol l(-1)，在-12 mV时负载达到半最大。β-肾上腺素能激动剂异丙肾上腺素（ISO）不影响最大SR Ca2+负载，但使半最大负载的电位向负移26 mV。在3 s去极化后，对照条件下最大SR Ca2+摄取速率（Vmax）为418 μmol l(-1) s(-1)。ISO不影响Vmax，但显著降低去极化期间平均游离Ca2+瞬变，并使SR Ca2+摄取与[Ca2+]i关系的K(0.5)从对照时的1.27移至ISO时的0.8 μmol l(-1)。在重复200 ms去极化后，ISO使L型Ca2+电流（ICa）幅度增加91±29%，使Ca2+瞬变峰值增加41±10%，并使Ca2+瞬变的半衰期从151±12 ms降至111±6 ms。利用[Ca2+]i与SR Ca2+摄取的关系计算由200 ms去极化引发的Ca2+瞬变期间SR Ca2+的总摄取量，可见SR Ca2+摄取量从对照时的37±6 μmol l(-1)显著增加至ISO时的68±4 μmol l(-1)。当以总Ca2+转运量进行归一化时，SR的贡献在不存在ISO（35±6%）或存在ISO（41±4%）时无显著差异。使细胞暴露于ISO和低细胞外[Ca2+]会使ICa增加67±40%（N = 5），但在膜电位高于-30 mV时显著降低SR Ca2+摄取。总之，这些结果表明：（i）ISO对SR Ca2+泵有刺激作用，这可能有助于Ca2+瞬变更快衰减；（ii）在虹鳟心房肌细胞中，ISO不会改变SR在舒张期对Ca2+清除的相对贡献。
