Role of Mg2+ in Ca2+-induced Ca2+ release through ryanodine receptors of frog skeletal muscle:: Modulations by adenine nucleotides and caffeine
Role of Mg2+ in Ca2+-induced Ca2+ release through ryanodine receptors of frog skeletal muscle:: Modulations by adenine nucleotides and caffeine
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DOI：
10.1016/s0006-3495(00)76731-2
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发表时间：
2000-04-01
影响因子：
3.4
通讯作者：
Ogawa, Y
中科院分区：
文献类型：
--
作者：
Murayama, T;Kurebayashi, N;Ogawa, Y
MG(2+) 在兰尼定受体 (RyR) 的高亲和力 Ca2+ 激活位点（A 位点）中充当 Ca2+ 的竞争性拮抗剂，并在低亲和力 Ca2+ 失活位点（I 位点）中充当 Ca2+ 的激动剂，介导 Ca2+ 诱导的 Ca2+ 释放 (CICR)。本文通过测量 [H-3]ryanodine 与纯化的 α- 和 β-RyR 的结合以及带皮纤维中的 CICR 活性，定量测定青蛙骨骼肌中 RyR 的 A-和 I-位点对 Ca2+ 和 Mg2+ 的亲和力。 α-RyR 和 β-RyR 之间的亲和力至多仅存在微小差异。 A 位点对 Ca2+ 的偏好程度是 Mg2+ 的 20 至 30 倍，而 I 位点在两种阳离子之间没有选择性。原位 RyR 对两个位点的 Ca2+ 和 Mg2+ 的亲和力比纯化的 α- 和 β-RyRs 高出五倍，而阳离子选择性未改变。腺嘌呤核苷酸的刺激作用被发现在游离形式和复合形式之间无法区分，除了增加 RyR 的最大活性外，它不会改变任一位点对阳离子的亲和力。咖啡因不仅增加了 A 位点对 Ca2+ 的亲和力，而且还增加了 RyR 的最大活性，而其他方面的变化很小。这里提出的结果表明，即使 Mg2+ 抑制消失，青蛙骨骼肌中的 CICR 率似乎太低，无法解释生理性 Ca2+ 释放。
MG(2+) serves as a competitive antagonist against Ca2+ in the high-affinity Ca2+ activation site (A-site) and as an agonist of Ca2+ in the low-affinity Ca2+ inactivation site (I-site) of the ryanodine receptor (RyR), which mediates Ca2+-induced Ca2+ release (CICR). This paper presents the quantitative determination of the affinities for Ca2+ and Mg2+ of A- and I-sites of RyR in frog skeletal muscles by measuring [H-3]ryanodine binding to purified alpha- and beta-RyRs and CICR activity in skinned fibers. There was only a minor difference in affinity at most between alpha- and beta-RyRs. The A-site favored Ca2+ 20-to 30-fold over Mg2+, whereas the I-site was nonselective between the two cations. The RyR in situ showed fivefold higher affinities for Ca2+ and Mg2+ of both sites than the purified alpha- and beta-RyRs with unchanged cation selectivity. Adenine nucleotides, whose stimulating effect was found to be indistinguishable between free and complexed forms, did not alter the affinities for cations in either site, except for the increased maximum activity of RyR. Caffeine increased not only the affinity of the A-site for Ca2+ alone, but also the maximum activity of RyR with otherwise minor changes. The results presented here suggest that the rate of CICR in frog skeletal muscles appears to be too low to explain the physiological Ca2+ release, even though Mg2+ inhibition disappears.