Store-refilling involves both L-type calcium channels and reverse-mode sodium calcium exchange in airway smooth muscle
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The aim of the present study was to examine the relative contributions to store-refilling of the two following voltage-regulated calcium ion influx pathways: 1) L-type-Ca(2+) channels; and 2) the reverse-mode of the sodium-calcium exchanger (NCX). Successive acetylcholine-induced contractions, triggered in bovine tracheal smooth muscle strips, were measured to determine the effect of intervention on contractions as an indication of the extent of store-refilling. Pre-treating the tissues with cromakalim significantly reduced the magnitude of successive contractions. Zero-Ca(2+) bathing media abolished the contractions, an effect that was completely reversed upon reintroduction of Ca(2+). Inhibition of L-type Ca(2+) channels, with nifedipine, significantly reduced the magnitude of the contractions. Similarly, inhibition of the reverse-mode of the NCX, with KB-R7943, significantly reduced the magnitude of the contractions. However, neither nifedipine nor KB-R7943 alone reduced contractions to the same extent as observed under zero-Ca(2+) conditions. Concurrent treatment with nifedipine and KB-R7943 almost abolished successive contractions. Furthermore, concurrent treatment with nifedipine and zero-Na(+) bathing media displayed a significantly greater effect than nifedipine alone. Probing the expression of NCX1 isoforms by Western blotting revealed the presence of three bands of 160, 120 and 110 kDa. The 120- and 110-kDa bands were identified as variably spliced NCX isoforms, NCX1.1 and NCX1.3, respectively. Taken together, these data suggest that influx of calcium ions through both L-type calcium channels and the reverse-mode of the sodium-calcium exchanger is necessary for complete store-refilling in airway smooth muscle.
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