Abstract
Ca2+-activated maxi K+ channels were studied in inside-out patches from smooth muscle cells isolated from either porcine coronary arteries or guinea-pig urinary bladder. As described by Groschner et al. (Pflügers Arch 417:517, 1990), channel activity (NP o) was stimulated by 3 μM [Ca2+]c (1 mM Ca-EGTA adjusted to a calculated pCa of 5.5) and was suppressed by the addition of 1 mM Na2ATP. The following results suggest that suppression of NP o by Na2ATP is due to Ca2+ chelation and hence reduction of [Ca2+]c and reduced Ca2+ activation of the channel. The effect was absent when Mg ATP was used instead of Na2ATP. The effect was diminished by increasing the [EGTA] from 1 to 10 mM. The effect was absent when [Ca2+]c was buffered with 10 mM HDTA (apparent pK Ca 5.58) instead of EGTA (pK Ca 6.8). A Ca2+-sensitive electrode system indicated that 1 mM Na2ATP reduced [Ca2+]c in 1 mM Ca-EGTA from 3 μM to 1.4 μM. Na2ATP, Na2GTP, Li4AMP-PNP and NaADP reduced measured [Ca2+]c in parallel with their suppression of NP o. After the Na2ATP-induced reduction of [Ca2+]c was re-adjusted by adding either CaCl2 or MgCl2, the effect of Na2ATP on NP o disappeared. In vivo, intracellular [Mg2+] exceeds free [ATP4−], hence ATP modulation of maxi K+ channels due to Ca2+ chelation is without biological relevance.
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Klöckner, U., Isenberg, G. ATP suppresses activity of Ca2+ -activated K+ channels by Ca2+ chelation. Pflugers Arch. 420, 101–105 (1992). https://doi.org/10.1007/BF00378648
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DOI: https://doi.org/10.1007/BF00378648