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Dissociation of P 2 purinoceptor‐mediated increase in intracellular Ca 2+ level from myosin light chain phosphorylation and contraction in rat aorta
Author(s) -
Kitajima Satoshi,
Harada KenIchi,
Hori Masatoshi,
Ozaki Hiroshi,
Karaki Hideaki
Publication year - 1996
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1996.tb15436.x
Subject(s) - adenosine triphosphate , contraction (grammar) , adenosine , chemistry , muscle contraction , biophysics , phosphorylation , extracellular , myosin light chain kinase , intracellular , egta , medicine , endocrinology , calcium , biochemistry , biology , organic chemistry
1 The effects of P 2 agonists, adenosine‐5′‐triphosphate (ATP), α, β‐methylene‐adenosine‐5′‐triphosphate (α, β‐me‐ATP) and adenosine‐5‐O‐(3‐thiotriphosphate) (ATPγS), on the intracellular free Ca 2+ level ([Ca 2+ ] i ), myosin light chain (MLC) phosphorylation and force of contraction were examined in vascular smooth muscle of rat aorta. 2 ATP (0.1 μ m ‐1 mM), α, β‐me‐ATP (0.1–100 μ m ) and ATPγS (1–100 μ m ) induced transient increases followed by sustained increase in [Ca 2+ ] i . The effects of these agonists were concentration‐dependent. Compared with the effects of a high concentration of KCl (17.5–72.4 mM), the contractions induced by these P 2 purinoceptor agonists were smaller at a given [Ca 2+ ] i . 3 In the absence of extracellular Ca 2+ (with 0.5 mM EGTA), ATPγS (10 μ m ) induced large transient increase in [Ca 2+ ] i with only small contraction in Ca 2+ ‐free solution. In contrast, α, β‐me‐ATP (10 μ m ) induced only a very small increase in [Ca 2+ ] i and contraction. 4 ATP (1 mM), α, β‐me‐ATP (10 μ m ) and ATPγS (10 μ m ), added during stimulation with 0.1 μ m noradrenaline, induced additional and transient increases in [Ca 2+ ] i which were also not associated with contraction. 5 High K + (72.4 mM) increased MLC phosphorylation with a similar time course to that of the increase in [Ca 2+ ] i (peak phosphorylation was 56% when [Ca 2+ ] i increased to 100%). In contrast, the time course of the increase in MLC phosphorylation due to ATP (1 mM) did not coincide with that of the large increases in [Ca 2+ ] i ; MLC phosphorylation increased to only 31% when [Ca 2+ ] i increased to 163%. The MLC phosphorylation due to α, β‐me‐ATP (10 μ m ) and ATPγS (10 μ m ), measured at peak [Ca 2+ ] i , were only 19% and 14%, respectively, irrespective of a large increase in [Ca 2+ ] i (138% and 188%, respectively). 6 The absence of a clear relationship between P 2 ‐purinoceptor‐mediated increase in [Ca 2+ ] i (either by Ca 2+ influx or Ca 2+ release) and MLC phosphorylation or force generation appears to imply that elevation in [Ca 2+ ] i does not contribute to these responses.