Involvement of reduced sensitivity to Ca in beta-adrenergic action on airway smooth muscle.
BACKGROUND
It is well known that beta-adrenoceptor agonists (beta-agonists) cause relaxation in airway smooth muscle mediated by a reduction in the concentration of intracellular Ca2+ ([Ca2+](i)). However, little is currently known regarding whether reduced sensitization to Ca2+ is involved in the beta-adrenergic relaxation.


OBJECTIVE
This study was designed to determine the intracellular mechanisms underlying suppression of Ca2+ sensitization in beta-adrenergic relaxation (Ca(2+)-independent relaxation by beta-agonists). Methods Isometric tension and [Ca2+](i) were simultaneously measured in fura-2-loaded strips isolated from guinea-pig tracheal smooth muscles. The relationships between tension and [Ca2+](i) were examined in the inhibitory action of isoprenaline (ISO) and other cAMP-related agents against methacholine-induced contraction.


RESULTS
The concentration-inhibition curve for ISO against methacholine in tension was significantly dissociated from the curve for ISO in [Ca2+](i). In ISO-induced relaxation, a reduction in tension was significantly greater than that in [Ca2+](i.) This phenomenon was mimicked by other cAMP-related agents: forskolin and dibutyryl-cAMP. In contrast, the inhibitory action of SKF-96365, a non-selective inhibitor of Ca(2+) channels, was associated with that in [Ca2+](i). In the presence of Rp-cAMPS, an inhibitor of protein kinase A (PKA), ISO caused an equivalent relaxation with less reduction in [Ca2+](i). The effects of ISO were not affected by Y-27632, an inhibitor of Rho-kinase, or by bisindolylmaleimide, an inhibitor of protein kinase C. ISO failed to inhibit contraction elicited by calyculin A, an inhibitor of myosin phosphatase. Conclusion beta-Adrenergic action antagonizes not only Ca2+ mobilization but also Ca2+ sensitization in methacholine-induced contraction. The cAMP/PKA-independent, G(s)-direct action is more potent in Ca(2+)-independent relaxation by beta-agonists than the cAMP/PKA-dependent pathway. Moreover, myosin phosphatase is a fundamentally affected protein in the reduced response to Ca2+ mediated by beta-agonist. Our results may provide evidence that this Ca2+ desensitization is a novel target for a reliever medication using rapid-acting beta-agonists in acute asthma management.
Oguma
T
T
Department of Respiratory Medicine, Nagoya University Graduate School of Medicine, Nagoya, Japan.
Kume
H
H
Ito
S
S
Takeda
N
N
Honjo
H
H
Kodama
I
I
Shimokata
K
K
Kamiya
K
K
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000577:Amides / Q000494:pharmacology; D000818:Animals; D001249:Asthma / Q000188:drug therapy; D003994:Bucladesine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D002373:Cyclic AMP Receptor Protein / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D003864:Depression, Chemical; D016257:Fura-2; D006168:Guinea Pigs; D007093:Imidazoles / Q000494:pharmacology; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D047908:Intracellular Signaling Peptides and Proteins; D007545:Isoproterenol / Q000494:pharmacology*; D008297:Male; D008301:Maleimides / Q000494:pharmacology; D008387:Marine Toxins; D016210:Methacholine Chloride / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects* / Q000378:metabolism; D043263:Myosin-Light-Chain Phosphatase / Q000037:antagonists & inhibitors; D010080:Oxazoles / Q000494:pharmacology; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors; D011725:Pyridines / Q000494:pharmacology; D014132:Trachea; D054460:rho-Associated Kinases
D000318:Adrenergic beta-Agonists; D000577:Amides; D002121:Calcium Channel Blockers; D002373:Cyclic AMP Receptor Protein; D007093:Imidazoles; D007211:Indoles; D047908:Intracellular Signaling Peptides and Proteins; D008301:Maleimides; D008387:Marine Toxins; D010080:Oxazoles; D011725:Pyridines; D016210:Methacholine Chloride; C108830:Y 27632; D005576:Colforsin; D003994:Bucladesine; C059041:calyculin A; D017346:Protein Serine-Threonine Kinases; D054460:rho-Associated Kinases; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D043263:Myosin-Light-Chain Phosphatase; C063159:1-(2-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenylethyl)-1H-imidazole; D007545:Isoproterenol; C088060:bisindolylmaleimide; D002118:Calcium; D016257:Fura-2