Involvement of Ca2+ mobilization in tachyphylaxis to beta-adrenergic receptors in trachealis.
We examined the mechanisms underlying tachyphylaxis to beta-adrenergic receptor agonists (beta-agonists) in tracheal smooth muscle. Simultaneous measurements of isometric tension and intracellular Ca2+ concentration ([Ca2+]i) using fura-2-loaded guinea pig tracheas showed that the inhibitory effects of isoproterenol (ISO) on tension and increases in [Ca2+]i induced by methacholine exhibited marked tachyphylaxis with repeated exposure to ISO at intervals of 15 min. Similarly, the activation of single Ca2+-activated K+ (KCa) channels in on-cell patches by 1 microM ISO was gradually attenuated after repeated extracellular application of ISO to single smooth cells of porcine tracheas. Desensitization of beta-adrenergic receptor/KCa channel stimulatory coupling and relaxation responses was prevented by separately antagonizing the voltage-dependent Ca2+ channel (VDCC) with verapamil, suggesting a surprising relationship between Ca2+ influx through VDCC and beta-adrenergic desensitization. Conversely, repeated exposure of 10 U/ml protein kinase A to inside-out patches did not result in desensitization of channel activation, and repeated exposure to 10 microM forskolin modestly augmented the inhibitory effects of forskolin on tension and [Ca2+]i by methacholine, indicating that the mechanism of desensitization is mediated by the beta-adrenergic receptor/G protein complex. These results indicate that an uncoupling of beta-adrenergic receptor from KCa channels augments Ca2+ mobilization through VDCC and stimulates tachyphylaxis.
Kume
Hiroaki
H
Division of Respiratory Medicine, Department of Medicine, Nagoya University Graduate School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya 466-8550, Japan. hkume@med.nagoya-u.ac.jp
Ishikawa
Takayuki
T
Oguma
Tetsuya
T
Ito
Satoru
S
Shimokata
Kaoru
K
Kotlikoff
Michael I
MI
D000262:Adenylyl Cyclases / Q000378:metabolism; D000704:Analysis of Variance; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D005576:Colforsin / Q000378:metabolism / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D005456:Fluorescent Dyes / Q000494:pharmacology; D016257:Fura-2 / Q000494:pharmacology; D006168:Guinea Pigs; D008297:Male; D016210:Methacholine Chloride / Q000378:metabolism / Q000494:pharmacology; D009130:Muscle, Smooth / Q000473:pathology; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000378:metabolism; D011485:Protein Binding; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*; D013552:Swine; D013618:Tachyphylaxis; D013696:Temperature; D013997:Time Factors; D014132:Trachea / Q000276:immunology / Q000473:pathology*; D014700:Verapamil / Q000494:pharmacology
D005456:Fluorescent Dyes; D015221:Potassium Channels; D011943:Receptors, Adrenergic, beta; D016210:Methacholine Chloride; D005576:Colforsin; D014700:Verapamil; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D000262:Adenylyl Cyclases; D011188:Potassium; D002118:Calcium; D016257:Fura-2