Effects of BKCa channels on the reduction of cytosolic Ca2+ in cGMP-induced relaxation of guinea-pig trachea.
1. In order to examine the mechanisms of cGMP-induced relaxation in airway smooth muscle, the effects of atrial natriuretic peptide (ANP) and 8-brom cGMP on muscle tone were studied by measuring isometric tension, while the effects on cytosolic Ca2+ concentrations were studied by measuring the spectra of fura-2 loaded in guinea-pig tracheal strips. 2. Atrial natriuretic peptide and 8-brom cGMP caused a concentration-dependent inhibition of spontaneous tone in the guinea-pig trachea. The relaxant effects of these agents on spontaneous tone were markedly suppressed in the presence of iberiotoxin (IbTX), a selective inhibitor of large-conductance Ca2(+)-activated K+ (BKCa) channels. Iberiotoxin (30 nmol/L) markedly affected the maximal effect induced by ANP and 8-brom cGMP and augmented EC70 values for ANP and EC50 values for 8-brom cGMP approximately 27- and 17-fold, respectively. The inhibitory effects of IbTX on relaxation induced by these agents were diminished in the presence of 1 mumol/L nifedipine, an antagonist of voltage-operated Ca2+ channels (VOCC). 3. The inhibitory action of ANP and 8-brom cGMP on spontaneous tone was not affected by the presence of 10 mumol/L glibenclamide, an inhibitor of ATP-sensitive K+ channels, and 100 nmol/L apamin, an inhibitor of small-conductance Ca2(+)-activated K+ channels. When these agents were applied to tissues precontracted by high (40 mmol/L) K+, the relaxant effects of these agents markedly diminished. 4. The extracellular Ca2(+)-dependent contraction was inhibited in the presence of 0.3 mumol/L ANP or 0.1 mmol/L 8-brom cGMP. Concentration-response curves to extracellular Ca2+ (0.03-2.4 mmol/L) were markedly diminished by exposure to these agents. The maximal effect induced by extracellular Ca2+ was affected by these agents. 5. Atrial natriuretic peptide caused an inhibition of spontaneous tone accompanied by a reduction in the intracellular Ca2+ concentration. In the presence of IbTX, the elimination of both muscle tone and cytosolic Ca2+ by ANP was suppressed. 6. We conclude that ANP and 8-brom cGMP activate BKCa channels and that the inhibition of Ca2+ influx through VOCC, mediated by BKCa channel activation, may be involved in cGMP-dependent bronchodilation.
Mikawa
K
K
Second Department of Internal Medicine, Nagoya University School of Medicine, Japan.
Kume
H
H
Takagi
K
K
D000818:Animals; D001030:Apamin / Q000494:pharmacology; D009320:Atrial Natriuretic Factor / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D006152:Cyclic GMP / Q000031:analogs & derivatives / Q000494:pharmacology*; D003600:Cytosol / Q000378:metabolism*; D005110:Extracellular Space / Q000187:drug effects / Q000502:physiology; D005905:Glyburide / Q000494:pharmacology; D006168:Guinea Pigs; D007424:Intracellular Fluid / Q000378:metabolism; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009126:Muscle Relaxation / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects*; D009543:Nifedipine / Q000494:pharmacology; D010455:Peptides / Q000494:pharmacology; D011188:Potassium / Q000502:physiology; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000502:physiology*; D012604:Scorpion Venoms / Q000494:pharmacology; D014132:Trachea / Q000378:metabolism*
D010455:Peptides; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D012604:Scorpion Venoms; D001030:Apamin; C016276:8-bromocyclic GMP; C064719:iberiotoxin; D009320:Atrial Natriuretic Factor; D006152:Cyclic GMP; D009543:Nifedipine; D011188:Potassium; D005905:Glyburide; D002118:Calcium