Myosin phosphorylation and calcium in tonic and phasic contractions of colonic smooth muscle.
The time dependence of lightly loaded shortening velocity, myosin phosphorylation, and changes in myoplasmic Ca2+ concentration ([Ca2+]i) were measured during tonic and phasic contractions of circular smooth muscle from the proximal colon of the dog. Shortening velocity was measured by quick release to a 10% afterload. Myosin phosphorylation was measured by an immunoblot method, and changes in [Ca2+]i were estimated by measuring fluorescence intensity at 550 nm in muscle strips loaded with fluo-3. During tonic contractions induced by 60 mM K+, phosphorylation increased monotonically from 0.11 +/- 0.011 to 0.29 +/- 0.015 mol Pi/mol light chain at 10 min. In contrast, lightly loaded shortening velocity increased rapidly within 10 s to 0.042 +/- 0.003 lengths/s and decreased exponentially to 0.013 +/- 0.001 lengths/s at 15 min. During transient contractions induced by 100 microM acetylcholine, phosphorylation increased from 0.16 +/- 0.03 to 0.30 +/- 0.06 mol Pi/mol light chain at 19 s. In contrast, shortening velocity increased to 0.068 +/- 0.015 lengths/s within 2.4 s and decreased significantly to 0.027 +/- 0.009 lengths/s at 22 s. Fluo-3 fluorescence increased in parallel with force during both tonic and transient contractions. In a smooth muscle that is able to contract both tonically and phasically we observed transient increases in shortening velocity without concurrent phosphorylation or [Ca2+]i transients. Therefore, there are factors in addition to myosin phosphorylation or changes in [Ca2+]i that regulate cross-bridge cycling rates in both tonic and phasic contractions.
Gerthoffer
W T
WT
Department of Pharmacology, University of Nevada School of Medicine, Reno 89557.
Murphey
K A
KA
Mangini
J
J
Boman
S
S
Lattanzio
F A
FA
D000109:Acetylcholine / Q000494:pharmacology; D000814:Aniline Compounds; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002122:Calcium Chloride / Q000494:pharmacology; D003106:Colon / Q000187:drug effects / Q000502:physiology*; D004285:Dogs; D004533:Egtazic Acid / Q000494:pharmacology; D005260:Female; D005456:Fluorescent Dyes; D015151:Immunoblotting; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D007700:Kinetics; D008297:Male; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D009218:Myosins / Q000302:isolation & purification / Q000378:metabolism*; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology; D012969:Sodium Fluoride / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D014966:Xanthenes
D000814:Aniline Compounds; D005456:Fluorescent Dyes; D014966:Xanthenes; C059715:Fluo-3; D004533:Egtazic Acid; D012969:Sodium Fluoride; D009218:Myosins; D002122:Calcium Chloride; D000109:Acetylcholine; D011188:Potassium; D002118:Calcium