Cytoplasmic free calcium, myosin light chain phosphorylation, and force in phasic and tonic smooth muscle.
The time course of [Ca2+]i, tension, and myosin light chain phosphorylation were determined during prolonged depolarization with high K+ in intact tonic (rabbit pulmonary artery) and phasic (longitudinal layer of guinea pig ileum) smooth muscles. [Ca2+]i was monitored with the 340 nm/380 nm signal ratio of the fluorescent indicator fura-2. The fluorescence ratio had a similar time course in both muscle types during depolarization with 109 mM [K+]o; after a transient peak, there was a decline to 70% of its peak value in tonic smooth muscle, and to 60% in phasic smooth muscle. Tension, however, continued to increase in the pulmonary artery, while in the ileum it declined in parallel with the [Ca2+]i. On changing [K+]o from 109 to 20 mM, tension and [Ca2+]i either remained unchanged or declined in parallel in the pulmonary artery. Phosphorylation of the 20-kD myosin light chain, measured during stimulation of muscle strips with 109 mM [K+]o in another set of experiments, increased from 3% to a peak of 50% in the intact pulmonary artery, and then declined to a steady state value of 23%. In the intact ileum, a very rapid, early transient phosphorylation (up to 50%) at 2-3 s was seen. This transient declined by 30 s to a value that was close to the resting level (7%), while tension remained at 55% of its peak force. A quick release during maintained stimulation induced no detectable change in the [Ca2+]i in either type of smooth muscle. We discuss the possibility that the slowly rising tonic tension in pulmonary artery could be due to cooperativity between phosphorylated and nonphosphorylated crossbridges.
Himpens
B
B
Pennsylvania Muscle Institute, University of Pennsylvania, School of Medicine, Philadelphia 19104-6083.
Matthijs
G
G
Somlyo
A V
AV
Butler
T M
TM
Somlyo
A P
AP
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D005260:Female; D006168:Guinea Pigs; D007082:Ileum / Q000378:metabolism / Q000502:physiology; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction; D009130:Muscle, Smooth / Q000378:metabolism* / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000378:metabolism* / Q000502:physiology; D009218:Myosins / Q000378:metabolism*; D010766:Phosphorylation; D011651:Pulmonary Artery / Q000378:metabolism / Q000502:physiology; D011817:Rabbits
D009218:Myosins; D002118:Calcium