AJP - Gastrointestinal and Liver Physiology, Vol 260, Issue 6 958-G964, Copyright © 1991 by American Physiological Society

ARTICLES

Myosin phosphorylation and calcium in tonic and phasic contractions of colonic smooth muscle

W. T. Gerthoffer, K. A. Murphey, J. Mangini, S. Boman and F. A. Lattanzio Jr
Department of Pharmacology, University of Nevada School of Medicine, Reno 89557.

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.

This article has been cited by other articles:

				N. H. Urban, K. M. Berg, and P. H. Ratz K+ depolarization induces RhoA kinase translocation to caveolae and Ca2+ sensitization of arterial muscle Am J Physiol Cell Physiol, December 1, 2003; 285(6): C1377 - C1385. [Abstract] [Full Text] [PDF]

				J. C. Hedges, I. A. Yamboliev, M. Ngo, B. Horowitz, L. P. Adam, and W. T. Gerthoffer p38 Mitogen-activated protein kinase expression and activation in smooth muscle Am J Physiol Cell Physiol, August 1, 1998; 275(2): C527 - C534. [Abstract] [Full Text] [PDF]