AJP - Cell Physiology, Vol 258, Issue 3 C524-C532, Copyright © 1990 by American Physiological Society
Dissociation between myosin phosphorylation and shortening velocity in canine trachea
L. Merkel, W. T. Gerthoffer and T. J. Torphy
Department of Pharmacology, Smith Kline & French Laboratories, King of Prussia, Pennsylvania 19406.
The relationship between glycogen phosphorylase activity (an index of
cytosolic Ca2+ content), myosin light-chain phosphorylation, isotonic
shortening velocity, and isometric tension was examined in canine
trachealis. Responses were measured in tracheal strips contracted with
various concentrations of methacholine or K+. Both agonists produced
prolonged and concentration-dependent increases in isometric tension that
reached 90% of the plateau level within 1 (methacholine) to 5 (K+) min and
remained stable over 60 min. In contrast to the monotonic increase in
isometric tension, shortening velocity reached a maximum almost immediately
(12-48 s) after the addition of either methacholine or K+ and then declined
over time to a steady-state level that was 25-40% of the peak.
Phosphorylase activity also increased transiently, reaching a maximum 1-2
min after the addition of either agonist before declining to near-basal
levels over the 60-min observation period. Unlike the increases in
shortening velocity and phosphorylase activity, agonist-induced myosin
phosphorylation was not markedly transient. Moreover, regardless of the
contractile agonist used, no correlation was found between myosin
phosphorylation and shortening velocity when these parameters were compared
at corresponding time points. This suggests that myosin phosphorylation is
not the sole determinant of shortening velocity in canine trachealis.
