Vol. 284, Issue 4, H1182-H1189, April 2003
Increased myofibrillar protein phosphatase-1 activity impairs
rat aortic smooth muscle activation after hypoxia
Hwee
Teoh,
Mary
Zacour,
Avraham D.
Wener,
Lakshman
Gunaratnam, and
Michael E.
Ward
Terrence Donnelly Laboratories, Division of Respirology and
Department of Critical Care, St. Michael's Hospital, University of
Toronto, Toronto, Ontario, Canada M5B 1W8
We hypothesized that
increased myofibrillar type 1 protein phosphatase (PP1) catalytic
activity contributes to impaired aortic smooth muscle contraction after
hypoxia. Our results show that inhibition of PP1 activity with
microcystin-LR (50 nmol/l) or okadaic acid (100 nmol/l) increased
phenylephrine- and KCl-induced contraction to a greater extent in
aortic rings from rats exposed to hypoxia (10% O2) for
48 h than in rings from normoxic animals. PP1 inhibition
also restored the level of phosphorylation of the 20-kDa myosin light
chain (LC20) during maximal phenylephrine-induced contraction to that observed in the normoxic control group.
Myofibrillar PP1 activity was greater in aortas from rats exposed to
hypoxia than in normoxic rats (P < 0.05). Levels of
the protein myosin phosphatase-targeting subunit 1 (MYPT1) that
mediates myofibrillar localization of PP1 activity were increased in
aortas from hypoxic rats (193 ± 28% of the normoxic control
value, P < 0.05) and in human aortic smooth muscle
cells after hypoxic (1% O2) incubation (182 ± 18%
of the normoxic control value, P < 0.05). Aortic
levels of myosin light chain kinase were similar in normoxic and
hypoxic groups. In conclusion, after hypoxia, increased MYPT1 protein and myofibrillar PP1 activity impair aortic vasoreactivity through enhanced dephosphorylation of LC20.
smooth muscle contraction; myosin light chain kinase; myosin light
chain phosphatase
