The
Role of RhoA Kinase in Circular
Smooth
Muscle Contraction in the Ileum
Jacqueline
M. Bartush
University
of Dallas
Summer
Mentor: Dr. Thomas Eddinger
Smooth
muscle exhibits great variability in terms of the force, duration, and
stimuli of its contractions. This implies that multiple pathways regulate
smooth muscle activity. RhoA kinase regulates contraction through inhibition
of myosin light chain phosphatase, an inhibitor of cross bridge cycling.
Thus, the net effect of RhoA kinase is an increase in contractile activity.
The research of this lab used changes in receptor-mediated and spontaneous
contractions due to the introduction of Y-27632, a Rho kinase inhibitor,
to determine the absence or presence of RhoA kinase in the pathway for
a given type of contraction. A significant decrease in contractile force
from control to experimental conditions (Y-27632 present) indicates the
presence of RhoA kinase in the pathway for that particular type of contraction
Samples
of ileum were obtained from New Zealand white hares and stored in physiological
salt solution (PSS). These samples were cut into strips and hung in a calibrated
smooth muscle isometric force chamber filled with PSS. Contractions were
measured by the degree of downward force exerted on the force transducer
by the hook connected to the muscle by a tissue clip. Following incubation,
contraction was induced through depolarization with KPSS and later through
receptor activation with carbachol and acetylcholine. Later, Y-27632 was
added to the bath prior to the addition of carbachol or acetylcholine to
determine its effects on peak force and spontaneous contractions. Y-27632
did not significantly lower peak force in carbachol or acetylcholine-induced
contraction. Also, it did not affect the peak force of spontaneous contractions.
However, Y-27632 did significantly lower the frequency of spontaneous contractions.
The first results suggest that either RhoA kinase is not present in the
circular muscle of the ileum, or, if present, it is not functioning as
an inhibitor of MLC phosphatase or another inhibitor of contraction. This
drug’s effects on the frequency of spontaneous contraction seem a bit puzzling
in light of other results and merit future research into the mechanisms
of spontaneous contraction.
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