Mechanotransduction and the Myogenic Response in Diabetes - The Mechanobiology of Obesity and Related Diseases - page 238

however, in the role of this mechanism may be evident between vascular beds
[ 83 , 84 ].
An additional SR release-driven Ca 2+ transient is evident in the form of cyclical
Ca 2+ waves with the frequency of propagating and asynchronous intracellular Ca 2+
waves increasing with arteriolar intraluminal pressure [ 85 , 86 ] suggesting a rela-
tionship with events underlying myogenic contraction. However, studies in arte-
rioles from skeletal muscle suggest that this relationship between intraluminal
pressure and wave frequency persists when myogenic tone is inactivated [ 86 ].
While at present it is uncertain whether myogenic reactivity is specifically mod-
ulated by the presence of cytosolic Ca 2+ waves, Welsh and Colleagues [ 87 , 88 ]
have suggested that the waves do, indeed, facilitate the development of myogenic
tone (especially at pressures \60 mmHg) via Ca 2+ -dependent phosphorylation of
MLC 20 .
3.4 Activation of Protein Kinases
3.4.1 Myosin Light Chain Kinase and Myosin Phosphatase
A number of protein phosphorylation events have been suggested to be involved in
myogenic signaling. As with agonist-induced contraction of smooth muscle direct
measurements have shown that increased arteriolar intraluminal pressure, and
active myogenic constriction, is associated with an increase in the phosphorylation
of the 20 kDa myosin regulatory light chain [ 23 , 88 , 89 ]. Further, the light chain is
phosphorylated at serine 19 [ 88 ]. This phosphorylation occurs via a Ca 2+ and
calmodulin-dependent mechanism. The extent of phosphorylation has been shown
to be directly related to the instantaneous level of wall tension as calculated by the
LaPlace relationship [ 23 ]. A causative relationship between myosin light chain
phosphorylation and myogenic contraction was suggested by the observation that
an increase in intraluminal pressure failed to elicit contraction in the presence of the
inhibitor, ML-7, despite a mechanically-induced increase in intracellular Ca 2+ [ 23 ].
Recent studies have demonstrated that the net level of myosin phosphorylation is
critically dependent on the activity of myosin phosphatase. While the phosphatase
was once thought to be unregulated it is now clear that its activity is modulated by
both Rho kinase and PKC. Rho kinase phosphorylation of the myosin targeting
subunit of the phosphatase (MYPT1) at threonine 855 inhibits binding to the
activated myosin molecule thereby decreasing its dephosphorylation [ 88 ]. PKC
phosphorylates CPI-17 that acts as an inhibitor of the phosphatase. Using a very
sensitive three-step Western blotting approach Cole and colleagues [ 90 , 91 ] have
shown in cerebral small arteries that increasing intraluminal pressure (10-60 and
100 mmHg) leads to Rho kinase-dependent phosphorylation of MYPT1 at Thr 855
while PKC-mediated phosphorylation of CPI-17 was not apparent.