Mechanotransduction and the Myogenic Response in Diabetes - The Mechanobiology of Obesity and Related Diseases - page 235

subunits only the b and c subunits were identified in native VSMCs [ 58 ]. By contrast,
Wang et al. [ 60 ] were unable to detect intact ENaC mRNA expression in afferent
arterioles and furthermore failed to demonstrate ENaC-specific effects of amiloride
or benzamil on afferent arteriole myogenic responses in the hydronephrotic rat. It is
possible that these apparently opposing results are a reflection of vessel heteroge-
neity and/or use of different preparations, or a requirement for ENaC subunits to
associate with other proteins to enable functionality (for example, the related acid
sensing ion channel protein ASIC-2 [ 61 ]). However, while the latter explanation may
suggest differences in the molecular composition between studies it does not appear
to explain the disparate results to the same pharmacological agents. While a
substantial amount of work is required to firmly establish ENaC as a myogenic
mechanosensor in arteriolar smooth muscle it is also of interest to note that the
b-ENaC-deficient mouse shows impaired renal autoregulation [ 62 ] together with
renal inflammation and chronically raised blood pressure [ 63 ].
In addition to cation channels a role for Cl - channels in stretch-dependent smooth
muscle cell dependent depolarization has been suggested. Initial studies used
pharmacological inhibitors (for example DIDS and IAA-94 [ 64 ]), however, these
have limited due to their apparent lack of selectivity. Further, a lack of knowledge of
the molecular identity of these channels has contributed to uncertainty. More
recently using a combination of electrophysiology, intact vessels and siRNA
manipulation of protein expression, Bulley et al. [ 65 ] have provided data suggesting
that a specific, Ca 2+ -activated, Cl - channel, TMEM16A, contributes to myogenic
responsiveness in cerebral arteries. Interestingly, it was concluded that the effect of
stretch was not directly on TMEM16A but occurred via upstream activation of a
NSCC, which provided Ca 2+ for subsequent activation of the Cl - channel.
3.2.2 G-proteins and Membrane Located Effectors
Early studies implicating a role for trimeric G-proteins in small artery myogenic
reactivity were largely indirect relying on the use of inhibitors of phospholipase C
(PLC) and measurements of the downstream production/accumulation of signaling
molecules [ 66 , 67 ]. Thus, Osol et al. showed myogenic responsiveness of cannulated
rat posterior cerebral arteries to be attenuated by the PLC inhibitor, U-73122, while
Narayanan et al. showed in dog renal vessels that an increase in intraluminal pressure
led to the time-dependent accumulation of inositol trisphosphate (IP 3 ) and DAG.
More recently, GPCRs, particularly the angiotensin II type 1 receptor (AT 1 R),
have been proposed to be mechanosensitive leading to the activation of specific
Ga q/11 -proteins and downstream phospholipases [ 20 , 68 ]. The direct mechanical
effect on the receptors was shown to be agonist independent although in the case of
the AT1R pressure-induced activation of Ga q/11 could be prevented by the receptor
blocker, losartan [ 69 ]. It is proposed that mechanical force directly alters the
conformation of the receptor such that it is placed in an activated configuration.
Interestingly, this mode of mechanical activation is not limited to the AT1R as it
could also be demonstrated for several other GPCRs including those for histamine