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Mary McGahon, Alexander V Zholos, Graham J McGeown, Tim M Curtis; Mechanosensitive TRP channel expression in rat retinal microvessels. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4353.
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Mechanosensitive TRP channels may play a role in the pressure-induced vasoconstriction, acting as pathway for Ca2+-entry in vascular smooth muscle. This “myogenic response” is a primary mechanism contributing to blood flow autoregulation in many vascular beds. Presently, we have begun to define the molecular and functional expression of mechanosensitive TRP channels in retinal arteriolar smooth muscle cells (SMCs).
Using mechanical dissociation, arteriolar enriched samples of rat retinas were screened (RT-PCR) for expression of mRNA encoding a number of known mechanosensitive TRP channels. Immunohistochemical (IHC) analysis of channel expression was performed on retinal flatmounts using 2 different antibodies per channel. Stretch sensitive responses in isolated arterioles were tested (fura-2 microfluorimetry) by reducing external osmolarity by 100mOsM in Cl- depleted conditions and by application of negative pressure to the area of membrane within the tip of a patch electrode in the cell-attached configuration. Whole-cell patch-clamp recordings were also made from rat retinal arteriolar SMCs embedded within isolated arteriolar segments.
mRNA transcripts for TRPC1, M7, V1, V2, V4 and P1 channels were detected. In contrast to larger resistance arteries, retinal arterioles do not appear to express TRPC6 or M4 channels. IHC revealed cytosolic and membrane expression of TRPV1, V2, and M7 in arteriolar SMCs surrounding arterioles, while TRPV1, V2, V4 and M7 were also detected in the endothelial cells lining arterioles, capillaries and venules. Hypo-osmotic stretch increased [Ca2+]i (dependent on external Ca2+) which was reversed by the TRPV2 inhibitor tranilast (10 μM; n=5), but not inhibitors of TRPV1 or M7. Pressure-induced stretch resulted in an increase in channel activity consistent with TRPV2 activation. Application of the TRPV2 agonist, delta-9-tetrahydrocannabinol (Δ9THC; 10 μM) increased both inward and outward currents in whole-cell patch-clamped SMCs. The current activated by Δ9THC was outwardly rectifying and reversed at -9.7 ± 4.0 mV (n=9). Both the inward and outward portions of the current elicited by Δ9THC were inhibited by pre-incubation of the vessel with tranilast (n=5).
Our results suggest that retinal arteriolar SMCs express a range of mechanosensitive TRP channels. TRPV2 channels may be molecular candidates underpinning the myogenic response in this vascular bed.
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