Dichotomous effects of smooth muscle TRPV4 channels on vascular contractility

Abstract

Ca2+ signaling mechanisms in smooth muscle cells (SMCs) determine vascular contractility. Transient receptor potential vanilloid 4 (TRPV4) ion channels have been described as a crucial Ca2+ influx pathway in SMCs from small, resistance-sized arteries. However, the impact of SMC TRPV4 (TRPV4SMC) channel activity on blood pressure remains unknown, mostly due to the lack of studies in SMC-specific knockout mice. We hypothesized that TRPV4SMC channels control the resting blood pressure. Radiotelemetric blood pressure recordings showed reduced daytime and nighttime blood pressures in the inducible, SMC-specific TRPV4 knockout (TRPV4SMC-/-) mice compared to the control mice. The effect of TRPV4SMC-/- on vascular contractility was assessed using two endogenous vasoconstrictor mechanisms- 1) SMC α1 adrenergic receptor (α1AR) stimulation and 2) increased intraluminal pressure. Phenylephrine (α1AR agonist)-induced constriction of pressurized mesenteric arteries (MAs, 80 mm Hg, 1 nM-10 mM phenylephrine) and blood pressure elevation (10 mg/kg phenylephrine, i.p.) were blunted in TRPV4SMC-/- mice, identifying a central role of TRPV4SMC channels in α1AR-induced vasoconstriction and blood pressure elevation. Individual Ca2+ influx signals through TRPV4SMC channels (TRPV4SMC sparklets) were recorded using spinning disk confocal imaging in fluo-4-loaded MAs pressurized to 80 mm Hg. Phenylephrine (1 mM) activated TRPV4SMC sparklets, an effect abolished by protein kinase C (PKC) inhibitor (Go6976, 1 mM). These results indicated that PKC-TRPV4SMC channel signaling is a major contributor to α1AR-induced vasoconstriction. In direct contrast with PE-induced constriction, intraluminal pressure-induced constriction was higher in MAs from TRPV4SMC-/- mice, suggesting that SMC TRPV4 channels limit pressure-induced vasoconstriction. Previous studies have shown that TRPV4SMC channels can activate large-conductance, Ca2+-sensitive K+ (BK) channels, thereby decreasing pressure-induced vasoconstriction. Consistent with these findings, vasoconstriction in response to BK channel inhibitor (paxilline, 1 mM) was reduced in MAs from TRPV4SMC-/- mice, supporting a dilatory TRPV4SMC channel-BK channel signaling under resting conditions. In situ proximity ligation assay revealed co-localization of TRPV4SMC channels with both α1ARs and BK channels. Overall, TRPV4SMC channels increase the resting blood pressure but have divergent effects on vascular contractility. TRPV4SMC channels contribute to α1AR-induced vasoconstriction but limit pressure-induced vasoconstriction. Therefore, selective vasoactive stimulus-TRPV4SMC channel coupling may determine the impact of TRPV4SMC channels on vascular resistance.