TRPV4 Forms a Novel Ca 2+ Signaling Complex With Ryanodine Receptors and BK Ca Channels

Abstract

Vasodilatory factors produced by the endothelium are critical for the maintenance of normal blood pressure and flow. We hypothesized that endothelial signals are transduced to underlying vascular smooth muscle by vanilloid transient receptor potential (TRPV) channels. TRPV4 message was detected in RNA from cerebral artery smooth muscle cells. In patch-clamp experiments using freshly isolated cerebral myocytes, outwardly rectifying whole-cell currents with properties consistent with those of expressed TRPV4 channels were evoked by the TRPV4 agonist 4α-phorbol 12,13-didecanoate (4α-PDD) (5 μmol/L) and the endothelium-derived arachidonic acid metabolite 11,12 epoxyeicosatrienoic acid (11,12 EET) (300 nmol/L). Using high-speed laser-scanning confocal microscopy, we found that 11,12 EET increased the frequency of unitary Ca²⁺ release events (Ca²⁺ sparks) via ryanodine receptors located on the sarcoplasmic reticulum of cerebral artery smooth muscle cells. EET-induced Ca²⁺ sparks activated nearby sarcolemmal large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels, measured as an increase in the frequency of transient K⁺ currents (referred to as “spontaneous transient outward currents” [STOCs]). 11,12 EET–induced increases in Ca²⁺ spark and STOC frequency were inhibited by lowering external Ca²⁺ from 2 mmol/L to 10 μmol/L but not by voltage-dependent Ca²⁺ channel inhibitors, suggesting that these responses require extracellular Ca²⁺ influx via channels other than voltage-dependent Ca²⁺ channels. Antisense-mediated suppression of TRPV4 expression in intact cerebral arteries prevented 11,12 EET–induced smooth muscle hyperpolarization and vasodilation. Thus, we conclude that TRPV4 forms a novel Ca²⁺ signaling complex with ryanodine receptors and BK_Ca channels that elicits smooth muscle hyperpolarization and arterial dilation via Ca²⁺-induced Ca²⁺ release in response to an endothelial-derived factor.