Docosahexaenoic Acid Targets TRPC3/6 Channels to Suppress Noradrenaline-Induced Contraction in Rat Vas Deferens Smooth Muscle

We investigated the effects of docosahexaenoic acid (DHA) on noradrenaline (NA)-induced contractile responses in rat vas deferens smooth muscle (VDSM). DHA (3 × 10^-6-3 × 10^-5 M) significantly inhibited phasic contractions induced by NA (3 × 10^-5 M) in a concentration-dependent manner. In contrast, under Ca²⁺-free conditions, DHA (3 × 10^-5 M) slightly enhanced NA-induced contractile responses. DHA did not inhibit NA-induced contractile responses in the prostate, which are mediated by α_1A-adrenoceptor stimulation, similar to those in VDSM. Although verapamil (10^-5 M), an L-type Ca²⁺ channel inhibitor, partially suppressed NA-induced contractile responses in VDSM, a substantial phasic component persisted. The residual phasic component was unaffected by LOE-908 (a receptor-operated Ca²⁺ channel inhibitor, 3 × 10^-5 M), but was markedly inhibited by SKF-96365 (an inhibitor of both receptor-operated and store-operated Ca²⁺ channels, 3 × 10^-5 M). Among inhibitors targeting store-operated Ca²⁺ channel candidates with high mRNA expression in VDSM, GSK417651A (10^-5 M) and Pyr10 (3 × 10^-5 M)-both putative inhibitors of transient receptor potential canonical 3/6 (TRPC3/6) channels-effectively inhibited the contractile response, whereas inhibitors targeting Orai1, transient receptor potential vanilloid 4 (TRPV4), or TRPC1/4/5 did not exert inhibitory effects. Similarly, under combined treatment with verapamil and LOE-908, contractile responses induced by cyclopiazonic acid (3 × 10^-5 M, a sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor) were reduced by DHA, SKF-96365, and putative TRPC3/6 inhibitors, but not by inhibitors of Orai1, TRPV4, or TRPC1/4/5. These findings indicate that DHA suppresses NA-induced contractile responses in VDSM, primarily through inhibition of TRPC3/6 channels.