Abstract
It has been proposed that activation of vanilloid receptor-1 (TRPV1) affects the vasotone of resistance arteries. One of the endogenous activators of TRPV1 is anandamide. The effects of anandamide on TRPV1 responsiveness were tested on isolated, pressurized (80 mm Hg) skeletal muscle (m. gracilis) arterioles (179 +/- 33 microm in diameter). We found that the TRPV1 agonist capsaicin (1 microM) elicited a substantial constriction in isolated arterioles (51 +/- 12%). In contrast, anandamide (0-100 microM) did not affect arteriolar diameter significantly (3 +/- 5%). Isolated vessels were also preincubated with anandamide (30 microM for 20 min). This anandamide pretreatment completely blocked capsaicin-induced arteriolar constriction (response decreased to 1 +/- 0.6%), and this inhibition was reversed by a protein phosphatase-2B inhibitor (cyclosporin-A; 100 nM, 5 min) treatment (constriction, 31 +/- 1%). An exogenous TRPV1-expressing cell line [Chinese hamster ovary (CHO)-TRPV1] was used to specifically evaluate TRPV1-mediated effects of anandamide. The efficacy of anandamide in this system, as determined by 45Ca2+ uptake, was 65 +/- 8% of that of capsaicin. Upon treatment of the cells with cyclosporin-A or the protein kinase C activator phorbol 12-myristate 13-acetate (PMA), anandamide was transformed to a full agonist. Anandamide treatment caused an acute desensitization in these cells as measured by intracellular Ca2+ imaging. Application of cyclosporin-A or PMA reversed this desensitization. Our data suggest that anandamide may cause a complete (albeit phosphorylation-dependent) desensitization of TRPV1 in skeletal muscle arterioles and in CHO-TRPV1 cells, which apparently transforms the ligand-gated TRPV1 into a phosphorylation-gated channel. This property of anandamide may provide a new therapeutic strategy to manipulate TRPV1 activity.
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