Recently, we suggested that relaxation to anandamide in rat coronary artery may involve an SR141716A-sensitive site other than a classical cannabinoid receptor and that vanilloid receptors and anandamide metabolism have a minor role (White et al., 2001). Although the underlying mechanisms were unclear, activation of large conductance Ca²⁺-activated K⁺ channels (BK_Ca) seemed to mediate responses to anandamide. Here, we report on the responses to some other cannabinoid receptor ligands in this vessel.

Male Wistar rats (300-400g) were killed with sodium pentobarbitone (120mg kg^-1, i.p.). Left anterior descending coronary arteries (2mm long) were mounted in a wire myograph under normalised tension in oxygenated Krebs-Henseleit solution with 10µM indomethacin (White et al., 2001). They were precontracted with 5-hydroxytryptamine (5-HT, 0.3-3µM) or KCl (60mM obtained by equimolar replacement of NaCl). Relaxations to cumulative addition of cannabinoids or the BK_Ca activator NS1619 to precontracted vessels are expressed as mean ± s.e.m.; n4 rats. SR141716A and iberiotoxin (IbTX) were incubated with tissues for 30min before determination of a concentration/ relaxation curve. Statistical analysis was by two-way analysis of variance followed by Bonferroni post-hoc tests; P<0.05 was taken as the level of significance.

Relaxation to the cannabinoid agonists R-(+)-WIN55,212-2 and CP55,940 was inhibited (P<0.01) by 50nM IbTX (R-(+)-WIN55,212-2 pEC₅₀=6.8±0.3 E_max=53±6%; + IbTX pEC₅₀=5.7±0.3 E_max=10±8%; CP55,940 pEC₅₀=5.9±0.2 E_max= 65±11%; + IbTX pEC₅₀=4.9±0.1 E_max=37±11%). The relaxation is also attenuated by 3µM SR141716A (Ho & Hiley, 2003) as previously shown for anandamide (White et al., 2001). This is consistent with a common relaxant mechanism for these cannabinoids and anandamide. In contrast, relaxation to NS1619 was reduced (P<0.01) by 50nM IbTX or precontraction with 60mM KCl, but not by 3µM SR141716A (NS1619 pEC₅₀=5.2±0.3 E_max=81±11%; + IbTX pEC₅₀=4.6±0.1 E_max=65±9%; KCl pEC₅₀=4.4±0.1 E_max= 43±15%; + SR141716A pEC₅₀=5.3±0.4 E_max=80±12%). Pertussis toxin (PTX; 400 ngml -1 for 2h) inhibited (P<0.01) responses to anandamide and R-(+)-WIN55,212-2 (anandamide pEC₅₀=6.8±0.2 E_max=59±8%; + PTX pEC₅₀=6.3±0.3 E_max= 48±14%; R-(+)-WIN55,212-2 pEC₅₀=7.0±0.3 E_max=47±6%; + PTX pEC₅₀=6.1±0.4 E_max= 37±7%) but did not affect relaxation to CP55,940 (+ PTX pEC₅₀=6.2±0.4 E_max= 67±7%). PTX also had no effect on relaxation induced by NS1619 (NS1619 pEC₅₀=5.1±0.1 E_max=85±6%; + PTX pEC₅₀=5.0±0.1 E_max=89±12%).

These results support the hypothesis that cannabinoids, including anandamide and R-(+)-WIN55212-2, relax the rat coronary artery by activation of a PTX- and SR141716-sensitive receptor which our previous work suggests is not a classical CB₁ or CB₂ receptor. Since the responses are sensitive to IbTX, activation of this receptor may lead to an increase in BK_Ca activity.

Ho W.-S.V. & Hiley C.R. (2003) Pharmacologist, 44, 130.24
White R. et al. (2001) Br. J. Pharmacol., 134, 921-929