CB1 receptor antagonist SR141716A inhibits Ca2+-induced relaxation in CB1 receptor-deficient mice

Mesenteric branch arteries isolated from cannabinoid type 1 receptor knockout (CB1-/-) mice, their wild-type littermates (CB1+/+ mice), and C57BL/J wild-type mice were studied to test the hypothesis that murine arteries undergo high sensitivity Ca2+-induced relaxation that is CB1 receptor dependent. Confocal microscope analysis of mesenteric branch arteries from wild-type mice showed the presence of Ca2+ receptor-positive periadventitial nerves. Arterial segments of C57 control mice mounted on wire myographs contracted in response to 5 mumol/L norepinephrine and responded to the cumulative addition of extracellular Ca2+ with a concentration-dependent relaxation that reached a maximum of 72.0+/-6.3% of the prerelaxation tone and had an EC50 for Ca2+ of 2.90+/-0.54 mmol/L. The relaxation was antagonized by precontraction in buffer containing 100 mmol/L K+ and by pretreatment with 10 mmol/L tetraethylammonium. Arteries from CB1-/- and CB1+/+ mice also relaxed in response to extracellular Ca2+ with no differences being detected between the knockout and their littermate controls. SR141716A, a selective CB1 antagonist, caused concentration-dependent inhibition of Ca2+-induced relaxation in both the knockout and wild-type strains (60% inhibition at 1 mumol/L). O-1918, a cannabidiol analog, had a similar blocking effect in arteries of both wild-type and CB1-/- mice at 10 mumol/L. In contrast, 1 mumol/L SR144538, a cannabinoid type 2 receptor antagonist, or 50 mumol/L 18alpha-glycyrrhetinic acid, a gap junction blocker, were without effect. SR141716A (1 to 30 mumol/L) was also assessed for nonspecific actions on whole-cell K+ currents in isolated vascular smooth muscle cells. SR141716A inhibited macroscopic K+ currents at concentrations higher than those required to inhibit Ca2+-induced relaxation, and appeared to have little effect on currents through large conductance Ca2+-activated K+ channels. These data indicate that arteries of the mouse relax in response to cumulative addition of extracellular Ca2+ in a hyperpolarization-dependent manner and rule out a role for CB1 or CB2 receptors in this effect. The possible role of a nonclassical cannabinoid receptor is discussed.