Cannabinoid (CB) Receptors are not Involved in A-955840 Induced Negative Inotropic Effects in Isolated Cardiac Myocytes

A-955840, a selective CB2 agonist, has been shown to elicit concentration-dependent decreases in cardiac contractility in the anesthetized dog (decreased maximal velocity of left ventricular pressure development [LV dP/dt max]). However, it is unknown whether this represents a direct effect or a response dependent on other factors (such as autonomic tone and neurohumoral factors) present in vivo. This study examined if A-955840 had a direct effect on contractility of isolated cardiac myocytes, and if so to determine the potential involvement of CB1 and CB2 receptors. Contractility was assessed in vitro using percent changes in maximal shortening velocity of sarcomeres (dL/dt max) and fractional shortening of sarcomere length (FS) in rabbit left ventricular myocytes. A-955840 reduced dL/dt max and FS in a reversible and concentration-dependent manner with an IC50 of 11.4 μg/mL (based on dL/dt max) which is similar to the IC50 value of 5.5 μg/mL based on the effects of A-955840 on LV dP/dt max in anesthetized dogs. A-955840 (4.0 μg/mL) reduced myocyte contractility (%FS) to a similar extent in the absence and presence of a CB2 antagonist, SR-2 (24.0 ± 3.4 vs. 23.1 ± 3.0 %, n=5) or a CB1 antagonist, rimonabant (18.8 ± 2.3 vs. 19.8 ± 2.7 %, n=5). A-955840 (4.0 μg/mL) also reduced L-type calcium current of rabbit ventricular myocytes (1.05 ± 0.11 vs. 0.70 ± 0.12 nA, n=5, P<0.01). These results suggest that A-955840 exerts direct negative inotropic effects on isolated rabbit ventricular myocytes, which is mediated by neither CB1 nor CB2 receptors, and consistent with off-target negative inotropy mediated by inhibition of the cardiac L-type calcium current.