Effects of chronic treatment with L-name on the cardiovascular effects of Δ9-tetrahydrocannabinol (THC) in conscious rats

Saoirse E. O’Sullivan*, Michael D. Randall & Sheila Gardiner, School of Biomedical Sciences, University of Nottingham, Queen’s Medical Centre, Nottingham NG7 2UH.

We have shown that chronic inhibition of NO synthase causes enhanced THC-induced vasorelaxation in isolated mesenteric resistance arteries (O’Sullivan et al., this meeting). We have now investigated the cardiovascular responses to THC in vivo in conscious rats, and the effects of chronic inhibition of NO synthase thereon.
Male, Sprague-Dawley rats (200-250g) were given either water (control) or NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mg ml-1 solution in water; ~7.5 mg kg-1 day-1) to drink. After 2 weeks of water or L-NAME drinking, rats underwent a two-stage procedure for the implantation of pulsed Doppler flow probes (renal (R), mesenteric (M), hindquarters (H)) and, subsequently, intravascular catheters (jugular vein, caudal artery). All surgery was under general anaesthesia (fentanyl and medetomidine, 300μg kg-1 of each i.p.) with post-operative anaesthetic reversal and analgesia provided by atipamezole and nalbuphine (1 mg kg-1 of each s.c.). At least 24h after catheter implantation, heart rate (HR), mean blood pressure (BP) and regional vascular conductances (VC) were measured in conscious, unrestrained animals. Experiments were performed over 4 days. On days 1 and 2, animals were given either THC (1 mg kg-1, 0.5 ml over 15 min) or vehicle (saline with 5% propylene glycol and 2% Tween 80), preceded by a 30 min infusion of the vehicle (saline with 5% propylene glycol and 2% Tween 80) for AM251, a CB1-receptor antagonist. On days 3 and 4, animals were given either THC or vehicle, preceded by AM251 (3 mg kg-1).

In control rats, at the end of the THC infusion, there was an increase in MAP (+14 ± 4 mmHg, mean ± SEM, P<0.001, Friedman’s test) accompanied by falls in RVC (-15 ± 4 %, P<0.001) and MVC (-32 ± 4 %, P<0.001) and an increase in HVC (+31 ± 8 %, P<0.001). After pre-treatment with AM 251, the pressor and regional vascular effects of THC were almost abolished and there was bradycardia (-25 ± 8 beats min-1, P<0.05). In L-NAME-treated rats, the pressor (+18 ± 3 mmHg, P<0.001) and vasoconstrictor (RVC -8 ± 3 %, P<0.001; MVC -29 ± 5 %, P<0.001) effects at the end of THC infusion were similar to those in control rats, but there was a significant bradycardia (-57 ± 1 beats min-1, P<0.001) and a smaller increase in HVC (7 ± 4 %, n.s.). Pre-treatment with AM251 reduced the pressor and mesenteric vasoconstrictor effects of THC in L-NAME-treated rats, and there was hindquarters vasoconstriction (-15 ± 3 %, P<0.05).

In conclusion, in vivo, the cardiovascular effects of THC are predominantly CB1-receptor-mediated. In contrast to the in vitro findings (O’Sullivan et al., this meeting), there is no evidence for enhanced vasodilator effects of THC in vivo after chronic L-NAME treatment. The reduced THC-induced hindquarters vasodilatation after L-NAME treatment is consistent with an involvement of NO in CB1-receptor mediated vasodilatation in that vascular bed (Gardiner et al., 2002).

Gardiner, SM et al. (2002) .Br. J. Pharmacol 136, 581-587.

S.O’S is funded by an Early Career Fellowship from the Leverhulme Trust.