Vascular activity of anandamide in spontaneously hypertensive rats

Amanda Wheal, Terence Bennett, Sheila Gardiner, Michael Randall. University of Nottingham, Nottingham, United Kingdom.

Anandamide is an endocannabinoid that causes vasorelaxation in vitro via multiple mechanisms which may be subject to regional variation within a vascular bed (O'Sullivan et al., 2004). In vivo, enhanced depressor effects of anandamide have been reported in spontaneously hypertensive rats (SHRs), which may, in part, be due to enhanced vasodilatation (BÁtkai et al., 2004). We have now investigated the vasorelaxant effects of anandamide in vitro in perfused mesenteric arterial beds and thoracic aortae from SHRs.

Male, SHRs (aged 20 weeks, 314-362g) and Wistar Kyoto rats (WKYs, 294-335g), were stunned and killed by cervical dislocation. The mesenteric arterial bed was cannulated, removed, and then perfused with gassed Krebs’ Henseleit solution containing 3μM indomethacin. Following an 80 min equilibration period, methoxamine (∼5μM) was added to increase the perfusion pressure to ∼120mmHg. A cumulative concentration-response curve to anandamide (1nM-30μM) was constructed. In addition, thoracic aortae from the same animals were dissected and mounted on wire clips as 4mm long rings in Krebs’ Henseleit buffer (plus 3μM indomethacin). Vessels were equilibrated under 9.8mN of tension and contracted with methoxamine (∼50μM) and the relaxant effects of anandamide (1nM-30μM) were examined. These experiments were also repeated in the presence of L-NAME (300μM), the cannabinoid CB1 receptor antagonist AM251 (1μM), or following 1 h capsaicin (10μM) pre-treatment used to deplete sensory nerves.

Anandamide was significantly (P<0.05, Student’s t-test) more potent as a vasorelaxant in perfused mesenteric arterial beds taken from WKYs (pEC50%=7.13±0.14, n=10), compared with SHRs (pEC50%=6.28±0.10, n=10). Conversely, in aortic rings from the same rats, anandamide caused greater (P<0.01, Student’s t-test) maximal relaxation in SHRs (pEC50=7.64±0.30, RMax=70.33±8.38, n=7) than in WKYs (pEC50=6.93±0.76, RMax=25.11±7.26, n=8). In both strains, the vasorelaxations were not significantly affected by L-NAME, AM251, or capsaicin pre-treatment.

This study has demonstrated variations in sensitivity to the vasorelaxant effects of anandamide in SHRs compared to WKYs which are regionally different. It also highlights differences between SHRs and the L-NAME-induced model of hypertension in which the vasorelaxant effects of anandamide in the mesenteric arterial bed are enhanced (MendizÁbal et al., 2001; Wheal et al., 2007). The mechanisms responsible for the vasorelaxant action of anandamide in SHRs remain under investigation.

BÁtkai et al. (2004). Circulation 110: 1996-2002

MendizÁbal et al. (2001). Eur J Pharmacol 427: 251-262

O'Sullivan et al. (2004). Br J Pharmacol 142: 435-442

Wheal et al. (2007). Br J Pharmacol [Epub ahead of print]

We thank the British Heart Foundation for funding this research.