The functional role of the calcium-sensing receptor in rat mesenteric arteries

P. N. Zadeh, E. Connor, H. Z. Greenberg, A. Albert. St George’s University of London, London, United Kingdom.

Introduction: Expression of the calcium-sensing receptor (CaSR), well-known for its role in Ca²⁺ homeostasis [1], has recently been discovered in the vasculature [2]. This project investigated this receptor’s role in vascular tone regulation in rat mesenteric arteries, specifically assessing the involvement of the endothelium. It was hypothesised that in pre-contracted rat mesenteric arteries, cumulatively increasing extracellular Ca²⁺concentrations ([Ca²⁺]_o) causes endothelium-dependent relaxations. It was also hypothesised that both endothelium derived nitric oxide (NO) and calcium-activated potassium (K_Ca) channels are involved in the mechanisms underlying endothelium-dependent relaxations.

Method: Changes in vascular tone were measured using wire myography. First order branches of the rat superior mesenteric artery were pre-contracted with 10μM methoxamine, following which Ca²⁺ (1.5mM to 10mM) was cumulatively added. Responses to [Ca²⁺]_o were compared in endothelium-intact and endothelium-denuded vessels. To examine the role of NO and K_Ca channels, endothelium-intact vessels were pre-treated with 300μM L-NAME (NO synthase inhibitor), 300nM apamin (small conductance K_Ca (SK_Ca) channel blocker) and 100nM charybdotoxin (intermediate (IK_Ca) and large (BK_Ca) conductance K_Ca channel blocker).

Results: In endothelium-intact arteries, cumulatively increasing [Ca²⁺]_o caused concentration-dependent relaxations (mean relaxation of ∼91% at 10mM, n=8). These relaxations were significantly inhibited when the endothelium was removed (mean relaxation of ∼23% at 10mM, n=3). L-NAME slightly reduced Ca²⁺-induced relaxations (mean relaxation of ∼79% at 10mM, n=5) whereas co-application of L-NAME, apamin and charybdotoxin significantly attenuated relaxations (mean relaxation of ∼60% at 10mM, n=2). In these experiments n represented the number of animals used, with 1-4 vessel segments used from each animal. For statistical analysis, two-way analysis of variance (2-way ANOVA) and Bonferroni post hoc tests were carried out.

Conclusions: In first order branches of the rat superior mesenteric artery, stimulation of CaSRs with cumulative increases in [Ca²⁺]_o causes concentration- and endothelium-dependent relaxations. Both endothelial derived NO and K_Ca channels are involved in the relaxation mechanisms, although the results suggest that K_Ca channels make a more dominant contribution. The individual actions and extent of contribution of SK_Ca, IK_Ca and BK_Ca channels require further investigation in the future. The endothelium-dependent relaxations and hence the decreased vascular tone caused by stimulating CaSRs indicates their potential use as novel therapeutic targets for treating hypertension in the future.

References:

1.Smajilovic S et al. (2011). Br J Pharmacol 164(3): 884-893.

2.Weston AH et al. (2011). Acta Physiol (Oxf) 203(1): 127-137.