Actions Of Calcium-sensing Receptor Ligands In Mesenteric, Femoral and Pulmonary Arteries From The Rat
The cell surface, extracellular calcium-sensing receptor (CaR), which is activated by Ca2+ ions, has been reported in the endothelium, smooth muscle and perivascular sensory nerves, and thus could contribute to vascular control. Increasing [Ca2+]o (from 1 to 5mM) was shown to relax rat mesenteric, basilar and renal arteries, thought to be mediated by CaR in perivascular sensory nerves expressing Transient Receptor Potential Vanilloid type1 receptors (TRPV1; Wang & Bukoski, 1988). Our recent study (Thakore & Ho, 2011) confirmed that mesenteric relaxation to Ca2+ is sensitive to the negative allosteric modulator of CaR, calhex231 and functional desensitization of sensory nerves by capsaicin (a potent TRPV1 agonist). However, we also reported that relaxation elicited by Ca2+ and positive allosteric modulators of CaR (calcimimetics) could differ in their mechanisms of action, including the involvement of CaR (Thakore & Ho, 2011). Here, we further investigated the relaxant effects of [Ca2+]o and the two calcimimetics, calindol and cinacalcet in different vascular regions. Small mesenteric, intrapulmonary and femoral arteries from male Wistar rats (250-350g; Charles river) were mounted in a wire-myograph for tension recording. Data are expressed as mean±s.e.m.(n≥4) and analysed by two-way analysis of variance of the whole data set. P<0.05 was considered statistically significant. All vessels were precontracted with 10µM methoxamine. In mesenteric arteries bathed in physiological solution containing 1mM Ca2+, incremental additions of CaCl2 (1.5-5mM) induced relaxation that was inhibited by 3µM calhex231 (control, EC50 = 2.3±0.2mM, Rmax = 69±9%; +calhex231, EC50 = 3.8±0.2mM, Rmax = 38±18%; P<0.01), confirming our previous data. Treatment with ruthenium red (10µM), which is a non-selective TRPV blocker, also reduced the maximal relaxation to Ca2+ (EC50 = 2.6±0.3mM, Rmax = 30±18%, P<0.01). By comparison, smaller Ca2+-induced relaxations and a smaller calhex231-sensitive component were seen in femoral arteries (control, EC50 = 2.7±0.3mM, Rmax = 42±5%; +calhex231, EC50 = 2.9±0.2mM, Rmax = 37±4%; P<0.05). Interestingly, femoral relaxation to Ca2+ was attenuated by ruthenium red but potentiated by capsaicin (control, EC50 = 2.5±0.3mM, Rmax = 40±6%; +10uM ruthenium red, EC50 = 2.8±0.6mM, Rmax = 29±10%, P<0.01; +10uM capsaicin, EC50 = 2.3±0.1mM, Rmax = 58±6%, P<0.01). On the other hand, in intrapulmonary artery, increasing [Ca2+]o predominantly resulted in small contraction, which was enhanced by calhex231 (at 3mM, control, -15±10%; +calhex231, -49±5%; P<0.01), perhaps due to an underlying, CaR-mediated relaxation. However, capsaicin treatment had no significant effect (+capsaicin, at 3mM, -5±11%). On the other hand, the relaxant effects of Ca2+ were mirrored by those of calcimimetics in the different vascular regions; with rank order of potency and efficacy, mesenteric>femoral>>pulmonary artery (data not shown). In fact, in intrapulmonary artery, calindol caused contraction (-36±6%), followed by relaxation at higher contractions (31±12%). To conclude, the vascular action of Ca2+ and calcimimetics differ greatly depending on the vascular regions. Our data also question the role of CaR and capsaicin-sensitive nerves in Ca2+-induced relaxation in regions beyond the mesenteric circulation.
Wang Y & Bukoski RD (1998). Br J Pharmacol 125: 1397.
Thakore P & Ho WS (2011). Br J Pharmacol 162: 749