Relative importance of the calcium-sensing receptor and GPRC6A in responses to calindol in the guinea pig common carotid artery

Erika Harno1, Arthur H Weston1, Martial Ruat2, Robert H Dodd2, Gillian Edwards1
1University of Manchester, Manchester, UK, 2CNRS, Gif-sur-Yvette, France

Calindol activates the endothelial extracellular calcium-sensing receptor (CaR) and the related GPRC6A which opens IKCa channels followed by smooth muscle hyperpolarization and relaxation (Weston et al. 2005; Harno et al. 2008). The present study aimed to determine whether CaR and GPRC6A have a functional role in the guinea pig carotid artery, a typical conduit artery. Male Duncan-Hartley guinea-pigs (∼500g) or Wistar-Kyoto rats (∼300g) were used. Guinea-pig carotid artery myocyte hyperpolarizations to calindol (300nM, 0.4±0.3mV; 1μM, 3.6±0.7mV, n=4) were smaller than those in rat mesenteric arteries (300nM, 8.0±1.7mV; 1μM, 12.8±1.0mV, n=4). The GPRC6A-selective agonist, L-ornithine (300μM), induced a small hyperpolarization of guinea-pig and rat myocytes (3.0±0.8mV, n=4 and 4.5±0.3mV, n=4, respectively). In both vessels, hyperpolarizations to 300nM calindol in the presence of L-ornithine were larger than in its absence (P<0.05). In tension studies (carotid arteries pre-contracted with 1μM phenylephrine), 3μM calindol produced a small relaxation (16.0±10.8%) despite an intact endothelium (subsequent relaxation to 3μM acetylcholine, 92.5±4.0%). Western blotting detected GPRC6A but failed to detect CaR protein in carotid artery lysates. These data suggest that the CaR may be absent from the carotid artery. In rat mesenteric arteries, calindol may cause hyperpolarisation of myocytes through GPRC6A activation (Harno et al., 2008) and we thus propose that the effects of calindol in carotid arteries may be due to the activation of GPRC6A alone.