Functional role of calcium-sensing receptors in the regulation of vascular tone

• Main Author: Greenberg, Harry Zvi Ephraim
• Published: St George's, University of London 2017
• Subjects: 615.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.706525

The calcium-sensing receptor (CaSR) has a well-established role in regulating Ca2+ homeostasis by controlling the release of parathyroid hormone from the parathyroid gland. CaSRs are also expressed in the vasculature where they regulate vascular tone, although the underlying cellular mechanisms are poorly understood. This thesis investigates the functional role and cellular mechanisms of CaSRs regulating vascular tone. Wire myography studies, nitric oxide imaging, western blotting, immunocytochemistry, proximity ligation assays, and patch-clamp electrophysiology experiments were performed using vessel segments and isolated endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) from rabbit and mouse mesenteric arteries. Initial experiments revealed that stimulating CaSRs by increasing the extracellular Ca2+ concentration induces endothelium-dependent vasorelaxations of precontracted arteries, mediated predominantly via nitric oxide (NO) signalling, which stimulates BKca channels in VSMCs. In vessels lacking a functional endothelium, CaSR activation mediates vasoconstrictions. CaSR expression was also demonstrated in isolated ECs and VSMCs. Further experiments revealed that CaSR-evoked NO synthesis and NO-mediated vasorelaxations involve the activation of a heteromeric cation channel composed of transient receptor potential vallinoid 4 (TRPV4) and transient receptor potential canonical 1 (TRPC1) proteins expressed in ECs. As a result, CaSR-induced vasorelaxations mediated by NO are impaired in TRPCT/_ mice lacking this channel. Additional studies revealed that stimulation of CaSRs also evokes vasorelaxations via the activation of IKca channels in ECs, and although cation channels are involved in mediating CaSR-IKca signalling, these channels are different to those involved in activating NO production. CaSR-evoked vasorelaxations were also revealed to be confined to larger diameter mesenteric artery branches. Finally, the vascular actions of synthetic CaSR modulators were examined as these have previously been insufficiently understood. The current study reveals that the negative CaSR modulators calhex-231 and NPS 2143, and the positive CaSR modulator calindol impair vascular reactivity via direct inhibition of voltage-dependent calcium channels, in a CaSR-independent manner.