Caloxin 1b3: A novel plasma membrane Ca2+-pump isoform 1 selective inhibitor that increases cytosolic Ca2+ in endothelial cells

The purpose of this study was to invent an extracellular inhibitor selective for the plasma membrane Ca2+ pump(s) (PMCA) isoform 1. PMCA extrude Ca2+ from cells during signalling and homeostasis. PMCA isoforms are encoded by 4 genes (PMCA1–4). Pig coronary artery endothelium and smooth muscle express the genes PMCA1 and 4. We showed that the endothelial cells contained mostly PMCA1 protein while smooth muscle cells had mostly PMCA4. A random peptide phage display library was screened for binding to synthetic extracellular domain 1 of PMCA1. The selected phage population was screened further by affinity chromatography using PMCA from rabbit duodenal mucosa which expressed mostly PMCA1. The peptide displayed by the selected phage was termed caloxin 1b3. Caloxin 1b3 inhibited PMCA Ca2+–Mg2+-ATPase in the rabbit duodenal mucosa (PMCA1) with a greater affinity (inhibition constant = 17 ± 2 μM) than the PMCA in the human erythrocyte ghosts (PMCA4, inhibition constant = 45 ± 4 μM). The affinity of caloxin 1b3 was also higher for PMCA1 than for PMCA2 and 3 indicating its selectivity for PMCA1. Consistent with an inhibition of PMCA1, caloxin 1b3 addition to the medium increased cytosolic Ca2+ concentration in endothelial cells. Caloxin 1b3 is the first known PMCA1 selective inhibitor. We anticipate caloxin 1b3 to aid in understanding PMCA physiology in endothelium and other tissues.