Caloxin 1B1, a bioactive peptide, exhibits selective inhibition of PMCA extracellularly by binding preferentially to the extracellular domain 1 of PMCA4 over PMCA1. Identified through a screening process for its high-affinity interaction with PMCA, Caloxin 1B1 is capable of direct application to cells and tissues, facilitating research into its impact on smooth muscle and endothelium.