The apelin receptor is a class A GPCR that binds two endogenous peptide ligands, apelin and elabela/Toddler (ELA), to induce positive cardiac inotropy and vasodilatation in the cardiovascular system. The apelin receptor provides a tractable therapeutic target for multiple cardiovascular diseases, such as pulmonary arterial hypertension, heart failure, and bleeding disorders, where the receptor also mediates antithrombotic responses. Here, four novel fluorescent ligands, designed as functional analogues of endogenous apelin and ELA peptides, were validated and used as a versatile tool for the qualitative and quantitative determination of pharmacological parameters of the apelin receptor. The fluorescent ligands were also used, with other techniques, to study apelin receptor variants identified in the genomes of human patients with rare cardiovascular diseases recruited to the NIHR BRIDGE project. The V38L, T89M, and R168H variants showed distinct effects on apelin receptor pharmacology. V38L reduced receptor localisation at the membrane, but was able to bind fluorescent apelin and showed little internalisation. T89M, uniquely, was able to bind fluorescent apelin but not fluorescent ELA. R168H was unable to bind apelin receptor ligands, and was consequently inactive, pointing to loss of pharmacological function. ACE2 is a zinc-metalloproteinase type 1 transmembrane protein that cleaves angiotensin peptides to counter-regulate the renin-angiotensin-aldosterone system in the cardiovasculature and mediate hypotensive responses. The protein also acts as the membrane receptor for human coronaviruses, including SARS-CoV-2, the cause of the COVID-19 pandemic. Here, ACE2 was identified in human left ventricle tissue, and in a human embryonic stem cell-derived cardiomyocyte model that was used to screen for novel inhibitors of viral infection. Benztropine (an inhibitor targeting B0AT1 that complexes with ACE2) and DX600 (a peptide antagonist of ACE2), were shown to exhibit antiviral properties for the first time. Additionally, the tissue expression and distribution of a recently identified novel short ACE2 isoform that does not bind SARS-CoV-2, revealed enrichment of the short isoform in respiratory and bile duct epithelia. The work in this thesis advancement our understanding of both the apelin receptor and ACE2 as regulators human cardiovascular health and disease.