Inhibits the binding of prostaglandin F2-alpha (PGF2-alpha) to its specific FP receptor, by decreasing the receptor number rather than the affinity constant. Functional coupling with the prostaglandin F2-alpha receptor seems to occur (By similarity). In myoblasts, associates with tetraspanins CD9 and CD81 to prevent myotube fusion during muscle regeneration (By similarity). {ECO:0000250, ECO:0000250|UniProtKB:Q9WV91}. This is the function of PTGFRN (prostaglandin F2 receptor inhibitor, ENSG00000134247).