Molecular properties of apelin: tissue distribution and receptor binding

We analyzed the tissue distribution of apelin mRNA in rats by a quantitativ e revere transcription-polymerase chain reaction and that of immunoreactive apelin (ir-apelin) by an enzyme immunoassay (EIA) using a monoclonal antib ody. The expression levels of apelin mRNA and ir-apelin seemed to be consis tent among tissues: they were highly expressed in the lung and mammary glan d. By the combination of gel filtration and EIA, nle found that the molecul ar forms of apelin differ among respective tissues: apelin molecules with s izes close to apelin-36 (long forms) were major components ill the lung, te stis, and uterus, but both long and short (whose sizes were close to [< Glu (65)]apelin-13) forms were detected in the mammary gland. In Scatchard anal yses, the radioiodinated apelin-36 analogue bound to the receptor. APJ, wit h high affinity. In competitive binding assays, apelin-36 and apelin-19 far more efficiently inhibited the binding of the labeled apelin-36 analogue w ith APJ than [< Glu(65)]apelin-13. In analyses for the dissociation of apel in from APJ, unlabeled apelin-36 replaced mole rapidly the labeled apelin-3 6 analogue bound with APJ than [ < Glu(65)]apelin-13. Our results demonstra te that the long and short forms of apelin differently interact with APJ. < (c)> Elsevier Science B.V. All rights reserved.