Molecular properties of apelin: tissue distribution and receptor binding

We analyzed the tissue distribution of apelin mRNA in rats by a quantitative revere transcription-polymerase chain reaction and that of immunoreactive apelin (ir-apelin) by an enzyme immunoassay (EIA) using a monoclonal antibody. The expression levels of apelin mRNA and ir-apelin seemed to be consistent among tissues: they were highly expressed in the lung and mammary gland. By the combination of gel filtration and EIA, nle found that the molecular forms of apelin differ among respective tissues: apelin molecules with sizes close to apelin-36 (long forms) were major components ill the lung, testis, 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 analyses, the radioiodinated apelin-36 analogue bound to the receptor. APJ, with high affinity. In competitive binding assays, apelin-36 and apelin-19 far more efficiently inhibited the binding of the labeled apelin-36 analogue with APJ than [< Glu(65)]apelin-13. In analyses for the dissociation of apelin from APJ, unlabeled apelin-36 replaced mole rapidly the labeled apelin-36 analogue bound with APJ than [ < Glu(65)]apelin-13. Our results demonstrate that the long and short forms of apelin differently interact with APJ. <(c)> Elsevier Science B.V. All rights reserved.