ANP(5-28) is the major molecular species in hypophysial portal blood of the rat.

We have previously demonstrated that the concentrations of immunoreactive atrial natriuretic peptide (IR-ANP) are significantly higher in hypophysial portal compared with peripheral blood of the rat, and that ANP suppresses the pituitary release of ACTH and beta-endorphin in vitro and in vivo. Using HPLC, we have now shown that the predominant species of IR-ANP in extracts of portal blood from adult male and female rats is ANP(5-28), whereas in peripheral blood, ANP(1-28) predominates. The ratio of ANP(5-28) in portal compared with peripheral blood was 4.2 in male and 4.8 in female animals.