The Carboxy‐Terminal Glycopeptide of the Vasopressin Precursor in the Guinea‐Pig: Release Studies Using a Specific and Sensitive Homologous Radioimmunoassay

Abstract

Abstract The glycopeptide corresponding to the C-terminal portion of the vasopressin precursor (CPP)has been isolated from guinea-pig posterior pituitary glands and used to generate a specific and sensitive radioimmunoassay. The antiserum is directed to the peptide rather than sugar moieties, and detects two components in pituitary extracts: CPP itself, and a biosynthetic intermediate (NP-CPP) containing both neurophysin and CPP sequences. Release of CPP from neurointermediate lobes incubated in vitro was stimulated five-fold by high K(+) in a Ca(2+) dependent manner: in vivo studies suggest that CPP is released under conditions stimulating vasopressin release. Chronic dehydration depleted neural lobe stores of CPP in parallel with other vasopressin-related components, and plasma levels of CPP were raised from 68+/-18 to 320 + 89 fmol/ml (SEM, n = 6). In anaesthetized guinea-pigs, intraperitoneal injections of increasingamounts of hypertonic saline increased plasma levels of CPP in a graded manner compared with isotonic saline injections. Acute haemorrhage also stimulated CPP release into plasma, and the half-time of clearance of CPP after infusion to steady state levels in guinea-pig plasma was 24 min. Cerebrospinal fluid withdrawn from the cisterna magna also contained detectable amounts of CPP (390 + 25 fmol/ml) suggesting that CPP is released from both hypophysical and extra-hypophysical projections. This assay may now be used to study the biosynthesis, processing and release of endogenous CPP under different physiological conditions.