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

Abstract

The glycopeptide corresponding to the C-terminal portion of the vasopressin precursor (CPP) h a s 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 w a s stimulated five-fold by high K + in a C a 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 320k89 fmol/ml (SEM, n=6). In anaesthetized guinea-pigs, intraperitoneal injections of increasing amounts 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 Cerebrospinal fluid withdrawn from the cisterna magna also contained detectable amounts of CPP (390 k 2 5 fmol/ml) suggesting that CPP is released from both hypophysial and extrahypophysial projections. This assay may now be used to study the biosynthesis, processing and release of endogenous CPP under different physiological conditions. The presence of glycosylated substances in the neurohypophysial systcm of a number of species has been demonstrated by a variety of techniques including in vivo incorporation of radiolabelled sugars, chemical and biochemical analysis of pituitary extracts (1)(2)(3)(4)(5). Although closely related amino-acid sequences of a 39-residue pituitary glycopeptide had been deduced for a number of species (4, 5), it was not until the structure of the vasopressin (AVP) precursor was elucidated from its cDNA sequence (6) that this pituitary glycopeptide was recognised as the C-terminal domain of the AVP precursor. linked to the C-terminus of neurophysin (NP) by a single basic residue. Since similar studies of the oxytocin (OT) precursor failed to show an equivalent glycopeptide moiety (7, it appeared that this glycopeptide is only Produced in conjunction with AVP, and immunohistochemical studies confirmed that glycopeptide immunoreactivity was indeed expressed in the AVP, but not OT, neurons in the hypothalamus Although these biochemical and immunocytochemical studies suggest that the glycopeptide is co-localized with AVP, to date there is little direct evidence to show that this glycopeptide is actually released under physiological conditions. Such studies would require a highly sensitive and specific RIA, and whilst antisera have been raised against sheep and human material, sequence comparisons reveal a much higher degree of homology between the glycopeptides from the larger animal species...

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