Activation and clearance of a hormonogen, triglycyl-oxytocin, in the conscious dog

Abstract

Triglycyl-oxytocin (TGOT) and oxytocin (OT) were administered iv and sc to conscious dogs and the plasma concentrations of these peptides determined simultaneously by RIA and bioassay. Constant iv infusions (1 h) at 23 pmol/kg/min produced plateau levels of 1.1 and 1.6 pmol/ml plasma for OT and TGOT respectively. Whilst the distribution space was similar for each peptide, TGOT persisted longer in the circulation ( t \ m=1/ 2\ , for TGOT 6.6 min; for OT 4.2 min). Bioactive values for OT followed RIA values whether this peptide was given iv or sc. Although very little bioactivity was generated by iv infusions of TGOT, considerable amounts of bioactive OT were found in plasma after injection of TGOT subcutaneously. The hormonogen proved relatively resistant to oxytocinase, an enzyme that destroys OT rapidly. Our results indicate that TGOT is converted to OT in vivo and that its action as a hormonogen is more pronounced after sc than iv administration. Small peptide hormones have a relatively short half-life in the circulation. The addition of a tripeptide extension to the amino-terminus of neurohypophysial peptides leads to a prolongation of biological action in a variety of systems (Beránková-Ksandrová et al. 1966;Kyncl & Rudinger 1970) by slow enzymic cleavage which releases the active principle and these analogues have been termed 'hormonogens' (see Rudinger et al. 1972).In the case of lysine vasopressin (LVP) the cor¬ responding analogue triglycyl-lysine vasopressin (TGLVP) has been shown to liberate LVP in both cat (Pliska et al. 1976) and man (Forsling et al. 1980) following intravenous injection. However, data on the steady state metabolic clearance of these peptides is not available. No similar studies have been performed with the corresponding oxytocin hormonogen, triglycyl-oxytocin (TGOT). In the present paper, the half-lives of oxytocin (OT) and TGOT have been compared in vivo following cessation of continuous intravenous (iv) infusions in conscious dogs. The conversion of TGOT to OT could be monitored by using radioimmunoassay and bioassay techniques (cf. Pliska et al. 1976). Materials and MethodsCannulations and sampling Male beagle dogs weighing 10-18 kg were prepared with two indwelling venous cannulae whose tips lay in the superior vena cava, the infusion cannula opening ap¬ proximately 3 cm nearer the atrium than the sampling cannula. 1.5 ml blood samples were withdrawn into heparinised syringes, immediately centrifuged and the plasma snap-frozen and stored at -20°C for subsequent assay. Full details of these procedures can be found in Stevenson et al. (1978). Administration ofpeptides Intravenous infusions at a rate of 1.12 ml/h were accom¬ plished using a miniature syringe pump (Mill Hill In¬ fuser; Parsons et al. 1977) mounted on the dog's back. Oxytocin (Batch No. 750919 and triglycyl-oxytocin, (Naglycyl-glycyl-glycyl-oxytocin ; Batch No. 720512) gene¬ rously provided by Dr. H. Vilhardt, Ferring AB, Malmö, were dissolved in 0.9% saline acidified to pH 4 with HC1and diluted before injection wi...