Vasopressin stimulates release of ?-lipotropin and ?-endorphin in conscious rats as measured by radioimmunoassay of unextracted plasma

Abstract

Using a newly developed radioimmunoassay to determine the β-endorphin-like immunoreactivity (β-EI) in unextracted plasma, the effect of vasopressin injections on plasma β-EI was investigated in conscious rats. Arginine vasopressin caused a dose-dependent increase of plasma β-EI from 34.5±7.8 fmol ml⁻¹ (n=6) in vehicle-treated animals to 205.0±36.1 fmol ml⁻¹ (n=7) after injection of the highest vasopressin dose employed (486 ng/100 g b.w.). In view of the appreciable cross-reactivity of β-lipotropin (β-LPH) in the radioimmunoassay used, plasma was extracted and subjected to gel chromatography on a Sephadex G-50 column. On average, about 70% of the β-EI co-eluted with human β-LPH and about 30% with human β-endorphin in plasma extracts obtained from both control and vasopressin-treated rats. No peripheral conversion of human β-LPH occurred under the experimental conditions, since after i.v. bolus injection of human β-LPH 97% of the β-EI comigrated with human β-LPH during gel filtration. A similar blood pressure increase to that induced by the vasopressin injections, when elicited by noradrenaline or angiotensin II i.v., was not followed by an elevation of plasma β-EI. These data indicate that vasopressin stimulates β-lipotropin and β-endorphin release into the systemic circulation in vivo.