The effect of isoprenaline on plasma concentrations of immunoreactive ?-endorphin and ?-lipotropin in the conscious rat

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Abstract
The effect of the β-adrenoceptor agonist isoprenaline on the plasma concentrations of β-endorphin (β-E) and β-lipotropin (β-LPH) was investigated in conscious rats. Isoprenaline (i.m.) elevated plasma β-endorphin-like immunoreactivity (β-EI) as measured by radioimmunoassay of unextracted plasma, with peak values 24 min after drug administration. This effect was dose-dependent. The lowest effective dose of isoprenaline was 15 μg kg−1; 240 μg kg−1 exerted a maximum effect, raising plasma β-EI about ten-fold above control values. Plasma vasopressin concentrations also increased in response to isoprenaline following a timecourse identical to that of plasma β-EI. (±)-Propranolol (1 mg kg−1) but not phentolamine (10 mg kg−1) rendered isoprenaline (240 μg kg−1) injections almost ineffective. Because of the cross-reactivity of β-LPH in the radioimmunoassay used, plasma was extracted by means of a cation exchange resin and subjected to gel chromatography on a Sephadex G-50 column, avoiding artefactual degradation of the peptides. In isoprenaline-treated rats about 50% of the β-EI behaved similar to human β-LPH, whereas 45% co-migrated with human β-E; immunoreactivity corresponding to β-LPH or β-E comprised about 70% or 30%, respectively, in the plasma extract of vehicle-treated rats. Dexamethasone pretreatment reduced the isoprenaline-induced increase in plasma β-EI by 87%, but left the simultaneous elevation of plasma vasopressin concentrations unchanged. These data demonstrate that isoprenaline stimulates β-LPH and β-E release in vivo. The possibility of an interrelationship between vasopressin and β-E release is discussed.