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

The effect of the beta-adrenoceptor agonist isoprenaline on the plasma concentrations of beta-endorphin (beta-E) and beta-lipotropin (beta-LPH) was investigated in conscious rats. Isoprenaline (i.m.) elevated plasma beta-endorphin-like immunoreactivity (beta-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 micrograms kg-1; 240 micrograms kg-1 exerted a maximum effect, raising plasma beta-EI about ten-fold above control values. Plasma vasopressin concentrations also increased in response to isoprenaline following a time-course identical to that of plasma beta-EI. (+/-)-Propranolol (1 mg kg-1) but not phentolamine (10 mg kg-1) rendered isoprenaline (240 micrograms kg-1) injections almost ineffective. Because of the cross-reactivity of beta-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 beta-EI behaved similar to human beta-LPH, whereas 45% co-migrated with human beta-E; immunoreactivity corresponding to beta-LPH or beta-E comprised about 70% or 30%, respectively, in the plasma extract of vehicle-treated rats. Dexamethasone pretreatment reduced the isoprenaline-induced increase in plasma beta-EI by 87%, but left the simultaneous elevation of plasma vasopressin concentrations unchanged. These data demonstrate that isoprenaline stimulates beta-LPH and beta-E release in vivo. The possibility of an interrelationship between vasopressin and beta-E release is discussed.