Effect of selective β
2-adrenergic stimulants, hexoprenaline and salbutamol, and other β-adrenergic (β) stimulants on the concentration of cyclic AMP in rat plasma and antagonistic activities of selective β
1-blockers, metoprolol and practolol, and other β-blockers were studied. The increases of cyclic AMP in the plasma occurred within 5 min, peaked between 15 and 30 min, after the intraperitoneal injection of isoproterenol, hexoprenaline, and salbutamol to rats. The potency of stimulation of cyclic AMP by β-stimulants tested at a dose of 100 μg/kg (i.p.), which was the highest dose employed, decreased in the order of hexoprenaline, isoproterenol, salbutamol, epinephrine, and norepinephrine. β-Blockers effectively inhibited the increase of cyclic AMP induced by isoproterenol or hexoprenaline when injected simultaneously with a β-stimulant. The inhibitory activities of practolol and metoprolol were less than these of propranolol and bufetolol. The dose required for 50% inhibition (ED
50) of β-blockers against isoproterenol (30 μg/kg, i.p.) was as follows : bufetolol, 10 ; propranolol, 50 ; metoprolol, 1500 ; practolol, 6000 μg/kg. ED
50 against hexoprenaline (10 μg/kg, i.p.) was 100 μg/kg for bufetolol and propranolol and >3000 μg/kg (i.p.) for practolol. These results suggest that the concentration of cyclic AMP in rat plasma may be useful for knowing the selectivity of β-blockes or stimulants.
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