Influence of isoprenaline and salbutamol on the threshold of electrically induced ventricular fibrillation in anaesthetized guinea pigs.

Fibrillation threshold and heart rate were measured after increasing doses of (-)-isoprenaline and (plus or minus)-salbutamol. The initial values of 212 plus or minus 10 muA (x plus or minus SEM) and 211 plus or minus 10muA were decreased to 74 plus or minus 10 muA by low doses of (-)-isoprenaline (10-11- to 10-10 moles/kg i.v.), and to 89 plus or minus 11 muA by (plus or minus)-salbutamol (10-9- to 10-8- moles/kg i.v.). Higher doses, paradoxically, increased fibrillation threshold to initial values, (-)-isoprenaline: 2x10-9- moles/kg; (plus or minus)-salbutamol: 2x10-6- moles/kg. A linear increase in heart rate per 10-fold increase of either drug was observed, (-)-isoprenaline: 25 beats - min-1-; (plus or minus)-salbutamol: 14 beats - min-1-. The apparent beta2-selective property of salbutamol is documented by its low potency in changing fibrillation threshold and heart rate.