Effects of adrenergic and muscarinic receptor stimulation on serum potassium concentrations and myocardial electrical stability.

The influence of adrenergic and muscarinic receptor activation on cardiac electrical stability and on serum potassium concentrations was studied in 23 anaesthetised dogs. The ventricular fibrillation threshold was assessed using the single stimulus technique. Adrenaline (1.0 microgram X kg-1 X min-1) caused a brief rise and a subsequent prolonged fall in serum potassium concentration, which was accompanied by a decline in ventricular fibrillation threshold when baroreceptor activation was prevented. After pretreatment with the beta1 adrenoceptor blocking agent metoprolol (0.5 mg X kg-1), adrenaline did not alter vulnerability to ventricular fibrillation but still elicited hypokalaemia. In contrast, selective beta2 adrenoceptor blockade (ICI 118551, 100 micrograms X kg-1) prevented the adrenaline induced lowering of serum potassium concentration but not of ventricular vulnerability. Muscarinic receptor activation by methacholine (3.0 micrograms X kg-1 X min-1) had no effect on serum potassium concentration but increased the ventricular fibrillation threshold by 30%. When methacholine was administered concomitantly with adrenaline the decline in serum potassium concentration persisted, but the increase in ventricular vulnerability was completely prevented. It is concluded that in the normal canine myocardium adrenaline produces an increase in vulnerability that is mediated through beta 1 adrenoceptors and that the beta 2 adrenoceptor mediated hypokalaemia is dissociated from electrophysiological effects of adrenaline. Parasympathetic nervous system activation does not influence serum potassium concentrations but opposes the effects of adrenaline on susceptibility to ventricular fibrillation.