Abstract
Isolated aortic strips from rabbits older than 6 months (1.7-2.5 kg) when subjected to contraction by noradrenaline or adrenaline were relaxed by isoprenaline. The relaxation was not blocked by propranolol. When these strips were contracted by potassium chloride, isoprenaline produced a small relaxation which was sensitive to propranolol. In aortic strips from rabbits 2-3 months old (0.75-0.95 kg), the contraction produced by noradrenaline or potassium chloride was relaxed by isoprenaline; isoprenaline was equally active against both stimulants. Propranolol antagonised the action of isoprenaline competitively, with a pA2 value, 7.05. In strips from rabbits from a lower age group, the pA2 values of phentolamine against isoprenaline and methoxamine contractions were 7.61 and 7.50 respectively. It is concluded that aortas of rabbits older than 6 months are almost devoid of beta adrenergic receptors whereas those of rabbits 2-3 months old contain both alpha and beta adrenergic receptors.
