ADRENOCEPTORS OF THE GUINEA‐PIG URINARY BLADDER

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Abstract
1 Adrenaline, noradrenaline and isoprenaline (5 μg/ml) did not affect the resting tone of the isolated urinary bladder of the guinea-pig. 2 The catecholamines (1–2 μg/ml) inhibited neuronally evoked contractions at various stimulation frequencies; the inhibition was maximum at 2 Hz and minimum at 50 Hz. Isoprenaline produced maximum inhibition. 3 Propranolol (0.5 μg/ml) completely blocked the catecholamine-induced inhibition at all the frequencies employed. The concentration-response curves of isoprenaline at 2, 10 and 50 Hz were characteristically shifted by propranolol (50 ng/ml). Phenoxybenzamine (0.2 μg/ml) was totally ineffective. 4 In some experiments adrenaline significantly raised the tone of the bladder exposed to propranolol; this effect could be blocked by phenoxybenzamine. 5 Acetylcholine-induced bladder contractions were inhibited by adrenaline (2 μg/ml); the inhibition was completely blocked by propranolol (0.5 μg/ml). 6 The results indicate the presence of an inhibitory β-adrenoceptor and suggest the possibility of an excitatory α-adrenoceptor in guinea-pig urinary bladder.