Abstract
Age-dependent changes in the effects of the α 1-adrenoceptor agonist, phenylephrine were investigated on neurally evoked contractile responses and basal tone in smooth muscle strips from rat urinary bladder. Phenylephrine facilitated the neurogenic contractions in both neonatal and 7-month-old adult rats. However, phenylephrine increased the basal tone in adult but not neonatal rats. In adult rats, phenylephrine-induced facilitation of neurally evoked contractions occurred before and after the block of cholinergic contractions with 1 μM atropine. In adult rats, the phenylephrine facilitation was reduced at stimulation parameters (20 Hz, 80 shocks and maximal voltage) which activated muscarinic receptor mediated facilitation of acetylcholine release. The results indicate that pre-synaptic α 1-adrenoceptors facilitate the release of both acetylcholine and the non-cholinergic non-adrenergic transmitter. In summary, α 1-adrenoceptor-mediated facilitation is less expressed when muscarinic M 1 receptor mediated facilitation is functioning; pre-junctional α 1-adrenoceptors are present in the bladder of both neonatal and adult rats, whereas post-junctional α 1-adrenoceptors are expressed only in older adult rats.
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