Effects of ageing on responses of rabbit iris smooth muscles to agonists and field stimulation

1. The pharmacological properties of contractile responses of sphincter and dilator to field stimulation did not change with age, so that innervation of both these muscles does not change apparently with age. 2. In the sphincter, no age-related change was observed in muscarinic cholinoceptor mechanisms. Tension induced by field stimulation increased with age from 5 to 13 weeks, decreased from 40 to 125 weeks and did not change thereafter. Age-related change in tension is due to change in the amount of acetylcholine released by stimulation. 3. In the dilator, the pD2 value of norepinephrine increased with age from 5 to 13 weeks, decreased from 13 to 125 weeks and did not change thereafter. The pD2 value of norepinephrine was proportional to the receptor reserve, suggesting that changes in alpha 1-adrenoceptor mechanisms are due to changes in receptor reserve. No age-related change was observed in affinity of alpha 1-adrenoceptors. 4. The tension of the dilator induced by field stimulation increased with age from 5 to 13 weeks did not change from 13 to 180 weeks. The age-related change in tension is due to change in the amount of norepinephrine released by stimulation.