Effect of aging on presynaptic alpha 2-adrenoceptor mechanisms in guinea pig ileum

1. Presynaptic alpha 2-adrenoceptor mechanisms in electrically stimulated longitudinal muscles of ilea isolated from 3, 10, 20 and 47 week-old guinea pigs were studied by analysis of the concentration-response curves of noradrenaline, a full agonist, and clonidine, a partial agonist, and the Scatchard plot of specific binding of [3H]-p-aminoclonidine to synaptosomal fractions from the longitudinal muscle of guinea pig ileum. 2. The pD2 value of noradrenaline and the maximum contraction induced by clonidine increased with age from 3 to 20 weeks and there after decreased to 47 weeks, while the pA2 value of yohimbine against noradrenaline did not alter with age. 3. The capacity of the maximum binding sites of [3H]-p-aminoclonidine increased with increasing age (3-20 weeks), while the dissociation constant (Kd) of [3H]-p-aminoclonidine did not change during the same period. 4. The changes in the presynaptic alpha 2-adrenoceptor mechanisms with age are considered to be due to the change in the total concentration of presynaptic of alpha 2-adrenoceptors.