The Contraction Mechanisms for Norepinephrine in Single Cells Prepared from Tracheal Smooth Muscles of Guinea Pig of Different Ages

Abstract

Single cells were prepared from guinea pig tracheal smooth muscle and used in an experiment on the contraction mechanisms for norepinephrine and a study on the change of α₂-adrenoceptors with age. Specific bindings of [³H]p-aminoclonidine to the single cells from the tracheal smooth muscles of 6- and 40-week-old guinea pigs were saturable and analyzed by Scatchard plot. The maximum number of [³H]p-aminoclonidine binding sites was larger in the preparation from 40-week-old guinea pigs than that from 6-week-old animals, while its dissociation constant did not change with age. The amount of prostaglandin F_2α released from the single cells was increased by norepinephrine, not affected by phenylephrine, and reduced by an α₂-antagonist such as yohimbine. The amount released by norepinephrine was significantly larger in the preparation from 6-week-old guinea pigs than that from 40-week-old animals. These results suggest that the age-related decrease in the potency of norepinephrine is due to reduction in the amount of excitable prostaglandin F_2α released by the drug, but not to a change in the total amount of α₂-adrenoceptors or the dissociation constant of the drug with respect to these adrenoceptors. Furthermore, α₂-adrenoceptors in the tracheal smooth muscle cell play an important role in the release of prostaglandin F_2α and the production of contractile responses of these muscles.