Physiological response to isoproterenol and coupling of beta-adrenergic receptors in young and elderly human subjects.

Diminished adrenergic responsiveness occurs in aged humans. The purpose of this study was to evaluate the coupling of beta-adrenergic receptors from human neutrophil preparations, and to investigate the relationship of this biochemical parameter to heart rate response after isoproterenol administration in young and elderly individuals. Fifteen young (aged 23-39 years) and eleven elderly (aged 59-82 years) volunteers participated in the study. The ratio, KL/KH, representing the ratio of dissociation constants for the low and high affinity state receptors, was calculated to determine coupling in the beta-adrenergic receptor system. An inverse correlation was observed between KL/KH and age. I25HR, representing the amount of isoproterenol required to elevate heart rate by 25 beats per minute, was determined; this parameter increased with age. Multiple regression analysis revealed that I25HR was most dependent upon age, but the changes with age were accompanied by diminution of both beta-adrenergic receptor affinity for agonist and in KL/KH.