Possible Changes in Tissue Sensitivity in the Age-Dependent Stimulation of DNA Synthesis In Vivo

Abstract

The time required to initiate salivary gland DNA synthesis in vivo following administration of isoproterenol increases progressively from 18 to 54 hours as rats age from 2 to 24 mo. The objective of this report is to investigate the role of cellular uptake and concentration of isoproterenol in the age-dependent stimulation of DNA synthesis. Experiments were performed in male Sprague-Dawley rats purchased from the Charles River Breeding Laboratories. Synthesis of DNA was assessed, as confirmed previously, as the time course and magnitude of incorporation of 3H-thymidine into extracted DNA. Cellular interaction with isoproterenol was assessed as the time course and magnitude of appearance of radioactivity derived from 3H-isoproterenol in homogenates of submandibular gland. Depending upon the dosage of isoproterenol administered, either young or old rats can be made to appear more responsive. This may indicate functional changes in the appropriate salivary gland receptors for isoproterenol.