Dose and age dependent variations in effect of tetracycline on enamel formation in rat

Incidence of rats with gross enamel lesions in response to tetracycline hydrochloride (TC) was studied in relationship to age, tooth type, and concentration of antibiotic in the injection fluid. Incidence was defined as percentage of animals with defects in at least one incisor or one molar. Serum levels were measured at various intervals in 4- and 75-day-old rats, which received a single i.p. injection with 130 mg/kg bw in concentrations of either 13 or 25 mg/ml saline. The study confirmed that the effect of TC on enamel varied with age of the animal and that the molar responded more readily than the incisor. Furthermore, the incidence was higher the higher the concentration of antibiotic. This effect appeared tied to a direct relationship between concentration and serum level. Serum levels declined more slowly in the younger than in the older rats, resulting in significantly higher levels 6, 24 and 48 h after dosing, which correlated with a more extensive labeling of the dentin in the young rats. The possible roles of serum levels, capillary supply and cell susceptibility are discussed in relation to differences in response with animal age, between tooth types, tooth surfaces and with age of the secretory cells.