Life span differences in digoxin uptake and excretion

Little is known about the underlying mechanisms for the altered susceptibility to digitalis with age. To this end, we investigated the digoxin uptake and excretion in mice and rats of different ages through the life span, including the periods of growth, maturity, and aging. Digoxin uptake by cardiac slices was linear from 0 to 15 min, with steady state occurring at 45 min. The rate in the mature 12-month mouse was significantly less than that of the senescent 30-month mouse. The kinetic parameters revealed a significant decrease in Km with a concomitant increase in Vmax during senescence. On the other hand, digoxin uptake by renal cortical slices was highest during growth, decreased to a maturation plateau and then declined further during senescence. Renal clearance and the secretory capacity for digoxin increased 30 and 62%, respectively, during growth and progressively decreased from maturity through senescence and were 59 and 77%, respectively, during aging. In summary, there was an increase in digoxin clearance and tubular activity during growth, and an increase in myocardial uptake of digoxin and a decrease in renal excretion during aging. Thus, these results may explain the clinical observations of altered susceptibility to digitalis with age.