Age-dependent decrease in the hepatic uptake and biliary excretion of ouabain in rats

Abstract

The biliary excretion of i.v. injected ouabain was examined in male and female Wistarderived rats in relation to age. The hepatic uptake velocity for ouabain was also determined in isolated hepatocyte preparations obtained from male rats of various ages. Biliary recovery values of ouabain (percent of the dose) were fairly comparable for young male and female rats (3–4-month old). Recovery progressively decreased with age, the first 10-min recoveries at 24 months being about one-third those of respective young values in both sexes. A significant linear relation was demonstrated between the first 10-min recovery (Y, percent of the dose) and rat age (X, month), yielding the relations of Y = 17.75 − 0.43X for males and Y = 18.99 − 0.43X for females respectively. Similarly, the initial uptake velocity (Y, nmol/mg/min) for ouabain decreased in a linear fashion with age (X, month), yielding a significant negative correlation (Y = 0.704 − 0.0021X, r = −0.839, P < 0.005, N = 21) at an ouabain concentration of 8 μM. Kinetic studies using non-linear regression analysis revealed a significantly lower V max value (0.533 ± 0.041 nmol/mg/min) in old (24–29 months) rats compared to the young (4–4.5 months) value (1.193 ± 0.105 nmol per mg/min, P < 0.05), while the affinity constant ( K m , μM) did not differ significantly between young and old animals (203.12 ± 25.42 μM in young rats vs 283.68 ± 28.90 μM in old rats, mean ± SE, 0.05 < P < 0.1). The results of the present study suggest that the age-dependent decrease in the biliary recovery of i.v. injected ouabain in rats can be largely explained by the decrease with age in the hepatic uptake of ouabain. Furthermore, the results provide further support for our previous thesis that the decrease in the lateral mobility of hepatocyte plasma membrane proteins, as revealed by the fluorescence recovery after photobleaching technique, may play a signfiicant role in the age-dependent decrease in the physiological function(s) of the hepatocyte plasma membrane, such as the hepatobiliary transport of ouabain.