Received for publication April 4, 2006.
Revised May 24, 2006.
Accepted for publication May 31, 2006.

Induction of rat intestinal p-glycoprotein by spironolactone and its effect on absorption of orally administered digoxin

Abstract

The effect of the diuretic spironolactone (SL) on expression and function of intestinal P-glycoprotein (P-gp), and its impact on intestinal absorption of digoxin, were explored. Rats were treated with daily doses of 200 µmol/kg b.w. of SL, i.p., for 3 consecutive days. The small intestine was divided into 4 equal segments of ~25 cm, with segment I being the most proximal. Brush border membranes were isolated and used in analysis of P-gp expression by western blotting. P-gp content increased in SL group by 526, 292, 210 and 622% over Controls for segments I, II, III and IV, respectively. Up-regulation of apical P-gp was confirmed by immunofluorescence microscopy. P-gp transport activity was explored in intestinal sacs prepared from segment IV, using two different model substrates. Serosal to mucosal transport (efflux) of rhodamine 123 was 140% higher and mucosal to serosal transport (absorption) of digoxin was 40% lower in SL group, both indicating increased P-gp function. In vivo experiments demonstrated that intestinal absorption of a single dose of digoxin administered orally was attenuated by SL pretreatment. Thus, concentration of digoxin in portal and peripheral blood was lower in SL vs Control groups, as well as its accumulation in kidney and liver. Urinary excretion of digoxin was significantly decreased in SL group, likely reflecting decreased systemic availability of digoxin for subsequent urinary elimination. We conclude that SL induces P-gp expression with potential impact on intestinal absorption of substrates with therapeutic application.

Key words: P-glycoprotein, digoxin, induction, intestine, rhodamine 123, spironolactone