Characterization of the rat colonic aldosterone receptor and its activation process

Aldosterone increases sodium absorption, short circuit current, and transmural potential difference in rat colon. We studied the rat colonic aldosterone receptor using the synthetic glucocorticoid, 11 beta, 17 beta-dihydroxy-17 alpha-propynylandrosta-1,4,6-triene-3-one, to prevent binding to the glucocorticoid receptor. Specific aldosterone binding was found in proximal and distal colon. Heating to 25 degrees C decreased binding within 15 min, but the protease inhibitor, phenylmethylsulfonyl fluoride, stabilized binding. Binding was highest in terminal distal colon. Competitive binding assay showed aldosterone specificity compared to other competitors was greater at 30 than at 4 degrees C, suggesting temperature-sensitive changes in receptor specificity. Scatchard analysis revealed a straight line with a KD of 2.5 nM at 0 degrees C and 4.1 nM at 30 degrees C. Bmax was higher in distal than in proximal colon (30 degrees C, 156 +/- 33 versus 65 +/- 9 fmol/mg protein) and increased by 36% in proximal and 180% in distal colon at 30 degrees C compared to 0 degrees C. DEAE-cellulose chromatography of unactivated receptor demonstrated a single peak eluting at 200-250 mM KCl. Heat, ATP, and gel filtration did not activate the receptor, whereas increasing cytosolic salt concentration to 300 mM KCl, raising the pH to 8, or adding EGTA and EDTA caused increased DNA-cellulose binding and a new peak eluting at 30-80 mM KCl on DEAE-cellulose chromatography. There is a specific aldosterone receptor in colon with increasing number of binding sites from proximal to most distal segments paralleling aldosterone's physiological effects. Absence of receptor activation with heat, gel filtration, or ATP suggests differences between activation of the aldosterone receptor and other steroid hormone receptors.