Corticosteroid Binding Globulin (CBG) is a relevant plasma carrier protein for cortisol/corticosterone. The protein is believed to keep the steroids inactive and to define the amount of free hormone acting on target tissues (free hormone hypothesis). Previous findings have shown that the delivery of corticosterone to peripheral tissues is insufficient in CBG^-/- mice despite very high concentrations of free corticosterone [1]. Since glucocorticoids synergistically enhance the effect of aldosterone on colonic ion transport [2] we hypothesized that the CBG^-/- mice show altered responsiveness to aldosterone.

Methods: 

Here, we used CBG deficient mice and their wild-type littermates to test its contribution on aldosterone-regulated ion transport. An Ussing chamber was used to measure luminal amiloride-sensitive Na⁺ absorption in freshly isolated distal colonic mucosa. It is well established that aldosterone up-regulates ENaC-mediated amiloride-sensitive colonic Na⁺ absorption. Here we use the amiloride-sensitive short circuit current (DI_sc(amil)) as a functional measure of aldosterone action. Total and free plasma aldosterone levels were measured with by RIA. A low sodium diet for 3 weeks was used to augment endogenous aldosterone and colonic Na⁺ absorption.

Results: 

No functional differences were observed in DI_sc(amil) or aldosterone levels in animals on control diet (resting DI_sc(amil):CBG^+/+: 6.3 1.0 and CBG^-/-: 6.0 1.7 mA cm^-2 n=8). Free and total aldosterone levels were: CBG^+/+: free 158.3 44.9 pg ml^-1 (n=4) and total 498.1 136.2 pg ml^-1 (n=10) and CBG^-/-: free 159.3 27.7 pg ml^-1 (n=3) and total 299.4 126.9 pg ml^-1 (n=8). When Na⁺ restricted, both genotypes up-regulated the DI_sc(amil). In CBG^+/+ up-regulation was 25-fold, contrasting to a much smaller degree (13-fold) in CBG^-/- tissue (CBG^+/+: 159.2 27.4, n=6 and CBG^-/-: 77.6 19.6 mA cm^-2, n=8). Interestingly, in CBG^+/+ mice the associated increase of aldosterone was only 2-fold (both free and total), whereas the CBG^-/- showed a 4-fold increase in aldosterone (CBG^+/+: free 329.1 71.0 pg ml^-1, n=7 and total 855.3 147.1 pg ml^-1, n=7 and CBG^-/-: free 655.8 93.7 pg ml^-1, n=5 and total 1482.0 247.4 pg ml^-1, n=14).

Conclusion: 

Thus, despite a dramatic increase of aldosterone in CBG^-/- mice on low Na⁺ diet the functional response in the aldosterone-sensitive target tissue (distal colon) remains strongly insufficient. Our data suggest that intact CBG is a necessary factor for aldosterone-responsiveness in the respective tissues.

References: [1] Petersen H. H. et al. Hyporesponsiveness to glucocorticoids in mice genetically deficient for the corticosteroid binding globulin Mol. Cell Biol. (2006) [2] Bastl C.P. et al. Role of glucocorticoids and aldosterone in maintenance of colonic cation transport Am. J. Physiol (1980).