Significant levels of estrogen and androgens circulate in men and women, and both play an important role in bone metabolism. While it is well established that either estrogen or androgen replacement therapy is effective at ameliorating bone loss associated with hypogonadism, recent evidence nevertheless suggests that estrogen and androgens have distinct molecular actions on the skeleton. In this study, we have employed normal rat calvarial osteoblast cultures to characterize relative expression profiles of estrogen (ER and ER ) and androgen receptors (AR) during osteoblast differentiation. Normal osteoblast cultures can proceed through in vitro differentiation with distinct stages of proliferation, matrix maturation and mineralization in the appropriate differentiation medium containing ascorbic acid. Expression profiles of AR, ER and ER in primary cultures during osteoblast differentiation were characterized both by semi-quantitative relative RT-PCR and by Western analysis. In cultures induced to differentiate by growth in the presence of ascorbic acid, the expression profile for each receptor was unique during the course of differentiation. ER levels were elevated during matrix maturation and then declined during mineralization. ER expression was relatively constant throughout differentiation, exhibiting more constitutive expression. In contrast, AR levels were lowest during proliferation, and then increased throughout differentiation with highest levels in the most mature mineralizing cultures. Since steroid hormone action is generally mediated by specific cognate receptors, these results suggest that androgen actions may target cells during the mineralization stage of osteoblast differentiation, while estrogen action through either receptor isoform is more likely to affect osteoblasts earlier during matrix maturation. Interestingly, sex steroid receptor expression profiles did not exhibit the same patterns of regulation if osteoblast cultures were grown without ascorbic acid in medium that did not support extracellular matrix deposition. Thus, sex steroids may distinctly influence skeletal health by differential modulation of function during osteoblast differentiation.

The proximal and distal segments of the colon are known to differ considerably in their ion transport functions and their responses to Na+ depletion. We examined the distribution of absorptive ion transporters in surface epithelial cells (SEC) of the cecum and colon (proximal, middle, distal) by confocal IF microscopy and Western blot of female CD‐1 mice maintained on a standard or low‐Na+ diet for 8 days. Mucosal layers were probed with antibodies selective for H,K‐ATPase‐α2 (H,K), K‐Cl cotransporter‐3 and ‐4 (KCC3, KCC4) and anion exchangers AE1, AE2, and DRA. H,K was restricted to the apical margin of SEC in the mid and distal colon, and its abundance increased >15‐fold with Na+ deprivation. Apical DRA and basolateral KCC3 and KCC4 expression increased distally along the colonic axis with augmented expression (~40%) in the mid to distal colon of Na+ restricted mice. Cecal SEC also highly expressed DRA, KCC3 and KCC4, and Na+ deprivation strongly reduced expression. AE1 was highly expressed in the distal colon and AE2 in the proximal colon; Na+ deprivation diminished expression of both these transporters. Our findings implicate coordinate expression of apical H,K and DRA together with basolateral KCC3/4 in distal colon transport (KCl absorption and/or NH4+ secretion), and establish the cecum as a target of divergent aldosterone control.