Hypercalcemia Selectively Decreases Rat Inner Medullary Collecting Duct(IMCD) Vasopressin (AVP)-Elicited Water Permeability (P_f) and Aquaporin-2 (AQP-2) Water Channel Protein Content While Increasing Its Urea Permeability (P_urea) and the UT-2 Urea Channel Protein Content • 1650

Previous work (Levi et al. Kidney Inter. 23:489, 1983) identified an AVP-independent decrease in urinary concentrating ability of rats given a standard diet, ad lib water and dihydrotachysterol (DHT) (4.25 mg/kg of diet). Since increases in IMCD P_urea and P_f are elicited by AVP, we used a similar experimental design to investigate the effects of DHT-induced hypercalcemia on IMCD components of the concentrating mechanism. As compared to matched controls (C), DHT rats (n=29) exhibited significant (p<0.05) increases in serum calcium (12.6±0.1 vs 9.6±0.1 mg/dl), sodium(137.4±1.0 vs 133.5±1.0 mM) and osmolality (288±1.6 vs 280.1±2.9 mOsm/L) accompanied by a reduced urine osmolality(771.5±56 vs 1067±81 mOsm/L). The basal P_f (252±33μM/sec n=6) of isolated perfused IMCD from DHT rats did not increase significantly after AVP (10^-10 or 10^-8 M). The IMCD of C rats(n=6) exhibited a significant 7 fold increase in P_f after AVP. In contrast, IMCD P_urea (10^-5 cm/sec n=5) from DHT vs C rats revealed significant increases in both basal (91±10 vs 48±8) and AVP (139±7 vs 101±7) values. Immunoblotting of DHT vs C IMCD showed a significant reduction in AQP-2 (87±4% n=4) protein but a 41±11% (n=5) increase in UT-2 content. Since our previous data (Sands, et al. J. Clin. Invest. In press. 1997) show that increases in IMCD intraluminal Ca²⁺ reduce AVP-elicited P_f, probably via an activated apical calcium/polyvalent cation receptor protein (CaR), we examined whether this CaR protein was altered in IMCD of DHT vs C rats. Immunoblotting with CaR antiserum reveals a marked shift of CaR bands to form a distinct high molecular weight complex in DHT not present in C rats. We conclude that AVP-elicited IMCD urinary concentrating components in DHT rats undergo a reduction in P_f and AQP-2 expression but an increase in P_urea and UT-2 expression as compared to normocalcemic C rats. These changes in IMCD transepithelial transport are accompanied by a specific alteration in CaR protein. These data provide support for a role of these adaptive responses in preventing renal stone formation in sustained hypercalcemia.