Regulatory Mechanism of Polarized Membrane Transport by Glucocorticoid in Renal Proximal Tubule Cells: Involvement of [Ca²⁺]i

Abstract

We examined the effect of glucocorticoids on brush border membrane transporters and, furthermore, the involvement of Ca²⁺ in its action in the primary cultured rabbit renal proximal tubule cells (PTCs). Dexamethasone (DEX, 10^-9 M) decreased Pi uptake by 17%; whereas DEX affected neither α-methyl-glucopyranoside (α-MG) uptake nor Na⁺ uptake. The DEX-induced inhibition of Pi uptake was due to a decrease of V_max. In contrast, other steroid hormones such as progesterone, testosterone, and 17β-estradiol (10^-9 M) did not induce inhibition of Pi uptake. In order to examine the involvement of Ca²⁺ in DEX-induced inhibition of Pi uptake, PTCs were treated with A 23187 (10^-6 M, Ca²⁺ ionophore). A 23187 also inhibited Pi uptake, mimicking DEX action in Pi uptake. Treatments with W-7 (10^-4 M, calmodulin dependent kinase inhibitor), KN-62 (10^-6 M, Ca²⁺/calmodulin-dependent protein kinase II inhibitor), and BAPTA/AM (10^-6 M) or TMB-8 (10^-4 M) (intracellular Ca²⁺ mobilization blockers) blocked the DEX-induced inhibition of Pi uptake. However, nifedifine, methoxyverapamil (10^-6 M, L-type Ca²⁺ channel blockers), and EGTA (1 mM, extracellular Ca²⁺ chelator) did not block it. In conclusion, DEX inhibited Pi uptake via, in part, Ca²⁺/calmodulin pathway mediated by intracellular Ca²⁺ mobilization in the PTCs.