CALBINDIN-D-28K FACILITATES CYTOSOLIC CALCIUM DIFFUSION WITHOUT INTERFERING WITH CALCIUM SIGNALING

The role of calbindin-D-28K, in transcellular Ca2+ transport and Ca2signaling in rabbit cortical collecting system was investigated. Rabbi t kidney connecting tubules and cortical collecting ducts, hereafter r eferred to as cortical collecting system, were isolated by immunodisse ction and cultured to confluence on permeable filters and glass covers lips. Calbindin-D-28K was present in the cytosol of principal cells, b ut was absent from the intercalated cells. 1,25(OH)(2)D-3 (48 h, 10(-7 ) M) significantly increased cellular calbindin-D-28K levels (194 +/- 15%) and stimulated transcellular Ca2+ transport (41 +/- 3%). This sti mulatory effect could be fully mimicked by the endogenous Ca2+ chelato r, BAPTA (30 mu M BAPTA/AM), which suggests that the presence of Ca2chelators alone is sufficient to enhance transcellular Ca2+ transport. Stimulation of Ca2+ transport was not accompanied by a rise in [Ca2+] (i). Isosmotic replacement of extracellular Na+ ([Na+](0)) for N-methy lglucamine (NMG) generated oscillations in [Ca2+](i) in individual cel ls of the monolayer. The functional parameters of these oscillations s uch as frequency of spiking, resting [Ca2+](i), increase in [Ca2+](i) and percentage of responding cells, were not affected by the level of calbindin-D-28K. In contrast, loading the cells with BAPTA abruptly st opped these [Ca2+](i) oscillations. This suggests that the kinetics of Ca2+ binding by calbindin-D-28K are slow relative to the initiation o f the [Ca2+](i) rise, so that calbindin-D-28K, unlike BAPTA, is unable to reduce [Ca2+](i) rapidly enough to prevent the initiation of Ca2+- induced Ca2+ release.