RELATIONSHIP BETWEEN EXTRACELLULAR AND INTRACELLULAR CALCIUM IN DISTAL SEGMENTS OF THE RENAL TUBULE - ROLE OF THE CA2+ RECEPTOR RAKCAR

The effect of extracellular calcium ([Ca2+](e)) on cytosolic calcium ( [Ca2+](i)) was investigated in thick ascending limbs and collecting du cts from the rat kidney, using the fluorescent dye fura-2. In cortical collecting ducts, basolateral but not apical changes in [Ca2+](e) wer e associated with parallel changes in [Ca2+](i). Basal [Ca2+](i) was h ardly modified by nifedipine and verapamil but was decreased by 60% by basolateral La3+. Increasing peritubular [Ca2+](e) triggered Ca2+ rel ease from intracellular stores. This effect was not reproduced by agon ists of the renal Ca2+-receptor RaKCaR, e.g., Ba2+, Mg2+, Gd3+, and ne omycin, but was reproduced by Ni2+. Ni2+-induced mobilization of intra cellular Ca2+ was larger in the inner medullary collecting duct, a seg ment which poorly responds to increasing [Ca2+](e). In the cortical th ick ascending limb, removing basolateral Ca2+ hardly altered [Ca2+](i) but increasing [Ca2+](e) or adding Ba2+, Mg2+, Gd3+ and neomycin rele ased intracellular calcium. These data demonstrate that (1) basolatera l influx of calcium occurs in cortical collecting ducts, under basal c onditions; (2) this influx occurs through nonvoltage gated channels, p ermeable to Ba2+, insensitive to verapamil and nifedipine, and blocked by La3+ (3) increasing [Ca2+](e) stimulates the influx and triggers i ntracellular calcium release, independently of the phospholipase C-cou pled receptor RaKCaR; (4) RaKCaR is functionally expressed in thick as cending limbs; (5) another membrane receptor, sensitive to Ni2+ but no t to Ca2+ is present in the collecting duct.