Calcium transport across the rabbit thick ascending limb of Henle's loop perfused in vitro

Ca²⁺ transport across the cortical thick ascending limb of Henle's loop (TALH) was studied on the isolated rabbit renal tubule perfused in vitro. Both the efflux (Ke) and influx coefficient (Ki) of Ca²⁺ were determined with⁴⁵Ca at three different levels of the transtubular electrical potential difference (PDt) caused by varying the transtubular Na⁺ concentration gradient. The flux ratios,Ke/Ki, always exceeded those predicted from observed PDt by simple passive diffusion, and the net Ca²⁺ efflux was shown to occur against an electro-chemical potential. An increase in Ca²⁺ concentration in the perfusate was associated with a decrease inKe. Efflux of Ca²⁺, therefore, tended to be saturated as luminal Ca²⁺ concentration was increased.Ke (10⁻⁷ cm²/s) decreased from 4.13±0.56 to 2.02±0.30 (P<0.02) along with a decrease in PDt when 10⁻³ M NaCN was added to the bathing fluid. By contrast, 10⁻³ M iodoacetamide did not affectKe in spite of a significant decrease in PDt. Similarly, neither 10⁻⁵ M ouabain added to the bathing fluid nor 10⁻⁴ M ruthenium red, added either to the bathing fluid or the perfusate, affectedKe despite significant decreases in PDt. Addition of 10⁻⁵ M furosemide in the lumen caused a decrease inKe from 4.31±0.43 to 3.17±0.25 (P<0.01) in association with a decrease in PDt from 6.3±1.04 to 1.7±0.37 mV. These findings suggest; (a) Ca²⁺ transport across the TALH is mainly an active process requiring aerobic metabolism; (b) dissociation of Ca²⁺ and Cl⁻ transport may occur under some experimental conditions.