Unidirectional calcium flux (J(ca)) in the superficial pars recta and thin descending limb of Henle (DLH) was examined by the isolated tubule microperfusion technic using 45Ca as the isotopic tracer. In the pars recta sequential measurements of lumen-to-bath flux (J(ca)(lb)) and bath-to-lumen flux (J(Ca)(bl)) revealed: J(Ca)(lb) 22.4 ± 4.18, J(Ca)(bl) 7.97 ± 1.95, and calculated net efflux of calcium (J(Ca)(net)) 13.0 ± 1.74 peq min-1 mm-1. To measure J(Ca)(net) directly, 45Ca of identical specific activity was used to bathe and perfuse the tubule. These studies revealed: J(Ca)(lb) 14.1 ± 1.33, J(Ca)(net) 11.2 ± 1.15, and calculated J(Ca)(bl) 2.91 ± 0.49 peq min-1 mm-1. The addition of ouabain (10 μM) resulted in a rise in potential difference and a fall in water absorption, but not a statistically significant change in J(Ca)(net). Tubules studied at 25°C bath temperature, showed no significant J(Ca)(net), and upon heating the bath to 37°C, showed J(Ca)(net) of 3.75-5.00 peq min-1mm-1. Unidirectional and net efflux studies in six DLH showed no significant transport of calcium. These studies demonstrate substantial active absorption of calcium by the superficial pars recta, which is not inhibitable by ouabain but is inhibited by lowering bath temperature to 25°C. No significant calcium transport was found in the DLH using identical technics.
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