Calcium transport in canine renal basolateral membrane vesicles. Effects of parathyroid hormone

The effects of parathyroid hormone were studied on Ca²⁺ fluxes in canine renal proximal tubular basolateral membrane vesicles (BLMV). Efflux of Ca²⁺ from preloaded BLMV was found to be stimulated by an external Na⁺ gradient, and this was inhibited by the Na⁺ ionophore, monensin, and enhanced by intravesicular negative electrical potentials, which indicated electrogenic Na⁺/Ca²⁺ exchange activity. There was a Na⁺ gradient independent Ca²⁺ flux, but membrane binding of Ca²⁺ was excluded from contributing to the Na⁺ gradient-dependent efflux. The Na⁺ gradient-dependent flux of Ca²⁺ was very rapid, and even 2- and 5-s points may not fully represent absolute initial rates. It was saturable with respect to the interaction of Ca²⁺ and Na⁺ with an apparent (5 s) K(m) for Na⁺-dependent Ca²⁺ uptake of 10 μM, and an apparent (5 s) V(max) of 0.33 nmol/mg protein per 5 s. The Na⁺ concentration that yielded half maximal Ca²⁺ efflux (2 s) was 11 mM, and the Hill coefficient was two or greater. Both Na⁺ gradient dependent and independent Ca²⁺ efflux were decreased in BLMV prepared from kidneys of thyroparathyroidectomized (TPTX) dogs, and both were stimulated by parathyroid hormone (PTH) infusion to TPTX dogs. BLMV from TPTX dogs exhibited significantly reduced maximal stimulation of Na⁺ gradient-dependent Ca²⁺ uptake with an apparent (5 s) V(max) of 0.23 nmol/mg protein per 5 s, but the apparent K(m) was 8 μM, which was unchanged from normal. The Na⁺ gradient independent Ca²⁺ uptake was also reduced in BLMV from TPTX dogs compared with normal. Thus, PTH stimulated both Na⁺/Ca²⁺ exchange activity and Na⁺ independent Ca²⁺ flux. In vivo, the latter could result in an elevation of cytosolic Ca²⁺ by PTH, and this might contribute to the observed decrease in solute transport in the proximal tubule.