Effects of parathyroid hormone and N6,O2′-dibutyryl cyclic AMP on Ca2+ transport across the rabbit distal nephron segments perfused in vitro

Effects on Ca²⁺ transport of parathyroid hormone (PTH) and N⁶,O²-dibutyryl adenosine 3,5-cyclic monophosphate (DB-cAMP) were examined in the rabbit distal nephron segments including the cortical thick ascending limb of Henle's loop (CAL), the connecting tubule (CNT) and the cortical collecting tubule (CCT) by the in vitro perfusion technique. When PTH (10^–8 mol·l^–1) was added to the bath, efflux of Ca²⁺ (pmol·mm^–1·min^–1) was increased from 6.29±1.46 to 7.96±1.66 (P<0.02) in the CAL, and from 8.55±1.30 to 13.73±1.24 (P<0.001) in the CNT, respectively, without changes in influx of Ca²⁺. The effect of PTH on Ca²⁺ transport in the CAL, however, was abolished when phosphate concentration in the medium was reduced from 3.0 to 1.0 mmol·l^–1. When DB-cAMP (10^–3 mol·l^–1) was added to the bath, efflux of Ca²⁺ was also increased from 7.01±0.83 to 9.40±0.82, (P<0.05) in the CAL, and from 13.11±0.89 to 19.74±0.52 (P<0.005) in the CNT, respectively. By contrast, neither PTH nor DB-cAMP affected efflux of Ca²⁺ in the CCT. PTH did not affect the transepithelial voltage either in the CAL or in the CNT. But in the CNT, DB-cAMP decreased the voltage from –14.1 to –9.4 mV. The response of adenylate cyclase activity to PTH in the collagenase treated isolated nephron segments was also examined. Significant increases in adenylase cyclase activity were observed in the CAL as well as in the CNT with 10^–6 mol·l^–1 PTH. These data indicate that PTH stimulates Ca²⁺ transport across the CNT probably via activation of the adenylate cyclase-cyclic AMP system. The hormone may also stimulate Ca²⁺ transport across the CAL in a special condition where plasma phosphate concentration is elevated.