Effect of angiotensin II on calcium reabsorption by the luminal membranes of the nephron

In the rat and the rabbit, a number of studies have reported the effects of angiotensin II (ANG II) on [Na.sup.+] reabsorption by the proximal (PT) and distal (DT) convoluted tubules of the kidney. The aim of the present study was to examine the effect of ANG II on [Ca.sup.2+] uptake by the luminal membranes of the PT and DT of the rabbit. Incubation of PT and DT with [10.sup.-12] M ANG II enhanced the initial [Ca.sup.2+] uptake in the two segments. Dose-response experiments revealed, for [Ca.sup.2+] as well as for [Na.sup.+] transport, a biphasic action with a maximal effect at [10.sup.-12] M. [Ca.sup.2+] transport by the DT luminal membrane presents a dual kinetic. ANG II action influenced the high-affinity [Ca.sup.2+] channel, increasing maximal velocity from 0.72 [+ or -] 0.03 to 0.90 [+ or -] 0.05 pmol [multiplied by] [micro][g.sup.-1] [multiplied by] 10 [s.sup.-1] (P [is less than] 0.05, n = 3) and leaving the Michaelis-Menten constant unchanged. The effect of ANG II was abolished by losartan, suggesting that the hormone is acting through [AT.sub.1] receptors. In the PT, calphostin C inhibited the effect of the hormone. It is therefore probable that protein kinase C is involved as a messenger. In the DT, however, neither Rp cAMP, calphostin C, nor econazole (a phospholipase A inhibitor) influenced the hormone action. Therefore, the mechanisms involved in the hormone action remain undetermined. Finally, we questioned whether ANG II acts in the same DT segment as does parathyroid hormone on [Ca.sup.2+] transport. The two hormones increased [Ca.sup.2+] transport, but their actions were not additive, suggesting that they both influence the same channels in the same segment of the distal nephron, i.e., the segment responsible for the high-affinity calcium channel. renal calcium transport