Ca2+ transport by the luminal membrane of the distal nephron: action and interaction of protein kinases A and C

Characterization of three types of calcium channel in the luminal membrane of the distal nephron

We previously reported a dual kinetics of Ca2+transport by the distal tubule luminal membrane of the kidney, suggesting the presence of several types of channels. To better characterize these channels, we examined the effects of specific inhibitors (i.e., diltiazem, an L-type channel; ω-conotoxin MVIIC, a P/Q-type channel; and mibefradil, a T-type channel antagonist) on 0.1 and 0.5 mM Ca2+uptake by rabbit nephron luminal membranes. None of these inhibitors influenced Ca2+uptake by the proximal tubule membranes. In contrast, in the absence of sodium (Na+), the three channel antagonists decreased Ca2+transport by the distal membranes, and their action depended on the substrate concentrations: 50 µM diltiazem decreased 0.1 mM Ca2+uptake from 0.65 ± 0.07 to 0.48 ± 0.06 pmol·µg–1·10 s–1(P < 0.05) without influencing 0.5 mM Ca2+transport, whereas 100 nM ω-conotoxin MVIIC decreased 0.5 mM Ca2+uptake from 1.02 ± 0.05 to 0.90 ± 0.05 pmol·µg–1·10 s–1(P < 0.02) and 1 µM mibefradil decreased it from 1.13 ± 0.09 to 0.94 ± 0.09 pmol·µg–1·10 s–1(P < 0.05); the latter two inhibitors left 0.1 mM Ca2+transport unchanged. Diltiazem decreased the Vmaxof the high-affinity channels, whereas ω-conotoxin MVIIC and mibefradil influenced exclusively the Vmaxof the low-affinity channels. These results not only confirm that the distal luminal membrane is the site of Ca2+channels, but they suggest that these channels belong to the L, P/Q, and T types.Key words: renal calcium transport, calcium channels, diltiazem, mibefradil, ω-conotoxin.

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(+) 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(2+) uptake by the luminal membranes of the PT and DT of the rabbit. Incubation of PT and DT with 10(-12) M ANG II enhanced the initial Ca(2+) uptake in the two segments. Dose-response experiments revealed, for Ca(2+) as well as for Na(+) transport, a biphasic action with a maximal effect at 10(-12) M. Ca(2+) transport by the DT luminal membrane presents a dual kinetic. ANG II action influenced the high-affinity Ca(2+) channel, increasing maximal velocity from 0.72 +/- 0.03 to 0.90 +/- 0.05 pmol x microg(-1) x 10 s(-1) (P < 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 AT1 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(2+) transport. The two hormones increased Ca(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.

Roles of ATP and cytoskeleton in the regulation of Na+/H+ exchanger along the nephron luminal membrane

Although in LLC-PK cells ATP depletion has been shown to result in alterations of cytoskeleton actin and an inhibition of Na+/H+ exchanger activity, there is little information concerning the regulation of this exchanger in the distal luminal membrane by ATP and actin filaments. The present study examined the direct effect of ATP and cytochalasin B on the Na+/H+ exchanger activity in the proximal and distal tubule luminal membranes. The presence of 100 microM ATP in the luminal membrane vesicles from rabbit proximal tubules did not influence the Ethyl Isopropyl Amiloride sensitive Na+ uptake by these membranes. In contrast, the same treatment of luminal membranes from distal tubules significantly enhanced the exchanger activity from 0.22 +/- 0.04 to 0.39 +/- 0.08 pM/microg/10 sec (P < 0.02). When ATP was replaced by its nonhydrolysable form, ATPgammas, the effect on the distal luminal membrane was strongly diminished suggesting that the action of the nucleotide implicates a phosphorylation step. Confirming this hypothesis, addition of 300-microM-Rp cAMP, a protein kinase A inhibitor, completely abolished the effect of ATP. In view of the fact that a tight relationship has been described between ATP, the cytoskeleton complex and the exchanger activity, we studied the effect of cytochalasin B on this activity. The presence of 20 microM cytochalasin B in the distal luminal membrane vesicles induced, as observed with ATP, a significant increase in the Na+ uptake. However, the actions of ATP and cytochalasin B were not additive. These results suggest that firstly, ATP and short actin filaments of the cytoskeleton regulate the distal luminal isoform through an intramembranous mechanism and secondly, a phosphorylation mechanism is, at least partially, implicated in the action of ATP. In contrast, the proximal tubule exchanger is regulated through different mechanisms.

Estradiol-17beta-BSA stimulates Ca(2+) uptake through nongenomic pathways in primary rabbit kidney proximal tubule cells: involvement of cAMP and PKC

The effect of estradiol-17beta-BSA (E(2)-BSA) on Ca(2+) uptake and its related signal pathways were examined in the primary cultured rabbit kidney proximal tubule cells. E(2)-BSA (10(-9) M) significantly stimulated Ca(2+) uptake from 2 h by 13% and at 8 h by 35% as compared to control, respectively. This stimulatory effect of E(2)-BSA was not inhibited by tamoxifen (10(-8) M, an intracellular estrogen receptor antagonist), actinomycin D (10(-7) M, a transcription inhibitor), and cycloheximide (4 x 10(-5) M, a protein synthesis inhibitor). However, E(2)-BSA-induced stimulation of Ca(2+) uptake was blocked by methoxyverapamil (10(-6) M, an L-type calcium channel blocker) and 5-(N-ethyl-N-isopropyl)-amiloride (10(-5) M, a Na(+)/H(+) antiporter blocker). These results suggest that E(2)-BSA stimulates Ca(2+) uptake through nongenomic pathways. Thus, we investigated which signal pathways were related to E(2)-BSA-induced stimulation of Ca(2+) uptake. 8-Br-cAMP (10(-6) M) alone increased Ca(2+) uptake by 22% compared to control. When E(2)-BSA combined with 8-Br-cAMP, Ca(2+) uptake was not significantly stimulated compared to E(2)-BSA. SQ 22536 (10(-6) M, an adenylate cyclase inhibitor) and myristoylated protein kinase A inhibitor amide 14-22 (10(-6) M, a protein kinase A inhibitor) blocked E(2)-BSA-induced stimulation of Ca(2+) uptake and E(2)-BSA also increased cAMP generation by 26% of that of control. In addition, TPA (0.02 ng/ml, an artificial PKC promoter) stimulated the Ca(2+) uptake by 14%, and the cotreatment of TPA and E(2)-BSA did not significantly stimulate Ca(2+) uptake compared to E(2)-BSA. E(2)-BSA-induced stimulation of Ca(2+) uptake was blocked by U 73122 (10(-6) M, a phospholipase C inhibitor) or bisindolylmaleimide I (10(-6) M, a protein kinase C inhibitor). Indeed, E(2)-BSA stimulated PKC activity by 26%. In conclusion, E(2)-BSA (10(-9) M) stimulated Ca(2+) uptake by nongenomic action, which is mediated by cAMP and PKC pathways.

ATP directly enhances calcium channels in the luminal membrane of the distal nephron

Calcium (Ca(2+)) transport by the distal tubule (DT) luminal membrane is regulated by the parathyroid hormone (PTH) and calcitonin (CT) through the action of messengers, protein kinases, and ATP as the phosphate donor. In this study, we questioned whether ATP itself, when directly applied to the cytosolic surface of the membrane could influence the Ca(2+) channels previously detected in this membrane. We purified the luminal membranes of rabbit proximal (PT) and DT separately and measured Ca(2+) uptake by these vesicles loaded with ATP or the carrier. The presence of 100 microM ATP in the DT membrane vesicles significantly enhanced 0.5 mM Ca(2+) uptake from 0.57 +/- 0.02 to 0.71 +/- 0.02 pmol/microg per 10 sec (P < 0. 01) in the absence of Na(+) and from 0.36 +/- 0.03 to 0.59 +/- 0.01 pmol/microg per 10 sec (P < 0.01) in the presence of 100 mM Na(+). This effect was dose dependent with an EC(50) value of approximately 40 microM. ATP action involved the high-affinity component of Ca(2+) transport, decreasing the Km from 0.08 +/- 0.01 to 0.04 +/- 0.01 mM (P< 0.02). Replacement of the nucleotide by the nonhydrolyzable ATPgammas abolished this action. Because ATP has been reported to be necessary for cytoskeleton integrity, we also investigated the effect of intravesicular cytochalasin on Ca(2+) transport. Inclusion of 20 microM cytochalasin B decreased 0.5 mM Ca(2+) uptake from 0.33 +/- 0.01 to 0.15 +/- 0.01 pmol/microg per 10 sec (P< 0.01). However, when both 100 microM ATP and 20 microM cytochalasin were present in the vesicles, the uptake was not different from that observed with ATP alone. Neither ATP nor cytochalasin had any influence on Ca(2+) uptake by the PT luminal membrane. We conclude that the high-affinity Ca(2+) channel of the DT luminal membrane is regulated by ATP and that ATP plays a crucial role in the integrity of the cytoskeleton which is also involved in the control of Ca(2+) channels within this membrane.