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

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Endocrine regulation of calcium transport in epithelia

1. Calcium (re)absorption occurs in epithelia, including the intestine, kidney, mammary glands, placenta and gills (in the case of fish). 2. Calcium is transported across epithelia by two transport mechanisms, paracellular and transcellular, and the movement is regulated by a complex array of transport processes that are mediated by hormonal, developmental and physiological factors involving the gastrointestinal tract, bone, kidney and the parathyroids. 3. Clear understanding of the calcium transport pathways and their endocrine regulation is critical for minimizing various metabolic and health disorders at different physiological stages. Here, we first briefly review the calcium transport mechanisms before discussing in detail the endocrine factors that regulate calcium transport in the epithelia.

Overexpression of regucalcin enhances its nuclear localization and suppresses L-type Ca2+ channel and calcium-sensing receptor mRNA expressions in cloned normal rat kidney proximal tubular epithelial NRK52E cells

The effect of regucalcin (RC), a regulatory protein in intracellular signaling pathway, on the gene expression of various mineral ion transport-related proteins was investigated using the cloned normal rat kidney proximal tubular epithelial NRK52E cells overexpressing RC. NRK52E cells (wild-type) and stable RC/pCXN2 transfectant were cultured for 72 h in medium containing 5% bovine serum (BS) to obtain subconfluent monolayers. After culture for 72 h, cells were further cultured 24-72 h in a medium containing either vehicle, aldosterone (10(-8) or 10(-7) M), or parathyroid hormone (PTH) (1-34) (10(-8) or 10(-7) M) without BS. RC was markedly localized in the nucleus of transfectants. Overexpression of RC caused a significant increase in rat outer medullary K(+) channel (ROMK) mRNA expression, while it caused a remarkable decrease in L-type Ca(2+) channel and calcium-sensing receptor (CaR) mRNA expressions. Overexpression of RC did not have an effect on epithelial sodium channel (ENaC), Na, K-ATPase (alpha-subunit), Type II Na-Pi cotransporter (NaPi-IIa), angiotensinogen, Na(+)-Ca(2+) exchanger, and glyceroaldehyde-3-phosphate dehydrogenase (G3PDH) mRNA expressions. Hormonal effect on gene expression, moreover, was examined. Culture with aldosterone (10(-8) or 10(-7) M) caused a significant increase in ENaC, Na, K-ATPase, and ROMK mRNA expressions in the wild-type cells. Those increases were weakened in the transfectants. Culture with PTH (10(-8) or 10(-7) M) significantly decreased NaPi-IIa mRNA expression in the wild-type cells. This effect was not altered in the transfectants. PTH significantly decreased angiotensinogen mRNA expression in the wild-type cells and the transfectants, while aldosterone had no effect. Culture with PTH (10(-8) or 10(-7) M) caused a significant decrease in L-type Ca(2+) channel and CaR mRNA expressions in the wild-type cells, while the hormone significantly increased Na(+)-Ca(2+) exchanger mRNA expression. The effects of PTH on L-type Ca(2+) channel, CaR, and Na(+)-Ca(2+) exchanger mRNA expressions were also seen in the transfectants. This study demonstrates that overexpression of RC caused a remarkable increase in its nuclear localization, and that it has suppressive effects on the gene expression of L-type Ca(2+) channel or CaR, which regulates intracellular Ca(2+) signaling, among various regulator proteins for mineral ions in NRK52E cells.

Plasma membrane Ca2+-ATPase and NCX1 Na+/Ca2+ exchanger expression in distal convoluted tubule cells

The plasma membrane Ca2+-ATPase (PMCA) and the NCX1 Na+/Ca2+ exchanger regulate intracellular Ca2+ concentrations and mediate Ca2+ efflux in absorptive epithelial cells. We characterized the PMCA isoforms and subtypes expressed in mouse distal convoluted tubule (mDCT) cells and Na+/Ca2+ exchanger protein expression in mDCT cells. In lysates of mDCT cells, immunoprecipitation and Western blot analysis, performed with a monoclonal antibody to PMCA, revealed a 140-kDa protein consistent with PMCA. Laser-scanning confocal fluorescence microscopy indicated that PMCA and NCX1 expression is restricted to basolateral membranes only in confluent mDCT cells, because subconfluent cultures predominately express intracellular localizations. PMCA isoform-specific PCR primers generated appropriately sized products only for PMCA1 and PMCA4 from DCT cells but PMCA1-4 from whole mouse kidney. Assessment of splice site C within the calmodulin-binding domain demonstrated the presence of PMCA1b and PMCA4b mRNAs in mDCT cells. Northern blot analysis of mDCT cell RNA revealed transcripts of 7.5 and 5.5 kb for PMCA1 and 8.5 and 7.5 kb for PMCA4. We conclude that DCT cells express PMCA transcripts encoding PMCA1b and PMCA4b. Basolateral localization of the Na+/Ca2+ exchanger and MCAs support the idea that multiple PMCA isoforms, in concert with the Na+/Ca2+ exchanger, mediate basal or hormone-stimulated Ca2+ efflux by distal tubules.

Physiological characterization of the Na(+)/Ca(2+) exchanger (NCX) in hepatopancreatic and antennal gland basolateral membrane vesicles isolated from the freshwater crayfish Procambarus clarkii

The purpose of this study was to physiologically characterize the basolateral Na(+)/Ca(2+) exchanger (NCX) in basolateral membrane vesicles (BLMVs) of hepatopancreas and antennal gland of intermolt crayfish. Conditions were optimized to measure Na(+)-dependent Ca(2+) uptake and retention in the BLMV including use of intravesicular (IV) oxalate and measuring initial uptake rates at 20 s. Na(+)-dependent Ca(2+) uptake rate into BLMV was temperature insensitive. Na(+)-dependent Ca(2+) uptake rate was dependent upon free Ca(2+) with saturable Michaelis-Menten kinetics determined as follows: hepatopancreas, maximal uptake rate (J(max))=2.45 nmol/mg per min, concentration at which carrier operates at half-maximal uptake rate (K(m))=0.69 microM Ca(2+); antennal gland, J(max)=13.2 nmol/mg per min, K(m)=0.59 microM Ca(2+). The two vesicle populations exhibited different sensitivity to putative NCX inhibitors. Benzamil had no effect on Na(+)-dependent Ca(2+) uptake rate in hepatopancreas; in antennal gland it was inhibitory at concentrations up to 30 microM and was stimulatory at higher concentrations. Conversely the inhibitor quinacrine was inhibitory at 10 microM in hepatopancreas and was stimulatory at 1000 microM; meanwhile it was ineffective in antennal gland BLMV. Short circuiting the BLMV had no effect on Na(+)-dependent Ca(2+) uptake rate suggesting that the process may be electroneutral. Compared with another prominent basolateral transporter in hepatopancreas the plasma membrane Ca(2+) ATPase (PMCA), the NCX has 70-fold greater J(max) (at comparable temperature) and a lower affinity. In antennal gland the NCX has 40-fold greater J(max) and a lower affinity. In hepatopancreas and antennal gland BLMV NCX appears to determine the rate of basolateral Ca(2+) efflux in intermolt.

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Age dependence of tolerance to anoxia and changes in cytosolic calcium in rabbit renal proximal tubules

Calcium(Ca2+)-dependent processes mediate, in part, anoxic cell injury. These may account for the difference in sensitivity to anoxia between certain immature and mature renal cells. To address this question, we studied the effects of anoxia on cytosolic free Ca2+ concentration ([Ca2+]i), cell integrity, and transport functions in microdissected proximal convoluted tubules (PCT) of < 3-week-old (newborn) and > 12-week-old (adult) rabbits. Tubules were loaded with 10 microM fura-2 AM by incubation for 60 min at 37 degrees C, and then superfused with isosmotic saline solution gassed with either 95%O2-5%CO2 (control group) or 95%N2-5%CO2 (anoxia group) for 30 min. [Ca2+]i was measured ratiometrically; cell damage was assessed by nuclear binding of propidium iodide (PI). Anoxia resulted in a fourfold increase in [Ca2+]i in adult tubules (from resting values of 245 +/- 10 to 975 +/- 100 nM, P < 0.001), whereas in newborn tubules the rise was significantly less (from resting values of 137 +/- 5 to 165 +/- 5 nM, P < 0.001 between anoxic groups). Transient exposure to 100 mM potassium chloride, which depolarizes the PCT cells, induced increases in [Ca2+]i from baseline, to 920 +/- 90 nM in tubules from adult and to 396 +/- 16 nM in those from newborn rabbits (P < 0.001 between age groups). After exposure to ligands such as parathyroid hormone (PTH) and ATP, [Ca2+]i increased in both newborn and adult tubules, but to lower levels in newborn tubules. The response to PTH and ATP was transient in both age groups, [Ca2+]i returning to baseline levels after 2 min. Following anoxia, tubules from adult animals exhibited staining of all cell nuclei by 1 min exposure to PI, indicative of gross permeabilization of the cells. Nuclei of anoxic immatures tubules did not stain with PI. The sodium-dependent uptakes of a glucose analogue (14C-alpha-methyl-glucopyranoside) and phosphate (32Pi) were preserved in agarose-filled tubules of newborns after anoxia, whereas in those of adults recovery from anoxia was associated with drastic reduction in the uptake of these solutes. Overall, our results suggest that: (1) during anoxia, cell Ca2+ rises to critical levels in PCTs of adults compared with those of < 3-week-old animals, (2) Ca2+ influx occurs via a pathway activated by exposure to high [K+]o, presumably voltage-sensitive Ca2+ channels or reversal of Na(+)-Ca2+ exchange, (3) these pathways are either less active or less abundant in proximal tubules of newborn compared with adult rabbits, and (4) secondary active transport activity and cellular integrity are well preserved after anoxia in PCT cells of newborn but not of adult rabbits.

Mechanisms of calcium transport across the basolateral membrane of the rabbit cortical thick ascending limb of Henle's loop

Although net Ca2+ absorption takes place in the thick ascending limb of Henle's loop, detailed mechanisms are unknown. Because it has been reported that the Ca2+ entry step across the luminal membrane is mediated by Ca2+ channels inserted by stimulation with parathyroid hormone, we studied the mechanism of Ca2+ transport across the basolateral membrane of rabbit cortical thick ascending limb (CTAL) perfused in vitro by using microscopic fluorometry of cytosolic Ca2+ ([Ca2+]i) with fura-2. The resting [Ca2+]i in this segment was 49.8 +/- 4.5 nmol/l. Neither Na+ removal from the bathing solution nor addition of ouabain (0.1 mmol/l) to the bath increased [Ca2+]i, indicating that a Na+/Ca2+ exchanger in the basolateral membrane may not contribute in any major way to [Ca2+]i of CTAL. To confirm our technical accuracy, similar protocols were conducted in the connecting tubule, where the existence of a Na+/Ca2+ exchanger has been reported. In this segment, Na+ removal from the bath increased cell Ca2+ from 148.6 +/- 6.4 nmol/l to 647.6 +/- 132.0 nmol/l, confirming the documented fact. [Ca2+]i in the CTAL was markedly increased when 1 mmol/l NaCN was added to the bath in the absence of glucose. Calmodulin inhibitors (trifluoperazine or W-7) increased [Ca2+]i. When the bath pH was made alkaline, [Ca2+]i was also increased. This response was abolished when Ca2+ was eliminated from the bath, indicating that the Ca2+ entry across the basolateral membrane is dependent on bath pH. Increase in [Ca2+]i induced by an alkaline bath was inhibited by increased the bath K+ from 5 nmol/l to 50 mmol/l, suggesting that the Ca2+ entry system is voltage-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone decreases HCO3 reabsorption in the rat proximal tubule by stimulating phosphatidylinositol metabolism and inhibiting base exit

The mechanism of inhibition of HCO3 transport by parathyroid hormone (PTH) in the proximal tubule is not clearly defined. Previous studies in vitro have suggested that this effect is mediated via cAMP generation, which acts to inhibit Na/H exchange, resulting in cell acidification. To examine this question in vivo, intracellular pH (pHi) was measured in the superficial proximal tubule of the rat using the pH-sensitive fluoroprobes 4-methylumbelliferone (4MU) and 2',7'-bis(carboxyethyl)-(5, and 6)-carboxyfluorescein (BCECF). PTH was found to alkalinize the cell. This alkalinization suggested inhibition of basolateral base exit, which was confirmed by in situ microperfusion studies: lowering HCO3 in peritubular capillaries acidified the cell, an effect blunted by PTH. Removal of luminal Na promoted basolateral base entry, alkalinizing the cell. This response was also blunted by PTH. Readdition of luminal Na stimulated the luminal Na/H exchanger, causing an alkalinization overshoot that was partially inhibited by PTH. cAMP inhibited luminal H secretion but did not alkalinize the cell. Stimulation of phosphatidylinositol-bis-phosphate turnover by PTH was suggested by the effect to the hormone to increase cell Ca. Blocking the PTH-induced rise in cell Ca blunted the effect of the hormone to alkalinize the cell, as did inhibition of phosphatidylinositol breakdown. Furthermore, stimulation of protein kinase C by a phorbol ester and a diacylglycerol applied basolaterally alkalinized the cell and inhibited luminal H secretion. The findings indicate that both arms of the phosphatidylinositol-bis-phosphate cascade play a role in mediating the effect of PTH on the cell pH. The results are consistent with the view that PTH inhibits base exit in the proximal tubule by activation of the phosphatidylinositol cascade. The resulting alkalinization may contribute, with cAMP, to inhibit apical Na/H exchange and the PTH-induced depression of proximal HCO3 reabsorption.

Parathyroid hormone stimulates ATP-dependent calcium pump activity by a different mode in proximal and distal tubules of the rat

A new technique was developed to isolate basolateral membrane vesicles individually from proximal and distal tubules of the rat cortex. This new technique enabled us to study differences in their kinetics and mechanisms of hormonal regulation of Ca pump between proximal and distal tubules. The Ca pump in distal tubule has very high affinity (42.6 nM Ca2+) and the one in proximal tubule has relatively low affinity (75.6 nM Ca2+). Parathyroidectomy (PTX) decreased the Vmax of Ca pump activity in proximal tubule (4.68 +/- 0.99 vs. 9.08 +/- 2.21 nmol 45Ca2+/min per mg protein BLMV, P less than 0.05), while it increased Km in distal tubule (93.1 +/- 11.0 vs. 35.1 +/- 16.1 nM Ca2+, P less than 0.05). Restoration of serum Ca2+ concentration by 1,25(OH)2D3 supplement could not reverse these changes by PTX in Ca pump activity in either the proximal or the distal tubule. In conclusion, this study strongly suggested that parathyroid hormone stimulated Ca pump activity by increasing the Vmax in proximal tubule and by increasing the affinity in distal tubule. 1,25(OH)2D3 does not have a direct effect on the basolateral membrane Ca pump activity.

Effects of guanine nucleotides and parathyroid hormone on inositol 1,4,5-trisphosphate metabolism in canine renal cortical tubular cell membranes

Parathyroid hormone (PTH) and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma S) increase levels of the second messenger inositol 1,4,5-triphosphate (IP3) and other inositol phosphates (IP) in several membrane preparations of PTH-responsive cells. We present evidence here indicating that in a membrane preparation of canine renal cortical tubular cells bPTH-(1-84), bPTH-(1-34), [N-Leu8,18Tyr34]bPTH-(3-34)NH2, and the human PTH related peptide fragment hPTHrP-(1-34)NH2 all increase levels of inositol phosphate (IP) but [Tyr34]-bPTH-(7-34)NH2 and hPTHrP-(7-34)NH2 have no significant effects on IP accumulation. Increases in IPs are generally attributed to increased formation of IPs and appear to be mediated by a G protein. However, increased levels of IPs may also result from inhibition of the phosphatases are responsible for their metabolism. We investigated the effect of PTH and GTP-gamma S on the metabolism of IP3 in canine renal cortical tubular membranes. These membranes rapidly metabolize [3H]IP3 (47% at 15 s). Decreases in [3H]IP3 at all time points are accounted for quantitatively by increases in the sum of its breakdown products: [3H]IP2, [3H]IP1, and [3H]inositol. After 5 minutes of exposure to membranes, the vast majority of [3H]IP3 (84%) is converted to its terminal metabolite, [3H]inositol. GTP-gamma S (100 microM) inhibits the amount of [3H]IP3 metabolized in 15 s by 70% and reduces the amount of [3H]inositol ultimately formed in 5 minutes by 64%. ATP-gamma S, ATP, and 2,3-bisphosphoglycerate (100 microM) also inhibit [3H]IP3 hydrolysis in this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol 1,4,5-trisphosphate may regulate rat brain Cai++ by inhibiting membrane bound Na(+)-Ca++ exchanger

The role of inositol 1,4,5-trisphosphate (1,4,5-IP3) in regulating cytosolic Ca++ by stimulating Ca++ release from intracellular organelles is well established. However, other modes of intracellular Ca++ regulation by 1,4,5-IP3 have not been determined. To determine if 1,4,5-IP3 may regulate cell cytosolic Ca++ by acting on plasma membrane bound Na(+)-Ca++ exchanger, we investigated Ca++ transport in synaptosomes using 45Ca++ as tracer. In the presence of either an inhibitor of voltage gated Na+ channels (tetrodotoxin) or the K+ ionophore (valinomycin), Ca++ uptake was significantly inhibited (P less than 0.05) by 1,4,5-IP3 in a concentration dependent manner, with half-maximal inhibition occurring at submicromolar concentrations between 10(-9) M and 10(-10) M 1,4,5-IP3. Similarly, Ca++ efflux by the exchanger was significantly inhibited 40% by 1,4,5-IP3. The inhibitory effect of 1,4,5-IP3 on the Na(+)-Ca++ exchanger was observed in the presence of Ca++ channel blockers, and in vesicles pretreated with caffeine to deplete the 1,4,5-IP3-sensitive stores of Ca++. These results suggest that during signal transduction in brain, 1,4,5-IP3 may increase cytosolic [Ca++] in part by inhibiting the Na(+)-Ca++ exchanger and thus, Ca++ efflux from cell.

Inhibition of the calcium pump by human parathyroid hormone-(1-34) and human calcitonin in liver plasma membranes

The effect of human parathyroid hormone-(1-34) (hPTH) and human calcitonin (hCT) on the activity of the Ca2(+)-extrusion pump in liver plasma membranes was studied. Both hormones were found to be potent inhibitors of Ca2+ transport and the related high-affinity (Ca2(+)-Mg2+)-ATPase activity, causing maximal inhibition of 25-30% at concentrations of 100 nM. Half-maximal inhibition was observed with 20 nM-hPTH and with 0.5 nM-hCT. By comparison, salmon calcitonin and intact bovine parathyroid hormone-(1-84) were inhibitory only at 10 microM. The effects of hCT and hPTH on the Ca2+ pump activity were not mimicked by cyclic AMP. Also, 10 microM of either hPTH-(1-34) or hCT did not alter the 45Ca2+ influx rate into isolated hepatocytes. We conclude that inhibition of Ca2+ efflux, rather than the stimulation of Ca2+ influx, may play a functional role in the control of hepatic calcium homeostasis by hPTH-(1-34) and hCT.

Stimulation of ATP-driven Ca2+ pump in the basal-lateral plasma membranes of kidney cortex during compensatory renal growth

During compensatory renal growth 45Ca2+ transport in basal-lateral plasma membrane vesicles isolated from the rat renal cortex have been investigated. Stimulation of Ca2(+)-ATPase activity was observed, without an effect of compensatory renal growth on Na+/Ca2+ exchanger activity and on passive Ca2+ permeability of the vesicles. Twelve hours following unilateral nephrectomy about 40% increase of Ca2(+)-ATPase activity above control value was observed and this effect was present until the end of the experimental period (7 days). When kinetic parameters for Ca2(+)-ATPase were studied in native membranes, an increase of Vmax was observed, whereas the Km for Ca2+ was similar in control vesicles and vesicles isolated from the remnant kidney. Depletion of endogenous calmodulin resulted in a decrease of Vmax and an increase of Km (Ca2+), while its addition reversed these parameters and increased the Hill coefficient from about 1 to about 2. Once again, only a significant increase of Vmax in vesicles isolated from the remnant kidney above the control value was observed. Finally, increase of Ca2(+)-ATPase activity during compensatory renal growth could be abolished by actinomycin D, indicating that its stimulation is due to protein synthesis.

Parathyroid hormone stimulates formation of inositol phosphates in a membrane preparation of canine renal cortical tubular cells

Recent studies have shown that, in addition to its well-known action to stimulate adenylate cyclase activity, parathyroid hormone (PTH) may stimulate the inositol phosphate second messenger system in its target tissues, bone and kidney. We have developed a membrane preparation of canine renal cortex to test this hypothesis. We also have examined the potential role of guanine nucleotides on the formation of inositol phosphates (IPs) in this tissue. Collagenase-dispersed tubules were labeled with [3H]inositol, and membranes containing labeled phospholipase C (PLC) substrates ([3H]phosphatidyl inositol, [3H]phosphatidylinositol monophosphate, and [3H]phosphatidylinositol bisphosphate) were prepared. bPTH-(1-34) (100 nM) rapidly increased levels of all measured [3H]IPs (IP1, IP2, and IP3) 1.6-1.7-fold within the first 30 s of stimulation. The half-maximal concentration for the response to bPTH-(1-34) was approximately 8 nM. GTP gamma S (100 microM), a nonhydrolyzable analog of GTP, also increased levels of the three [3H]IPs (1.8 to 2.8-fold). The half-maximal concentration for the response to GTP gamma S was approximately 30 microM. In the presence of GTP gamma S, bPTH-(1-34) increased levels of IPs by up to 2.7 times more than GTP gamma S alone. The results indicate that bPTH-(1-34) can stimulate the formation of inositol phosphates in the kidney and suggest that PTH may activate a receptor coupled to this effect through a guanine nucleotide regulatory protein.

Desensitization to parathyroid hormone in renal cells from aged rats is associated with alterations in G-protein activity

Parathyroid hormone (PTH)-stimulated Na+/Ca2+ exchange activity, but not forskolin-sensitive Na+-dependent Ca2+ efflux, was blunted in renal cortical cells from aged rats. PTH-sensitive adenylate cyclase activity in renal membranes from senescent rats also declined, but forskolin-stimulated activity did not change. In addition, cholera toxin- and pertussis toxin-stimulated Na+-dependent Ca2+ efflux and cAMP formation were blunted in cells from aged animals. Further, cells from aged rats had decreased Gs-alpha and Gi-alpha proteins, as detected by ADP-ribosylation. These findings would be consistent with the proposal of an age-associated heterologous desensitization that involved the G-proteins. Serum concentrations of iPTH were increased in the old rat, suggesting that the desensitization to PTH in the aging rat represented an adaptive response to prolonged stimulation by the hormone. This hypothesis was supported by the findings that the attenuated PTH-sensitive Na+/Ca2+ exchange activity, cAMP formation, and adenylate cyclase activity in cells from old rats could be reversed by parathyroidectomy. The decreased label in cholera toxin-catalyzed ADP-ribosylated Gs-alpha and pertussis toxin catalyzed ADP-ribosylated Gi-alpha found in cells from aged rats was also largely negated by the surgery. In conclusion, the results suggest that the age-related blunting in the responses of renal cells to PTH was associated with a deficit in G-protein function and that this alteration could be reversed by removal of the parathyroid gland.

The renal Na+/Ca2+ exchange system is located exclusively in the distal tubule

The movement of Ca2+ across the basolateral plasma membrane was determined in purified preparations of this membrane isolated from rabbit proximal and distal convoluted tubules. The ATP-dependent Ca2+ uptake was present in basolateral membranes from both these tubular segments, but the activity was higher in the distal tubules. A very active Na+/Ca2+ exchange system was also demonstrated in the distal-tubular membranes, but in proximal-tubular membranes this exchange system was not demonstrable. The presence of Na+ outside the vesicles gradually inhibited the ATP-dependent Ca2+ uptake in the distal-tubular-membrane preparations, but remained without effect in those from the proximal tubules. The activity of the Na+/Ca2+ exchange system in the distal-tubular membranes was a function of the imposed Na+ gradient. These results suggest that the major differences in the characteristics of Ca2+ transport in the proximal and in the distal tubules are due to the high activity of a Na+/Ca2+ exchange system in the distal tubule and its virtual absence in the proximal tubule.

ATP-driven Ca2+ pump in the basolateral membrane of rat kidney cortex catalyzes an electroneutral Ca2+/H+ antiport

An ATP-driven Ca2+ pump in the basolateral membrane of rat kidney cortex pumps Ca2+ out of the cell at the expense of MgATP (Km = 0.191 mM). This pump has a high affinity for free Ca2+ (26 nM). Vanadate, lanthanum, N-ethylmaleimide and calmodulin inhibitor R24571 inhibited this pump activity. Dimethyl[2-14C]oxazolidine-2,4-dione [( 14C]DMO) was entrapped in the vesicles in association with the ATP-driven Ca2+ influx. The ATP-driven Ca2+ influx was stimulated by the intravesicular acid pH and an upper convex Lineweaver-Burk reciprocal plot suggested two possible kinetics; one is that this Ca2+ pump is an allosteric enzyme with more than 1.72 H+ binding sites and another is the presence of two Ca2+ pumps with different affinities for H+. Valinomycin study indicated that the ATP-dependent Ca2+ transport by the BLMV was electroneutral and voltage independent. These results strongly suggest that the ATP-driven Ca2+ pump in the renal basolateral membrane catalyzes an electroneutral Ca2+/H+ antiport.

Na+-Ca2+ exchange and calcium permeability in canine basolateral membrane vesicles: the effects of dibutyryl cAMP and specific inhibitors

The role of dibutyryl 3',5'-cyclic adenosine monophosphate (dibutyryl cAMP) as putative second messenger for parathyroid hormone (PTH) in regulating canine proximal tubular basolateral membrane Na+-Ca2+ exchange and passive calcium permeability was assessed, as was the nature of this passive calcium permeability. Dibutyryl cAMP (50 mg) infused in vivo over 30 min increased fractional phosphate excretion from 4.9 +/- 1.8% to 20.5 +/- 4.6%, P less than 0.05, n = 6, but had no effect on either passive Ca2+ efflux or sodium-stimulated Ca2+ efflux from Ca2+-preloaded basolateral membrane vesicles (BLMV). Both of these mechanisms have been previously shown to be stimulated by PTH. Further studies were performed to investigate the mechanism of the passive calcium flux. Calcium uptake by BLMV was blocked by lanthanum (La3+) but not by the calcium-channel blocker verapamil. La3+ blocked efflux of Ca2+ from preloaded vesicles when it was placed in the external solution. This La3+-blockable efflux was larger in potassium equivalent BLMV prepared from normal dogs than in BLMV prepared from thyroparathyroidectomized dogs. Benzamil produced 50% inhibition of sodium-stimulated Ca2+ uptake at 250 microM whereas neither amiloride nor diltiazem achieved 50% inhibition at the maximal doses studied. Benzamil, 1 mM, had no effect on passive calcium efflux and neither did the substitution of sucrose for potassium, which has been shown to affect Ca2+-Ca2+ exchange by the Na+-Ca2+ exchanger. This suggests that the calcium flux under potassium equivalent conditions was not mediated by Ca2+-Ca2+ exchange by the Na+-Ca2+ exchanger. These results demonstrate that the basolateral membrane of proximal tubular cells possesses both a Na+-Ca2+ exchanger inhibitable by benzamil and a passive calcium permeability not inhibited by benzamil nor by verapamil but by La3+. Neither of these two mechanisms of calcium flux was affected by dibutyryl cAMP whereas both have been shown to be stimulated by PTH.

Kinetic transport model for cellular regulation of pH and solute concentration in the renal proximal tubule

An open circuit kinetic model was developed to calculate the time course of proximal tubule cell pH, solute concentrations, and volume in response to induced perturbations in luminal or peritubular fluid composition. Solute fluxes were calculated from electrokinetic equations containing terms for known carrier saturabilities, allosteric dependences, and ion coupling ratios. Apical and basolateral membrane potentials were determined iteratively from the requirements of cell electroneutrality and equal opposing transcellular and paracellular currents. The model converged to membrane potentials accurate to 0.05% in one to four iterations. Model variables included cell concentrations of Na, K, HCO3, glucose, pH (uniform CO2), volume, and apical and basolateral membrane potentials. The basic model contained passive apical membrane transport of Na/H, Na/glucose, H and K, basolateral transport of Na/3HCO3, K, H, and glucose, and paracellular transport of Na, K, Cl, and HCO3; apical H and basolateral 3Na/2K-ATPases were present. Apical Na/H and basolateral K transport were regulated allosterically by pH. Apical Na/H transport, basolateral Na/3HCO3 transport, and the 3Na/2K-ATPase were saturable. Model parameters were chosen from data in the rat proximal tubule. Model predictions for the magnitude and time course of cell pH, Na, and membrane potential in response to rapid changes in apical and peritubular Na and HCO3 were in excellent agreement with experiment. In addition, the model requires that there exist an apical H-ATPase, basolateral Na/3HCO3 transport saturable with HCO3, and electroneutral basolateral K transport.

Stimulation of inositol trisphosphate and diacylglycerol production in renal tubular cells by parathyroid hormone

Parathyroid hormone (PTH) produced a dose-dependent immediate stimulation of inositol triphosphate and diacylglycerol production in the opossum kidney cell line, primary culture proximal tubular cells, and basolateral membranes from canine proximal tubular segments. The increase in inositol triphosphate production was accompanied by a minor increase in inositol phosphate and no significant increase in inositol bisphosphate production. Associated with the changes in inositol triphosphate and diacylglycerol, there was an immediate hydrolysis of phosphatidylinositol 4'5-bisphosphate. The effect on phospholipid hydrolysis was followed by stimulation of phosphorylation of phosphatidylinositol 4' monophosphate and phosphatidylinositol. PTH produced a sudden increase in cytoplasmic Ca2+ in opossum kidney cells that persisted for approximately 1 min. Inositol triphosphate transiently increased cytoplasmic Ca2+ in saponin-treated opossum kidney and primary culture proximal tubule cells. The effects of PTH were not mimicked by cyclic nucleotides. In fact, cyclic AMP appeared to diminish inositol triphosphate production. These results demonstrate that PTH may activate renal tubular epithelial cells by the production of inositol triphosphate and diacylglycerol.

Electrogenic sodium/bicarbonate cotransport in rabbit renal cortical basolateral membrane vesicles

The present studies examined the mechanism of bicarbonate transport across basolateral membrane vesicles prepared from rabbit renal cortex. Isotopic sodium uptake was stimulated by bicarbonate when compared with gluconate (2.5 nmol/mg protein per 5 s versus 1.4 nmol/mg protein per 5 s), and this process was inhibited by disulfonic stilbenes. Imposition of an interior-positive potassium diffusion potential further stimulated isotopic sodium uptake to 3.4 nmol/mg protein per 5 s, an effect that occurred only in the presence of bicarbonate and was blocked by disulfonic stilbenes. Kinetic analysis of the rate of bicarbonate-dependent sodium uptake as a function of sodium concentration revealed saturable stimulation with a Vmax of 2.7 nmol/mg protein per 2 s and a Km of 10.4 mM. The effect of bicarbonate concentration on bicarbonate-dependent sodium uptake was more complex. The present results demonstrate an electrogenic (negatively charged) sodium/bicarbonate cotransporter in basolateral membrane vesicles from the rabbit renal cortex. The electrogenicity implies a stoichiometry of at least two bicarbonate ions for each sodium ion.

Calcium transport systems in the LLC-PK1 renal epithelial established cell line

ATP-dependent calcium uptake was measured in membrane vesicles prepared from the renal epithelial LLC-PK1 established cell line. The relative contribution of the nonmitochondrial versus the mitochondrial calcium uptake is larger in LLC-PK1 cell homogenates than in homogenates from renal cortex. Two types of calcium pump, characterized by the formation of calcium-dependent phosphointermediates of 135 kDa and 115 kDa, were found in membrane fractions from LLC-PK1 cells. The 135 kDa calcium pump was also detected by 125I-labelled calmodulin overlay. Although the subcellular localization in LLC-PK1 cell membranes could not be unambiguously determined, it is conceivable that the 135 kDa and the 115 kDa molecules represent the plasma membrane calcium pump and the endoplasmic reticulum calcium pump respectively, in agreement with what was found for renal cortex preparations. Extravesicular sodium partially inhibits ATP-driven calcium uptake in a plasma-membrane-enriched fraction of the LLC-PK1 cells. The effect is potentiated by a vesicle inside-negative membrane potential. Although the effect is less pronounced than in renal cortex basal-lateral membranes, this observation suggests that an Na+-Ca2+ exchange mechanism is also present in LLC-PK1 cells. ATP-dependent calcium uptake in nonmitochondrial intracellular stores was investigated, using saponin-permeabilized cells. Permeabilized LLC-PK1 cells lowered the free calcium concentration in the medium to less than 0.4 microM. More than 60% of the accumulated calcium can be released by addition of inositol 1,4,5-trisphosphate. Our data indicate that the LLC-PK1 cell line can be successfully used as model system for the study of renal calcium handling.