Kinetic properties of the ATP-dependent Ca2+ pump and the Na+/Ca2+ exchange system in basolateral membranes from rat kidney cortex

Homeostasis and secretion of calcium in the oviductal mucosa of toad Rhinella arenarum

The presence of a calcium pump, calbindin D-28KD, and calmodulin in the secretory cells (SC) of the oviductal pars convoluta (PC) of Rhinella arenarum was established for the first time in amphibians using immunohistochemical techniques. Marked variations were observed in the localization and degree of expression of these proteins according to the duct segment and the period of the sexual cycle analyzed. During the preovulatory and ovulatory periods the calcium pump colocalized with calbindin D-28KD can be seen mainly in the apical border of the SC, which are located in the first zones of PC and synthesize and secrete the components of the inner jelly coat layers. These envelopes, which surround the oocytes, contain the molecules indispensable for fertilization, probably inducing the sperm acrosome reaction (AR). Our results suggest that calmodulin, colocalized with the calcium pump at the SC cytoplasmic level, would be involved in the active transport of the cation inside the secretory granules, maintaining adequate levels of intracellular Ca(2+) . During the postreproductive period, a calcium pump colocalized with calbindin D-28KD appears for the first time in the cycle in the basal zones of the SC. This system may be related to the replenishing of intracellular Ca(2+) stores. In contrast, in R. arenarum the Ca(2+) present in the jelly coats that surround the oocytes participates in the AR during fertilization, suggesting that this secretion system of the cation provided by the oviductal mucosa is functionally more active during the reproductive period of this species.

Targeted ablation of the histidine-rich Ca(2+)-binding protein (HRC) gene is associated with abnormal SR Ca(2+)-cycling and severe pathology under pressure-overload stress

The histidine-rich Ca(2+)-binding protein (HRC) is located in the lumen of the sarcoplasmic reticulum (SR) and exhibits high-capacity Ca(2+)-binding properties. Overexpression of HRC in the heart resulted in impaired SR Ca(2+) uptake and depressed relaxation through its interaction with SERCA2a. However, the functional significance of HRC in overall regulation of calcium cycling and contractility is not currently well defined. To further elucidate the role of HRC in vivo under physiological and pathophysiological conditions, we generated and characterized HRC-knockout (KO) mice. The KO mice were morphologically and histologically normal compared to wild-type (WT) mice. At the cellular level, ablation of HRC resulted in significantly enhanced contractility, Ca(2+) transients, and maximal SR Ca(2+) uptake rates in the heart. However, after-contractions were developed in 50 % of HRC-KO cardiomyocytes, compared to 11 % in WT mice under stress conditions of high-frequency stimulation (5 Hz) and isoproterenol application. A parallel examination of the electrical activity revealed significant increases in the occurrence of Ca(2+) spontaneous SR Ca(2+) release and delayed afterdepolarizations with ISO in HRC-KO, compared to WT cells. The frequency of Ca(2+) sparks was also significantly higher in HRC-KO cells with ISO, consistent with the elevated SR Ca(2+) load in the KO cells. Furthermore, HRC-KO cardiomyocytes showed significantly deteriorated cell contractility and Ca(2+)-cycling caused possibly by depressed SERCA2a expression after transverse-aortic constriction (TAC). Also HRC-null mice exhibited severe cardiac hypertrophy, fibrosis, pulmonary edema and decreased survival after TAC. Our results indicate that ablation of HRC is associated with poorly regulated SR Ca(2+)-cycling, and severe pathology under pressure-overload stress, suggesting an essential role of HRC in maintaining the integrity of cardiac function.

The effect of different kinds of electrolyte and non-electrolyte solutions on the survival rate and morphology of zebrafish Danio rerio embryos

The effect of electrolyte and non-electrolyte solutions on the survival and on the morphology of zebrafish Danio rerio embryos was investigated. Embryos in different ontogenetic stages were incubated in electrolyte (NaCl, KCl, MgCl2 and CaCl2) and non-electrolyte solutions [sucrose and polyvinylalcohol (PVA)] of different concentrations for 5-15 min. The embryos were hatched to the long-pec stage and the effective concentrations which caused a 50% decrease in embryo development (EC50) were determined. The morphometric changes, which were caused by the test solutions, were measured. Ion channel blockers were used to see if active ion transport played a role for embryo survival. Finally, dechorionated embryos were exposed to the test solutions to get indications about the importance of chorion and perivitelline space. For 12 hours post fertilization (hpf) embryos and a 15 min exposure period, EC50 was highest for MgCl2 (1.60 mol l(-1)), followed by sucrose (0.73 mol l(-1)), NaCl (0.49 mol l(-1)), KCl (0.44 mol l(-1)), CaCl2 (0.43 mol l(-1)) and PVA [0.0005 mol l(-1) (2.2%)]. EC50 were lower for early embryonic stages than for advanced stages for all solutions with exception of MgCl2 and sucrose. At the EC50, MgCl2 and CaCl2 solutions did not induce morphometric changes. NaCl and sucrose solutions induced reversible morphometric changes, which were compensated within 10 min. Only the EC50 of KCl and PVA solutions induced permanent morphometric changes, which could not be compensated. Incubation of embryos in electrolyte and non-electrolyte solutions together with ouabain (blocker of Na+- K+ ATPase), HgCl3 (dose-dependent inhibition of aquaporine channels), verapamil (inhibition of calcium and magnesium uptake) and amiloride (inhibition of sodium uptake) significantly decreased the per cent of embryos developing to the long-pec stage in comparison to the same solutions without blockers. Ouabain and HgCl(3) also induced morphometric changes. For dechorionated embryos the survival rates in water and in the different test solutions were similar to untreated embryos.

Cyclic AMP/cAMP-GEF pathway amplifies insulin exocytosis induced by Ca2+ and ATP in rat islet beta-cells

BACKGROUND

Cyclic AMP (cAMP) plays a pivotal role in insulin secretion induced by incretins. The effects of the second messenger extend to many sites and there has been much controversy on the mechanisms. The aim of this study was to examine how cAMP amplified insulin exocytosis.

METHODS

Rat islets were permeabilized with alpha-toxin to measure insulin exocytosis in the fixed conditions of Ca(2+) and ATP. The effects of several agents on insulin exocytosis were observed in perifusion experiments.

RESULTS

Cyclic AMP enhanced the Ca(2+)-induced insulin release by around 30%, independent of Ca(2+) concentrations between 10 and 3000 nmol/L. A cAMP-GEF selective cAMP analogue, 8-(4-chloro-phenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate, also amplified insulin release. The effect disappeared in the absence of ATP. Conversely, cAMP-independent gradual increase in insulin release was observed with ATP. These results suggested that the site of action of cAMP-GEF existed proximal to that of ATP. An analogue selective to PKA, N(6)-Benzoyladenosine-3',5'-cyclic monophosphate, had little effect. Also, a PKA-selective inhibitor, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide, reduced insulin releases induced by 1000 nmol/L Ca(2+), but did not influence the relative increase produced by Ca(2+) and cAMP.

CONCLUSION

Cyclic AMP potentiated Ca(2+) and ATP-induced exocytosis to a similar relative extent independent of Ca(2+) concentrations. The process appeared to be mainly mediated by cAMP-GEF. In addition, the cAMP/cAMP-GEF pathway may enhance insulin release by replenishing the readily releasable pool.

Hormonal regulation on regucalcin mRNA expression in cloned normal rat kidney proximal tubular epithelial NRK52E cells

Regucalcin is a regulatory protein in cell signaling. This study was undertaken to determine whether regucalcin mRNA expresses in the cloned normal rat kidney proximal tubular epithelial NRK52E cells and its expression regulates due to hormones and cell signaling-related factors. Cells with subconfluency were cultured for 24, 48, or 72 h in a Dulbecco's modified Eagle medium supplemented with non-essential amino acid without bovine serum (BS). The result of Western blot analysis showed that regucalcin protein was present in the NRK52E cells. The expression of regucalcin mRNA in the cells was determined using reverse transcription-polymerase chain reaction (RT-PCR). Regucalcin mRNA expression in the NRK52E cells was significantly increased by culture with parathyroid hormone (PTH, 10(-8) or 10(-7) M), aldosterone (10(-8) or 10(-7) M), or dexamethasone (10(-8) M). The presence of 1,25-dihydroxyvitamin D(3) (1,25(OH)2D3, 10(-8) or 10(-7) M) or calcitonin (10(-9) or 10(-8) M) did not have a significant effect on regucalcin mRNA levels in the cells. Culture with dibutyryl cyclic AMP (DcAMP, 10(-5) or 10(-4) M) or phorbol 12-myristate 13-acetate (PMA, 10(-6) M), an activator of protein kinase C, caused a significant increase in regucalcin mRNA expression. The presence of staurosporine (10(-8) M) caused a significant decrease in regucalcin mRNA expression. Dibucaine (10(-7) M), PD98059 (10(-7) M), or vanadate (10(-6) or 10(-5) M) did not have an effect on regucalcin mRNA levels. The present study demonstrates that regucalcin mRNA and its protein are expressed in the cloned normal rat kidney proximal tubular epithelial NRK52E cells, and that the expression is enhanced by hormones which regulate ion transport in the proximal tubule.

Evidence for facilitated diffusion of urea across the gill basolateral membrane of the rainbow trout (Oncorhynchus mykiss)

Recent in vivo evidence suggests that the mechanism of branchial urea excretion in the ammoniotelic rainbow trout (Oncorhynchus mykiss) is carrier-mediated. Further characterization of this proposed mechanism was achieved by using an in vitro isolated basolateral membrane vesicle (BLMV) preparation in which isolated gill membranes were used to determine a variety of physiological properties of the transporter. BLMV demonstrated two components of urea uptake, a linear component at concentrations up to 17.5 mmol x l(-1) and a saturable component (K(0.5)=0.35+/-0.01 mmol x l(-1); V(max)=0.14+/-0.02 micromol mg protein(-1) h(-1)) with a Hill constant of 1.35+/-0.18 at low, physiologically relevant urea concentrations (<2 mmol x l(-1)). Saturable uptake of urea at 1 mmol x l(-1) by BLMV was reduced by 88.5% when incubated with 0.25 mmol x l(-1) phloretin, a potent blocker of UT-type facilitated diffusion urea transport mechanisms. BLMV also demonstrated differential handling of urea versus urea analogues at 1 mmol x l(-1) concentrations and total analogue/total urea uptake ratios were 32% for acetamide and 84% for thiourea. Saturable urea uptake at 1 mmol x l(-1) was significantly reduced by almost 100% in the presence of 5 mmol x l(-1) thiourea but was not affected by 5 mmol x l(-1) acetamide or 5 mmol x l(-1) N-methylurea. Lastly, total urea uptake at 1 mmol x l(-1) by BLMV was sensitive to temperatures above and below the temperature of acclimation with a Q(10)>2 suggesting a protein carrier-mediated process. Combined, this evidence indicates that a facilitated diffusion urea transport mechanism is likely present in the basolateral membrane of the rainbow trout gill.

In vivo treatment with stobadine prevents lipid peroxidation, protein glycation and calcium overload but does not ameliorate Ca2+ -ATPase activity in heart and liver of streptozotocin-diabetic rats: comparison with vitamin E

Hyperglycemia leads to excess production of reactive oxygen species (ROS), lipid peroxidation and protein glycation that may impair cellular calcium homeostasis and results in calcium sequestration and dysfunction in diabetic tissues. Stobadine (ST) is a pyridoindole antioxidant has been postulated as a new cardio- and neuroprotectant. This study was undertaken to test the hypothesis that the treatment with ST inhibits calcium accumulation, reduces lipid peroxidation and protein glycation and can change Ca2+,Mg2+-ATPase activity in diabetic animals. The effects of vitamin E treatment were also evaluated and compared with the effects of combined treatment with ST. Diabetes was induced by streptozotocin (STZ, 55 mg/kg i.p.). Some of diabetic rats and their age-matched controls were treated orally with a low dose of ST (24.7 mg/kg/day), vitamin E (400-500 IU/kg/day) or ST plus vitamin E for 10 weeks. ST and vitamin E separately produced, in a similar degree, reduction in diabetes-induced hyperglycemia. Each antioxidant alone significantly lowered the levels of plasma lipid peroxidation, cardiac and hepatic protein glycation in diabetic rats but vitamin E treatment was found to be more effective than ST treatment alone. Diabetes-induced increase in plasma triacylglycerol levels was not significantly altered by vitamin E treatment but markedly reduced by ST alone. The treatment with each antioxidant completely prevented calcium accumulation in diabetic heart and liver. Microsomal Ca2+,Mg2+-ATPase activity significantly decreased in both tissues of untreated diabetic rats. ST alone significantly increased microsomal Ca2+,Mg2+-ATPase activity in the heart of normal rats. However, neither treatment with ST nor vitamin E alone, nor their combination did change cardiac Ca2+,Mg2+-ATPase activity in diabetic heart. In normal rats, neither antioxidant had a significant effect on hepatic Ca2+,Mg2+-ATPase activity. Hepatic Ca2+,Mg2+-ATPase activity of diabetic rats was not changed by single treatment with ST, while vitamin E alone completely prevented diabetes-induced inhibition in microsomal Ca2+,Mg2+-ATPase activity in liver. Combined treatment with ST and vitamin E provided more benefits in the reduction of hyperglycemia and lipid peroxidation in diabetic animals. This study describes potential mechanisms on cellular effects of ST in the presence of diabetes-induced hyperglycemia that may delay or inhibit the development of diabetic complications. The use of ST together with vitamin E can better control hyperglycemia-induced oxidative stress.

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.

Ca(2+) channels activated by endothelin-1 in CHO cells expressing endothelin-A or endothelin-B receptors

We compared the Ca(2+) channels activated by endothelin-1 (ET-1) in Chinese hamster ovary (CHO) cells stably expressing endothelin type A (ET(A)) or endothelin type B (ET(B)) receptors using the Ca(2+) channel blockers LOE-908 and SK&F-96365. In both CHO-ET(A) and CHO-ET(B), ET-1 at 0.1 nM activated the Ca(2+)-permeable nonselective cation channel-1 (NSCC-1), which was sensitive to LOE-908 and resistant to SK&F-96365. ET-1 at 1 nM activated NSCC-2 in addition to NSCC-1; NSCC-2 was sensitive to both LOE-908 and SK&F-96365. ET-1 at 10 nM activated the same channels as 1 nM ET-1 in both cell types, but in CHO-ET(A), it additionally activated the store-operated Ca(2+) channel (SOCC), which was resistant to LOE-908 and sensitive to SK&F-96365. Up to 1 nM ET-1, the level of the formation of inositol phosphates (IPs) was low and similar in both cell types, but, at 10 nM ET-1, it was far greater in CHO-ET(A) than in CHO-ET(B). These results show that, in CHO-ET(A) and CHO-ET(B), ET-1 up to 10 nM activated the same Ca(2+) entry channels: 0.1 nM ET-1 activated NSCC-1, and ET-1 > or = 1 nM activated NSCC-1 and NSCC-2. Notably, in CHO-ET(A), 10 nM ET-1 activated SOCCs because of the higher formation of IPs.

Partial ischemia reduces the efficiency of sarcoplasmic reticulum Ca2+ transport in rat EDL

To investigate the hypothesis that prolonged partial ischemia would result in a depression in homogenate sarcoplasmic reticulum (SR) Ca2+-sequestering and mechanical properties in muscle, a cuff was placed around the hindlimb of 8 adult Sprague-Dawley rats (267+/-5.8 g; x +/- S.E.) and partially inflated (315 mm Hg) for 2 h. Following occlusion, the EDL was sampled both from the ischemic (I) and contralateral control (C) leg and SR properties compared with the EDL muscles extracted from rats (n = 8) immediately following anaesthetization (CC). Ischemia was indicated by a lower (p < 0.05) concentration (mmol.kg dry wt(-1)) of ATP (19.0+/-0.7 vs. 16.7+/-0.7) and phosphocreatine (58.1+/-5.7 vs. 35.0+/-4.6) in I compared to C. Although Ca2+-ATPase activity (micromol x g protein(-1) x sec(-1)), both maximal and submaximal, was not different between C and I (19.7+/-0.4 vs. 18.5+/-1.3), reductions (p < 0.05) in Ca2+-uptake (mmol x g protein(-1) x sec(-1)) of between 18.2 and 24.7% across a range of submaximal free Ca2+-levels were observed in I compared to C. Lower submaximal Ca2+-ATPase activity and Ca2+-uptake were also observed in the EDL in C compared to CC animals. Time dependent reductions (p < 0.05) were found in peak twitch and maximal tetanic tension in EDL from I but not C. It is concluded that partial ischemia, resulting in modest reductions in energy state in EDL, induces a reduction in Ca2+-uptake independent of changes in Ca2+-ATPase activity. These changes reduce the coupling ratio and the efficiency of Ca2+-transport by SR.

Superfusion of synaptosomes to study presynaptic mechanisms involved in neurotransmitter release from rat brain

Neurotransmitter release, as the primary way for neuron signaling, represents the target of a staggering number of studies in order to understand complex neural functions. The corpus striatum is a brain area especially rich in neurotransmitters where cholinergic neurons are supposed to play an associative role between different neuronal types, and therefore their activity is modulated by multiple neurotransmitter systems [Trends Neurosci. 17 (1994) 228; Trends Neurosci. 18 (1995) 527] [13,25]. In this regard, superfusion of synaptosomes is a useful in vitro approach to study the neurotransmitter release allowing an unequivocal interpretation of results obtained under accurately specified experimental conditions. Synaptosomes are sealed presynaptic nerve terminals obtained after homogenating brain tissue in iso-osmotic conditions [J. Physiol. 142 (1958) 187] [22]. Synaptosomes have been extensively used to study the mechanism of neurotransmitter release in vitro because they preserve the biochemical, morphological and electrophysiological properties of the synapse [J. Neurocytol. 22 (1993) 735] [42]. The superfusion, strictly a perfusion, allows both the continuous removal of the compounds present in the biophase of the presynaptic proteins and the easy exchange of the medium. We herein describe the method of superfusion of rat striatal synaptosomes to study the [(3)H]ACh release under basal and stimulated conditions. To depolarize the synaptosomal preparation three different strategies were employed: high extracellular concentration of K(+) (15 mM), a K(+) channel-blocker (4-aminopyridine, 1-30 microM), or veratridine (10 microM) which blocks the inactivation of voltage-dependent Na(+) channels.

Differential effect of quinpirole and 7-OH-DPAT on the spontaneous [(3)H]-dopamine efflux from rat striatal synaptosomes

The effect of quinpirole and 7-OH-DAPT, two D(2)-like agonists, were examined using superfused rat striatal synaptosomes to study the autoregulation of spontaneous [(3)H]-dopamine ([(3)H]-DA) release. Basal [(3)H]-DA efflux was Ca(2+)-dependent by approximately 45% and was inhibited by cadmium 10 microM by 24%. Quinpirole (1 nM to 3 microM) inhibited spontaneous [(3)H]-DA efflux in a concentration-dependent manner (pEC(50) = 7.56 +/- 0.07 and E(max) = 26 +/- 0.09%) and this effect was competitively antagonized by haloperidol (0.3-1 nM) (apparent pA(2) = 9.61 +/- 0.08). In addition, activation of the D(2) DA autoreceptor by quinpirole only modulates the calcium-dependent component of [(3)H]-DA efflux. Low concentrations of a putative-selective D(3) DA agonist, (+/-)-7-OH-DPAT (0.03-0.1 microM), inhibited spontaneous [(3)H]-DA release by 13% (P < 0.05), but higher drug concentrations (> or =1 microM) increased basal [(3)H]-DA efflux in a concentration-dependent, nonsaturable, but reversible manner. Haloperidol (1-10 nM) reversed the (+/-)-7-OH-DPAT-induced inhibition, but not the increase in [(3)H]-DA outflow. The effect of (+/-)-7-OH-DPAT was mimicked by (+)-7-OH-DPAT. However, another putative D(3) DA agonist, PD 128,907 (1 nM to 3 microM), decreased spontaneous tritium efflux (maximal inhibition of 19 +/- 3.06% at 3 microM, P < 0.01). The effect of 7-OH-DPAT 10 microM was independent of the presence of extracellular Ca(2+), since its effect on basal [(3)H]-DA outflow was not significantly modified in a 200 nM free-Ca(2+) medium. In addition, the 7-OH-DPAT-induced enhancement of basal [(3)H]-DA efflux does not involve depolarization of nerve terminals or the reversal of the DA uptake system, as tetrodotoxin (1 microM) and nomifensine (1microM) did not modify the effect of 7-OH-DPAT 10 microM. The present data indicate that activation of D(2) DA autoreceptor subtype by quinpirole inhibits Ca(2+)-dependent spontaneous [(3)H]-DA efflux. 7-OH-DPAT activates the D(2) DA autoreceptor at low concentrations, whereas its action in releasing [(3)H]-DA effect is not receptor-mediated and could involve other mechanisms other than either conventional vesicular exocytosis or the DA uptake system.

State-dependent modification of ATP-sensitive K+ channels by phosphatidylinositol 4,5-bisphosphate

With inside-out patch recordings in ventricular myocytes from the hearts of guinea pigs, we studied ATP-sensitive K+ (K(ATP)) channels activated by phosphatidylinositol 4,5-bisphosphate (PIP2) with respect to sensitivity to ATP when in either a rundown state (RS) or a non-rundown state (NRS). Rundown of K(ATP) channels was induced by exposure either to ATP-free solution or to ATP-free solution containing 19 microM Ca2+. Exposure of membrane patches to 10 microM PIP2 reactivated channels with both types of rundown. The reactivation by PIP2 did not require ATP in the bath. The IC50 of channels recovered from RS and before the rundown was 37.1 and 31.1 microM, respectively. PIP2 irreversibly increased the mean current when the channel was in the NRS. This was associated with a shift of IC50 to 250.6 microM after PIP2 exposure. PIP2 activates NRS K(ATP) channels by decreasing their sensitivity to ATP, whereas PIP2 reactivates RS-K(ATP) channels independently of ATP without changing ATP sensitivity.

Active urea transport and an unusual basolateral membrane composition in the gills of a marine elasmobranch

In elasmobranch fishes, urea occurs at high concentrations (350-600 mM) in the body fluids and tissues, where it plays an important role in osmoregulation. Retention of urea by the gill against this huge blood-to-water diffusion gradient requires specialized adaptations to the epithelial cell membranes. Experiments were performed to determine the mechanisms and structural features that facilitate urea retention by the gill of the spiny dogfish Squalus acanthias. Analysis of urea uptake by gill basolateral membrane vesicles revealed the presence of a phloretin-sensitive (half inhibition 0.09 mM), sodium-coupled, secondary active urea transporter (Michaelis constant = 10.1 mM, maximal velocity = 0.34 micromol. h(-1). mg protein(-1)). We propose that this system actively transports urea out of the gill epithelial cells back into the blood against the urea concentration gradient. Lipid analyses of the basolateral membrane revealed high levels of cholesterol contributing to the highest reported cholesterol-to-phospholipid molar ratio (3.68). This unique combination of active urea transport and modification of the phospholipid bilayer membrane is responsible for decreasing the gill permeability to urea and facilitating urea retention by the gill of Squalus acanthias.

Presynaptic effect of 7-OH-DPAT on evoked [3H]-acetylcholine release in rat striatal synaptosomes

The objective of the present experiments was to study the presynaptic effect of 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT, a D(2)-like dopamine receptor agonist) on [3H]-acetylcholine ([3H]-ACh) release induced by potassium (15 mM, 25 mM and 60 mM), potassium channel-blockers (4-aminopyridine, 4-AP; tetraethylammonium, TEA and quinine) and veratridine to gain insight into the mechanisms involved in the activation of the D(2) dopamine-receptor subtype located at striatal cholinergic nerve terminals. 7-OH-DPAT (1 microM) inhibited the evoked [3H]-ACh release induced by K(+) 15 mM in a similar percentage than that obtained during basal conditions (30% and 27%, respectively). Nevertheless, in the presence of 25 mM and 60 mM of K(+) the inhibitory effect of 7-OH-DPAT was completely abolished. 4-AP (1-100 microM) and TEA (1 and 5 mM) significantly enhanced [3H]-ACh release, showing 69.32%+/-7.60% (P<0.001) and 52.27%+/-5.64% (P<0.001), respectively, at the highest concentrations tested. In these conditions, 7-OH-DPAT (1 microM) inhibited the release induced by potassium channel-blockers approximately 25-27%. Quinine (0.1-1 microM) did not alter [3H]-ACh release either in the presence or absence of 7-OH-DPAT. Veratridine 10 microM evoked [3H]-ACh release in the presence of a low-calcium medium, but in such conditions 7-OH-DPAT (1 microM) did not modify the neurotransmitter release in the absence or presence of veratridine. Present data indicate that activation of the presynaptic D(2) dopamine receptor inhibits the [3H]-ACh release by increasing K(+) conductance, as high K(+) concentrations abolished the inhibitory control of 7-OH-DPAT on [3H]-ACh release. This effect could be mediated by potassium channels different from those sensitive to 4-AP, TEA and quinine. In addition, the presynaptic D(2) dopamine-receptor activation seems to not involve changes in intracellular Ca(2+).

Characterization of Na/Ca exchange in plasmalemmal vesicles from zona fasciculata cells of the bovine adrenal gland

The presence of an Na/Ca exchange system in fasciculata cells of the bovine adrenal gland was tested using isolated plasmalemmal vesicles. In the presence of an outwardly Na(+) gradient, Ca(2+) uptake was about 2-fold higher than in K(+) condition. Li(+) did not substitute for Na(+) and 5 mM Ni(2+) inhibited Ca(2+) uptake. Ca(2+) efflux from Ca(2+)-loaded vesicles was Na(+)-stimulated and Ni(2+)-inhibited. The saturable part of Na(+)-dependent Ca(2+) uptake displayed Michaelis-Menten kinetics. The relationship of Na(+)-dependent Ca(2+) uptake versus intravesicular Na(+) concentration was sigmoid (apparent K(0.5) approximately 24 mM; Hill number approximately 3) and Na(+) acted on V(max) without significant effect on K(m). Na(+)-stimulated Ca(2+) uptake was temperature-dependent (apparent Q(10) approximately 2.2). The inhibition properties of several divalent cations (Cd(2+), Sr(2+), Ni(2+), Ba(2+), Mn(2+), Mg(2+)) were tested and were similar to those observed in kidney basolateral membrane. The above results indicate the presence of an Na/Ca exchanger located on plasma membrane of zona fasciculata cells of bovine adrenal gland. This exchanger displays similarities with that of renal basolateral cell membrane.

Role of regucalcin in calcium signaling

Regucalcin was discovered in 1978 as a calcium-binding protein that does not contain EF-hand motif of Ca(2+)-binding domain [M. Yamaguchi and T. Yamamoto, Chem. Pharm. Bull. 26 1915-1918 (1978)]. In recent years, regucalcin has been demonstrated to play an important role as a regulatory protein in Ca2+ signaling in rat liver and kidney cells. The organization of the rat regucalcin gene consists of seven exons and six introns. The mRNA is mainly present in liver and kidney with a size of 1.8 kb. Hepatic regucalcin mRNA expression has been shown to be stimulated by various factors including calcium, calcitonin, insulin, and estrogen in rats. The mRNA is also expressed in hepatoma cells (Morris hepatoma, HepG2, and rat hepatoma H4-II-E cells). Regucalcin plays a role in the maintenance of intracellular Ca2+ homeostasis due to activating Ca2+ pump enzymes in the plasma membrane (basolateral membrane) and microsomes of liver and renal cortex cells. Moreover, regucalcin has an inhibitory effect on the activation of Ca2+/calmodulin-dependent enzymes and protein kinase C. Also, regucalcin has been demonstrated to regulate nuclear function in liver cells; it can inhibit Ca(2+)-activated DNA fragmentation, DNA and RNA synthesis, protein kinase and protein phosphatase activities in the nuclei. Such an effect is also seen in the nuclei of regenerating rat liver. Regucalcin may play a physiological role in the control for overexpression of proliferative cells. Regucalcin has been proposed to be an important regulatory protein in Ca2+ signaling system, and it plays a multifunctional role in liver and kidney cells.

Role of calcium on the modulation of spontaneous acetylcholine efflux by the D2 dopamine receptor subtype in rat striatal synaptosomes

The role of calcium in the modulation of spontaneous [3H]acetylcholine ([3H]ACh) efflux through presynaptic D2 dopamine hetero-receptors was investigated in rat striatal synaptosomes. The kinetic studies of [3H]ACh efflux in the presence or absence of Ca2+ were carried out in nonstimulating conditions. When Ca2+ was omitted from the superfusion medium, a notable and significant (P<0.001) decrease of tritium efflux (39%) was obtained. While [3H]ACh efflux was insensitive to tetrodotoxin (TTX) 1 microM, cadmium (10 microM), a nonselective antagonist of calcium channels, significantly reduced the tritium efflux by 24% (P<0.001), while the L-type calcium antagonist, nifedipine, (30 microM) inhibited the tritium efflux by only 10% (P<0.02). 2-(4-Fenylpiperidine)cyclohexanol (vesamicol), an inhibitor of the vesicular [3H]ACh carrier, significantly depressed the spontaneous tritium efflux in the presence of Ca2+ (60%; P<0.001) and in a low-calcium medium (20%; P<0.001). Although 1 microM of 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT) inhibited spontaneous [3H]ACh efflux in the presence of calcium, this dopaminergic agonist did not modify the neurotransmitter release in either the low-Ca2+ medium or in the presence of vesamicol. These results suggest that the spontaneous [3H]ACh efflux is a process involving a Ca2+-dependent component (39%), sensitive to calcium channel-blockers and vesamicol, in rat striatal synaptosomes. In addition, activation of the D2 dopamine hetero-receptor only modulates the calcium-dependent component of spontaneous [3H]ACh efflux.

Effects of sea water and stanniectomy on branchial Ca(2+) handling and drinking rate in eel (Anguilla anguilla L.)

We examined the effects of seawater adaptation and extirpation of the Stannius corpuscles on branchial Ca(2+) flows, gill plasma membrane Ca(2+) transporters and drinking rate of European eels, Anguilla anguilla. Transepithelial Ca(2+) inflow in the gills increased 2 weeks after transfer of the eels from fresh water to sea water and after stanniectomy. Neither of these treatments changed the membrane density or the affinity of the Ca(2+)-extrusion mechanisms (Ca(2+)-ATPase and Na(+)/Ca(2+)-exchanger) in the gill cells, as measured in basolateral plasma membrane vesicles. We conclude that the increase in the Ca(2+)-transporting capacity observed in the gills of fish exposed to the larger transepithelial Ca(2+) fluxes, resulting from exposure to sea water or stanniectomy, involves an increase in number and/or size of the Ca(2+)-transporting cells, but not in the membrane density of Ca(2+) transporters. Branchial Ca(2+) outflow was higher in stanniectomised than in sham-operated fish. Changes in electrochemical driving forces as well as plasma stanniocalcin or teleocalcin levels may be the basis for the observed differences. Stanniectomy enhanced drinking in stanniectomised eels. Drinking was not further affected by transfer to sea water. These observations suggest that the corpuscles of Stannius are involved in the water balance.

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.

Activation of three types of voltage-independent Ca2+ channel in A7r5 cells by endothelin-1 as revealed by a novel Ca2+ channel blocker LOE 908

1. We have shown that in addition to voltage-operated Ca2+ channel (VOC), endothelin-1 (ET-1) activates two types of Ca2+-permeable nonselective cation channel (NSCC) in A7r5 cells: its lower concentrations (< or = 1 nM; lower [ET-1]) activate only an SK&F 96365-resistant channel (NSCC-1), whereas its higher concentrations (> or = 10 nM; higher [ET-1]) activate an SK&F 96365-sensitive channel (NSCC-2) as well. 2. We now characterized the effects of a blocker of Ca2+ entry channel LOE 908 on NSCCs and store-operated Ca2+ channel (SOCC) in A7r5 cells, and using two drugs, clarified the involvement of these channels in the ET-1-induced increase in the intracellular free Ca2+ concentrations ([Ca2+]i). Whole-cell recordings and [Ca2+]i monitoring with fluo-3 were used. 3. LOE 908 up to 10 microM had no effect on increases in [Ca2+]i induced by thapsigargin or ionomycin, but SK&F 96365 abolished them. 4. In the cells clamped at -60 mV, both lower and higher [ET-1] induced inward currents with linear iv relationships and the reversal potentials of -15.0 mV. Thapsigargin induced no currents. 5. In the presence of nifedipine, lower [ET-1] induced a sustained increase in [Ca2+]i, whereas higher [ET-1] induced a transient peak and a sustained increase. The sustained increases by lower and higher [ET-1] were abolished by removal of extracellular Ca2+, and they were suppressed by LOE 908 to 0 and 35%, respectively, with the LOE 908-resistant part being abolished by SK&F 96365. 6. These results show that LOE 908 is a blocker of NSCCs without effect on SOCC, and that the increase in [Ca2+]i at lower [ET-1] results from Ca2+ entry through NSCC-1 in addition to VOC, whereas the increase at higher [ET-1] involves NSCC-1, NSCC-2 and SOCC in addition to VOC.

ATP-dependent calcium uptake into basolateral vesicles from transporting epithelia of intermolt crayfish

ATP-dependent Ca2+ uptake was determined into inside-out basolateral membrane vesicles (BLMV) from intermolt crayfish (Procambarus clarkii) Ca2+-transporting epithelia: gill, hepatopancreas (liver), and antennal gland (kidney). Extravesicular (EV) ATP (5 mM) increased 45Ca2+ uptake (free Ca2+ 5 microM) by fivefold but was abolished by pretreatment with either vanadate or the ionophore A-23187. Addition of A-23187 to Ca2+-loaded vesicles produced 70% efflux. The saturable carrier exhibited a Km for Ca2+ of 0.11-0.27 microM and maximal influx of 20-123 pmol. mg-1. min-1. The Km for ATP was 0.01-0.04 mM. The temperature coefficient ranged from 1.43 to 2.06. EGTA treatment of hepatopancreas and antennal gland vesicles decreased 45Ca2+ uptake by 50-90%; uptake was restorable by calmodulin. However, in gill, 45Ca2+ uptake was unaffected by EGTA treatment and calmodulin decreased uptake in both EGTA-treated and untreated vesicles. Addition of EV Na+ (5 mM) increased ATP-dependent Ca2+ uptake into hepatopancreas and antennal gland BLMV by 60%; in hepatopancreas BLMV, this increase was inhibitable by ouabain. However, ATP-dependent Ca2+ uptake in gill vesicles was Na+ independent. The relative role of each epithelium in whole animal Ca2+ homeostasis has been interpreted based on in vitro characteristics.

Ion transport processes of crustacean epithelial cells

Epithelial cells of the gut, antennal glands, integument, and gills of crustaceans regulate the movements of ions into and across these structures and thereby influence the concentrations of ions in the hemolymph. Specific transport proteins serving cations and anions are found on apical and basolateral cell membranes of epithelia in these tissues. In recent years, a considerable research effort has been directed at elucidating their physiological and molecular properties and relating these characteristics to the overall biology of the organisms. Efforts to describe ion transport in crustaceans have focused on the membrane transfer properties of Na+/H+ exchange, calcium uptake as it relates to the molt cycle, heavy metal sequestration and detoxification, and anion movements into and across epithelial cells. In addition to defining the properties and mechanisms of cation movements across specific cell borders, work over the past 5 yr has also centered on defining the molecular nature of certain transport proteins such as the Na+/H+ exchanger in gill and gut tissues. Monovalent anion transport proteins of the gills and gut have received attention as they relate to osmotic and ionic balance in euryhaline species. Divalent anion secretion events of the gut have been defined relative to potential roles they may have in hyporegulation of the blood and in hepatopancreatic detoxification events involving complexation with cationic metals.

Activation of two types of Ca2+-permeable nonselective cation channel by endothelin-1 in A7r5 cells

1. In A7r5 cells loaded with the Ca2+ indicator fura-2, we examined the effect of a Ca2+ channel blocker SK&F 96365 on increases in intracellular free Ca2+ concentrations ([Ca2+]i) and Mn2+ quenching of fura-2 fluorescence by endothelin-1 (ET-1). Whole-cell patch-clamp was also performed. 2. Higher concentrations (> or = 10 nM) of ET-1 (higher [ET-1]) evoked a transient peak and a subsequent sustained elevation in [Ca2+]i: removal of extracellular Ca2+ abolished only the latter. A blocker of L-type voltage-operated Ca2+ channel (VOC) nifedipine at 1 microM reduced the sustained phase to about 50%, which was partially sensitive to SK&F 96365 (30 microM). 3. Lower [ET-1] (< or = 1 nM) evoked only a sustained elevation in [Ca2+]i which depends on extracellular Ca2+. The elevation was partly sensitive to nifedipine but not SK&F 96365. 4. In the presence of 1 microM nifedipine, higher [ET-1] increased the rate of Mn2+ quenching but lower [ET-1] had little effect. 5. In whole-cell recordings, both lower and higher [ET-1] induced inward currents at a holding potential of -60 mV with linear I-V relationships and reversal potentials close to 0 mV. The current at lower [ET-1] was resistant to SK&F 96365 but was abolished by replacement of Ca2+ in the bath solution with Mn2+. The current at higher [ET-1] was abolished by the replacement plus SK&F 96365. 6. In a bath solution containing only Ca2+ as a movable cation, ET-1 evoked currents: the current at lower [ET-1] was sensitive to Mn2+, whereas that at higher [ET-1] was partly sensitive to SK&F 96365. 7. These results indicate that in addition to VOC, ET-1 activates two types of Ca2+-permeable nonselective cation channel depending on its concentrations which differ in terms of sensitivity to SK&F 96365 and permeability to Mn2+.

Comparative study of p120 GTPase-activating protein and its point mutant in the pleckstrin homology domain

GTPase-activating protein (GAP) enhances the intrinsic GTPase activity of cellular Ras. In addition to two Src homology 2 (SH2) domains and one Src homology 3 (SH3) domain, it contains a pleckstrin homology (PH) domain. The wild-type or point mutant in the PH domain of p120 GAP (W568A) was expressed by using the baculovirus/Sf9 cell system. Direct effects of the G protein beta gamma subunit (G beta gamma) and several sphingolipids and the effects of phosphorylation by c-Src on the GTPase-stimulating activity of these GAPs on Ras were examined by using immunoprecipitates of these GAPs. The activities of neither of these GAPs were affected by the addition of G beta gamma, although the W568A mutant bound less to G beta gamma compared with the wild type. Several sphingolipids had no effect on the activity of these GAPs. Only in the W568A mutant was GTPase-stimulating activity reduced by tyrosine phosphorylation by c-Src.

Stimulatory effect of calcium administration on regucalcin mRNA expression is attenuated in the kidney cortex of rats ingested with saline

The expression of calcium-binding protein regucalcin mRNA in the kidney cortex of rats ingested with saline was investigated. The alteration in regucalcin mRNA levels was analyzed by Northern blotting using liver regucalcin complementary DNA (0.9 kb of open reading frame). Rats were freely given saline as drinking water for 7 days. Regucalcin mRNA levels in the kidney cortex were suppressed by saline ingestion. When calcium chloride (10 mg Ca/100 g body weight) was intraperitoneally administered to rats ingested with saline for 7 days, the effect of calcium administration to increase regucalcin mRNA levels was weakened by saline ingestion. Such effect was also seen by the administration of 2.5 and 5 mg Ca/100 g. Regucalcin mRNA levels in the kidney cortex of spontaneous hypertensive rats (SHR) were not appreciably increased by the administration of calcium (10 mg/100 g). Meanwhile, calcium content in the kidney cortex was significantly elevated by the administration of calcium (10 mg/100 g) to normal rats. This increase was weakened in saline-ingested rats. Moreover, Ca2+/calmodulin-dependent protein kinase activity in the cytosol of kidney cortex was significantly decreased by saline ingestion. These results suggest the possibility that saline ingestion-induced suppression of regucalcin mRNA expression in the kidney cortex is partly involved in the attenuation of Ca2+ signalling.

Intracellular Ca2+ homeostasis in trypomastigotes of Trypanosoma cruzi

Trypomastigotes of Trypanosoma cruzi maintain an intracellular Ca2+ concentration ([Ca2+]i) of 64 +/- 30 nM. Equilibration of trypomastigotes in an extracellular buffer containing 0.5 mM [Ca2+]o (preloaded cells) increased [Ca2+]i < 20 nM whereas total cell Ca2+ increased by 1.5 to 2.0 pmole/cell. This amount of Ca2+ would be expected to increase [Ca2+]i to > 10 microM suggesting active sequestration of Ca2+. We tested the hypothesis that maintenance of [Ca2+]i involved both the sequestration into intracellular storage sites and extrusion into the extracellular space. Pharmacological probes known to influence [Ca2+]i through well characterized pathways in higher eukaryotic cells were employed. [Ca2+]i responses in the presence or absence of [Ca2+]o were measured to asses the relative contribution of sequestration or extrusion processes in [Ca2+]i homeostasis. In the presence of 0.5 mM [Ca2+]o, the ability of several agents to increase [Ca2+]i was magnified in the order ionomycin >>> nigericin > thapsigargin > monensin > valinomycin. In contrast, preloading markedly enhanced the increase in [Ca2+]i observed only in response to monensin. Manoalide, an inhibitor of phospholipase A2, enhanced the accumulation of [Ca2+]i due to all agents tested, particularly ionomycin and thapsigargin. Our results suggest that sequestration of [Ca2+]i involved storage sites sensitive to monensin and ionomycin whereas extrusion of Ca2+ may involve phospholipase A2 activity. A Na+/Ca2+ exchange mechanism did not appear to contribute to Ca2+ homeostasis.

Inhibitory effect of regucalcin on Ca2+/calmodulin-dependent protein kinase activity in rat renal cortex cytosol

The effect of regucalcin on Ca2+/calmodulin-dependent protein kinase activity in the cytosol of rat renal cortex was investigated. Regucalcin is a calcium-binding protein which exists in rat liver and renal cortex. Protein kinase activity in renal cortex cytosol was markedly increased by the addition of CaCl2 (0.5 mM) plus calmodulin (10 microg/ml) in the enzyme reaction mixture . This increase was completely prevented by the addition of trifluoperazine (25 microM), an antagonist of calmodulin. The cytosolic Ca2+/calmodulin-dependent protein kinase activity was clearly inhibited by the addition of regucalcin; an appreciable effect of regucalcin was seen at 0.01 microM. The cytosolic Ca2+/calmodulin-dependent protein kinase activity was fairly increased by increasing concentrations of added Ca2+ (100-1000 microM). This increase was markedly blocked by the presence of regucalcin (0.1 microM). The inhibitory effect of regucalcin on the protein kinase activity was also seen with varying concentrations of calmodulin (2-20 microg/ml). These results demonstrate that regucalcin can regulate Ca2+/calmodulin-dependent protein kinase activity in renal cortex cells.

Alterations in Ca2+-ATPase activity and calcium-binding protein regucalcin mRNA expression in the kidney cortex of rats with saline ingestion

The alteration in calcium metabolism in rats ingested with saline was investigated. Rats were freely given saline as drinking water for 2 and 7 days. Calcium concentration in the serum was significantly elevated by saline ingestion for 2 and 7 days, while serum inorganic phosphorus concentration was not altered. Serum urea nitrogen concentration was significantly increased by saline ingestion for 7 days. Calcium content in the femoral-diaphyseal and metaphyseal tissues was not altered by saline ingestion for 7 days. Calcium content in the kidney cortex was significantly elevated by saline ingestion for 7 days. Ca2+-ATPase activity in the basolsateral membranes of kidney cortex was clearly increased by saline ingestion for 2 and 7 days. The enzyme activity was not altered by the addition of sodium chloride (10(-3) and 10(-2) M), parathyroid hormone (10(-7) and 10(-6) M), and calcitonin (3 x 10(-8) and 3 x 10(-7) M) in the enzyme reaction mixture. A calcium-binding protein regucalcin mRNA expression in the kidney cortex was markedly suppressed by saline ingestion for 7 days, although such a suppression was not seen for 2 days. These results suggest that saline ingestion causes the disturbance of calcium transport system in the kidney cortex of rats, and that the renal disorder may induce hypercalcemia.

Activatory effect of calcium-binding protein regucalcin on ATP-dependent calcium transport in the basolateral membranes of rat kidney cortex

The effect of regucalcin, a calcium-binding protein, on ATP-dependent Ca2+ transport in the basolateral membranes isolated from rat kidney cortex was investigated. The prepared membranes were in inside-out oriented and membrane vesicles. Ca(2+)-ATPase activity in the basolateral membranes was progressively elevated by increasing concentrations of regucalcin (10(-8) to 10(-6) M) in the reaction mixture. This increase was dependent on Ca2+ addition. The activatory effect of regucalcin on the enzyme is inhibited by the presence of digitonin (5 x 10(-3)%) which can solubilize the membranous lipids. Moreover, the regucalcin effect was clearly abolished by the presence of vanadate (0.1 mM) or N-ethylmaleimide (5.0 mM). However, the effect of calmodulin (6 x 10(-7) M) to increase Ca(2+)-ATPase activity was not significantly inhibited by vanadate or N-ethylmaleimide, indicating that the action mode of regucalcin differs from that of calmodulin. Also, the activatory effect of regucalcin on Ca(2+)-ATPase was appreciably inhibited by addition of dibutyryl cAMP (10(-5) and 10(-3) M), while inositol 1,4,5-trisphosphate (10(-7) and 10(-5) M) had no effect. Dibutyryl cAMP itself did not have an effect on the enzyme activity. Furthermore, the 45Ca2+ uptake by the basolateral membranes was clearly increased by the presence of regucalcin (10(-7) and 10(-6) M). This increase was completely blocked by the presence of vanadate (0.1 mM), N-ethylmaleimide (5.0 mM) or dibutyryl cAMP (10(-4) and 10(-3) M) in the reaction mixture. These results clearly demonstrate that regucalcin, which is expressed in rat kidney cortex, can increase Ca(2+)-ATPase activity and Ca2+ uptake in the basolateral membranes. Regucalcin may play a cell physiologic role as an activator in the ATP-dependent Ca2+ pumps in the basolateral membranes from rat kidney cortex.

Inhibitory effect of nitrovasodilators and cyclic GMP on ET-1-activated Ca(2+)-permeable nonselective cation channel in rat aortic smooth muscle cells

1. In single vascular smooth muscle cells (VSMCs) isolated from the aortae of male Wistar rats, we examined the effects of nitric oxide (NO) donors such as sodium nitroprusside (SNP) and S-nitroso-N-acetyl-DL-penicillamine (SNAP), and 8-bromo-guanosine-3':5'-cyclic monophosphate (8-bromo-cyclic GMP) on endothelin-1 (ET-1)-activated Ca(2+)-permeable nonselective cation channel by use of whole-cell recordings of patch-clamp technique and monitoring of intracellular free Ca(2+)-concentration ([Ca2+]i) with fura-2 real-time digital microfluorometry. 2. ET-1 evoked an initial transient peak and a subsequent sustained elevation in [Ca2+]i. After removal of extracellular Ca2+. ET-1 evoked only an initial transient peak without a sustained phase. Nifedipine (1 microM), a specific blocker of the L-type voltage-operated Ca2+ channel (VOC), reduced the sustained phase to about 40% of the control level. The remaining part of the sustained phase was abolished by 30 microM SK&F 96365, a blocker of nonselective cation channels. 3. The nifedipine-resistant sustained elevation in [Ca2+]i was abolished by 100 microM SNP, 10 microM SNAP and 300 microM 8-bromo-cyclic GMP. Neither SNP, SNAP nor 8-bromo-cyclic GMP significantly affected the basal level of [Ca2+]i. 4. In a VSMC clamped at a holding potential of -60 mV with K+ in the pipette solution replaced by Cs+, application of 10(-8) M ET-1 induced an inward current with an increase in baseline fluctuation. With fluctuation analysis, unit conductance of the ET-1-induced current was calculated to be about 21 pS. The ET-1-induced current was linearly related to the membrane potentials with its reversal potential of -5.5 mV. 5. The ET-1-induced current was reversibly and completely inhibited by 30 microM SK&F 96365 or 500 microM Cd2+. The current inhibited by SK&F 96365 or Cd2+ was linearly related to membrane potential with a reversal potential of about -5 mV. 6. The ET-1-induced current was reversibly and completely inhibited by 100 microM SNP, 10 microM SNAP and 300 microM 8-bromo-cyclic GMP. The current inhibited by SNP, SNAP or 8-bromo-cyclic GMP showed linear voltage-dependence and reversed at about -5 mV. 7. In a bath solution in which all cations were replaced by 30 mM Ca2+ and 100 mM nonpermeant cation N-methyl-D-glucamine (NMDG), ET-1 evoked a current with a reversal potential of -11 mV, from which PCa2+/Pcs1 was calculated to be 2.1. This Ca2+ current was also abolished by 100 microM SNP, 10 microM SNAP and 300 microM 8-bromo-cyclic GMP. The current inhibited by SNP, SNAP or 8-bromo-cyclic GMP showed linear voltage-dependence and reversed at about -11 mV. 8. These results taken together indicate that NO through a cyclic GMP signalling pathway inhibits ET-1-activated Ca(2+)-permeable nonselective cation channels, thereby suppressing the sustained increase in [Ca2+]i. Thus, the present study indicates that this Ca(2+)-permeable nonselective cation channel is an important target for nitrovasodilators.

Molecular dissection of Ca2+ efflux in immortalized proximal tubule cells

Plasma membrane Ca(2+)-ATPase (PMCA) and the Na+/Ca2+ exchanger participate in regulating cell function by maintaining proper intracellular Ca2+ concentrations ([Ca2+]i). In renal epithelial cells these proteins have been additionally implicated in cellular calcium absorption. The purpose of the present studies was to determine the Ca2+ extrusion mechanisms in cells derived from the proximal tubule. Homology-based RT-PCR was used to amplify PMCA transcripts from RNA isolated from mouse cell lines originating from the S1, S2, and S3 proximal tubule segments. S1, S2, and S3 cells exhibited only PMCA1 and PMCA4 products. PCR product identity was confirmed by sequence analysis. Northern analysis of proximal tubule cell RNAs revealed appropriate transcripts of 7.5 and 5.5 kb for PMCA1 and 8.5 and 7.5 kb for PMCA4, but were negative for PMCA2 and PMCA3. Western analysis with a monoclonal antibody to PMCA showed that all proximal cell lines expressed a reacting plasma membrane protein of 140 kD, the reported PMCA molecular mas. Na+/Ca2+ exchanger (NCX1) mRNA expression, analyzed by RT-PCR, protein expression by Western analysis, and functional exchange activity were uniformly absent from all proximal tubule cell lines. These observations support the idea that immortalized cells derived from the proximal tubule express PMCA1 and PMCA4, which may serve as the primary mechanism of cellular Ca2+ efflux.

Arachidonate mobilization is coupled to depletion of intracellular calcium stores and influx of extracellular calcium in differentiated U937 cells

We have previously reported that dimethylsulfoxide-differentiation of U937 cells induced significant A23187-stimulatable arachidonate mobilization, consistent with characteristics of cytosolic phospholipase A2 (Rzigalinski, B.A. and Rosenthal, M.D. (1994) Biochim. Biophys. Acta 1223, 219-225). The present report demonstrates that differentiated cells attained higher elevations of intracellular free calcium in response to A23187 and thapsigargin, consistent with enhancement of the capacitative calcium influx pathway. Differentiation induced as significant increase in the size of the intracellular calcium stores, as well as in the capacity for store-activated calcium influx. Alterations in the capacitative calcium influx pathway were coupled to differentiation-induced activation of cPLA2 and mobilization of arachidonate in response to thapsigargin and fMLP stimulation. Although cPLA2 activity is often associated with influx of extracellular calcium, arachidonate mobilization in response to thapsigargin or fMPL was not simply a consequence of calcium influx. Assessment of intracellular free calcium elevations during thapsigargin or fMPL-induced stimulation suggest that a low level of arachidonic acid release was initiated upon release of intracellular store calcium. This initial release of arachidonate was unaffected by inhibition of calcium influx with nickel, EGTA, or SKF96365. Arachidonate release was observed when extracellular calcium was replaced with extracellular strontium, suggesting activation of the cytosolic PLA2 rather than secretory PLA2. Inhibition of PLA2 with prostaglandin B oligomer prevented both thapsigargin and fMLP-stimulated influx of extracellular calcium. Furthermore, exogenous free arachidonate stimulated influx of extracellular calcium in differentiated U937 cells. These results suggest that cPLA2-mediated release of free arachidonate may participate in the formation of a calcium influx factor which controls influx of extracellular calcium through store-controlled channels in the plasma membrane.

Effect of the aminosteroid, U73122, on Ca2+ uptake and release properties of rat liver microsomes

The putative phospholipase C inhibitor, U73122, transiently increases the cytosolic free Ca2+ concentration in rabbit pancreatic acinar cells by stimulating the release of Ca2+ from intracellular stores [Willems, Van de Put, Engbersen, Bosch, Van Hoof & De Pont (1994) Pflügers Arch. 427, 233-243]. In order to elucidate the exact mechanism of action of U73122 we studied its effects on both Ca(2+)-stimulated Mg(2+)-dependent ATPase activity and Ca(2+)-stimulated ATP-dependent Ca2+ uptake in rat liver microsomes. In addition, we studied its effects on Ca2+ release from steady-state loaded microsomes. The effects of U73122 were compared with those of thimerosal, described in the literature as inhibiting Ca(2+)-ATPases and sensitizing inositol 1,4,5-trisphosphate-operated Ca2+ release channels, and thapsigargin, a specific inhibitor of sarcoplasmic and endoplasmic reticulum Ca(2+)-ATPases. Both U73122 (IC50 = 9 microM) and thimerosal (IC50 = 11 microM) dose-dependently inhibited Ca(2+)-stimulated Mg(2+)-dependent ATPase activity, without significantly affecting Mg(2+)-stimulated ATPase activity. Similarly, both U73122 (IC50 = 9 microM) and thimerosal (IC50 = 14 microM) dose-dependently inhibited ATP-dependent Ca2+ uptake. At concentrations beyond 20 microM, U73122 stimulated Ca2+ release from steady-state loaded microsomes at a rate considerably higher than obtained with a maximally inhibitory concentration of thapsigargin (1 microM). This observation, which was not reached with equally inhibitory concentrations of thimerosal, demonstrates that higher U73122 concentrations cause an additional increase of passive Ca2+ leak. The data presented demonstrate that U73122 stimulates the release of actively stored Ca2+ primarily through inhibition of the internal Ca2+ pump.

Isolation of syncytial microvillous membrane vesicles from human term placenta and their application in drug-nutrient interaction studies

The initial step in placental uptake of nutrients occurs across the syncytial microvillous membrane of the trophoblast. This study was designed to isolate syncytial microvillous membrane vesicles (SMMV) of human term placenta, to validate their purity and viability, and to investigate the interaction of several commonly used drugs with the transport of two essential nutrients: alanine and choline. SMMV were isolated according to an established procedure, but instead of homogenization the initial preparation step was replaced by mincing of placental tissue followed by gently stirring to loosen the microvilli. These modifications doubled the protein recovery and increased the enrichment in alkaline phosphatase, whereas no substantial contamination with basal membranes nor interfering subcellular organelles was found. The functional viability of the vesicles was evaluated through the transport of alanine. In accordance with literature, uptake was sodium-dependent, inhibitable by structural analogues, and saturable. A number of cationic drugs were were able to able to inhibit choline uptake, whereas no effect on alanine transport was observed. Anionic drugs, drugs of abuse, and catecholamines did not interfere with alanine transport either. In conclusion, our isolated SMMV provide a suitable tool for screening drug-nutrient interactions at the level of membrane transport. In view of the very low susceptibility of the alanine transporter to drug inhibition and the relatively high drug concentrations necessary to inhibit choline transport, it seems unlikely that clinically important drug interactions may occur with these nutrients.

Long-lasting activation of cation current by low concentration of endothelin-1 in mouse fibroblasts and smooth muscle cells of rabbit aorta

1. Recombinant human ETA receptors were expressed in a mouse fibroblast cell line (Ltk- cell) and functional coupling of the receptors with Ca2+ permeable channels at low concentrations of endothelin-1 (ET-1) was investigated using whole-cell recordings and monitoring the changes in intracellular free Ca2+ concentrations ([Ca2+]i) with a Ca2+ indicator, fluo-3. A similar type of coupling was investigated in freshly dispersed vascular smooth muscle cells (VSMCs) of rabbit thoracic aorta by use of whole-cell recordings. 2. In Ltk- cells expressing recombinant human ETA receptors, concentrations of ET-1 (10(-8) M, 10(-9) M) evoked an initial transient peak and a subsequent sustained elevation in [Ca2+]i whereas a lower concentration of ET-1 (10(-10) M) evoked only a sustained elevation of [Ca2+]i. After removal of extracellular Ca2+, ET-1 evoked only an initial peak without a sustained elevation of [Ca2+]i. The sustained elevation induced by 10(-10) M ET-1 was blocked by 300 microM mefenamic acid (a cation channel blocker) but not by 10 microM nifedipine (a blocker of voltage-operated Ca2+ channel). 3. In whole-cell recordings with Ltk- cells, a brief (3-5 min) application of ET-1 (10(-10) M) induced a sustained inward current at a holding potential of -60 mV. The current-voltage relationship revealed that the reversal potential of the ET-1-induced current was close to 0 mV (1.9 mV) and was not altered by reducing the concentration of Cl- in the bath solution, indicating that the current is carried by cations. The current was reversibly blocked by 300 microM mefenamic acid, and it persisted after all cations in the bath solution had been replaced by Ca2+ (5 or 30 mM) and nonpermeant cation N-methyl-D glucamine,indicating that the ET-1-activated channel is permeable to Ca2+. Activation of the current was independent of membrane potential and the current was induced even after addition of a high concentration (10 mM) of a Ca2+ chelator, EGTA, to the pipette solution.4. In whole-cell recordings from rabbit aortic VSMCs, ET-l (101-10 M) induced a sustained inward current at a holding potential of -60 mV. The reversal potential was - 12 mV and was not altered when the concentration of Cl- in the pipette solution was decreased, indicating that the current is carried by cations. Again activation of the current was independent of membrane potential and was observed even after addition of a high concentration (10 mM) of a Ca2+ chelator, EGTA to the pipette solution. The current was reversibly blocked by 300 microM mefenamic acid and was permeable to Ca2+,showing marked similarities to ET-1-induced cationic current in Ltk- cells.5. These results indicate that in Ltk- cells transfected with cDNA for recombinant ETA receptors andVSMCs, ETA receptors can functionally couple with a nonselective cation channel permeable to Ca2+.Thus the present data suggest that the cation channel plays an essential role in the sustained elevation of[Ca2+]i at low concentrations of ET-l by causing Ca2+ entry through the channel.

Na/Ca exchange in the basolateral membrane of the A6 cell monolayer: role in Cai homeostasis

The presence of a Na/Ca exchanger in A6 cells was investigated by measuring intracellular calcium (Cai) fluctuations and the 45Ca fluxes through the basolateral membranes (blm) of the cell monolayer. Removal of Na+ from the medium produced a transient increase in Cai followed by a regulatory phase returning Cai to control levels in 3-4 min, this phase being greatly accelerated (< 60 s) by NaCl addition (apparent Km of approximately 5 mM Na+). The Cai increase was only found with the Na(+)-free medium on the basolateral side of the cell monolayer. A twofold increase in the 45Ca influx was observed under these conditions. In Ca(2+)- depleted cells, the initial Cai increase after Ca2+ addition to the medium was greater when the putative Na/Ca exchanger was not functioning (i.e. in a Na(+)-free medium). 45Ca effluxes through the blm of the monolayer were greatly and transiently increased by a Na(+)-free medium on the serosal side and blocked by orthovanadate (1 mM). The Cai increased induced by a hypo-osmotic shock was greater in cells bathed in a Na(+)-medium, conditions expected to block the activity of the Na/Ca exchanger. These findings support the hypothesis that a Na/Ca exchanger is present on the blm of A6 cells and affirm its role in Cai homeostasis in steady-state conditions and following osmotic shock. In addition, a Ca2+ pump also located on the blm and Ca2+ stores sensitive to inositol 1,4,5-trisphosphate were found to be implicated in Cai homeostasis.

Characterization of Gq family G proteins GL1 alpha (G14 alpha), GL2 alpha (G11 alpha), and Gq alpha expressed in the baculovirus-insect cell system

The alpha subunits of Gq family G proteins, GL1 alpha (G14 alpha), GL2 alpha(G11 alpha), and Gq alpha were expressed with G protein beta 1 and gamma 2 subunits in insect cells using a baculovirus system. The trimeric forms of G proteins, GL1 (GL1 alpha beta gamma), GL2 (GL2 alpha beta gamma), and Gq (Gq alpha beta gamma), were solubilized by 1% sodium cholate and purified by sequential chromatography on three kinds of columns. GL1, GL2, and Gq activated phospholipase C-beta purified from bovine brain in the presence of aluminum fluoride to the same extent. Muscarinic acetylcholine receptor m1 subtype stimulated the guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) binding to GL1, GL2, and Gq in the presence of similar concentrations of carbamylcholine. When m1 receptor, G protein, and phospholipase C-beta were reconstituted in lipid vesicles, each subtype of Gq family G proteins mediated the activation of phospholipase C-beta by carbamylcholine in the presence of either 1 microM GTP gamma S or 1 mM GTP. Phospholipase C-beta stimulated the GTPase activity of GL1, GL2, and Gq in the presence of m1 receptor and carbamylcholine but did not stimulate the GTPase activity of GO. Protein kinase C phosphorylated m1 receptor and phospholipase C-beta, but the phosphorylation did not significantly affect the ability of the m1 receptor to stimulate phospholipase C-beta in the reconstitution system of purified proteins.

Differential effects of heavy metal ions on Ca(2+)-dependent K+ channels

1. The ability of various divalent metal ions to substitute for Ca2+ in activating distinct types of Ca(2+)-dependent K+ [K+(Ca2+)] channels has been investigated in excised, inside-out membrane patches of human erthrocytes and of clonal N1E-115 mouse neuroblastoma cells using the patch clamp technique. The effects of the various metal ions have been compared and related to the effects of Ca2+. 2. At concentrations between 1 and 100 microM Pb2+, Cd2+ and Co2+ activate intermediate conductance K+(Ca2+) channels in erythrocytes and large conductance K+(Ca2+) channels in neuroblastoma cells. Pb2+ and Co2+, but not Cd2+, activate small conductance K+(Ca2+) channels in neuroblastoma cells. Mg2+ and Fe2+ do not activate any of the K+(Ca2+) channels. 3. Rank orders of the potencies for K+(Ca2+) activation are Pb2+, Cd2+ > Ca2+, Co2+ >> Mg2+, Fe2+ for the intermediate erythrocyte K+(Ca2+) channel, and Pb2+, Cd2+ > Ca2+ > Co2+ >> Mg2+, Fe2+ for the small, and Pb2+ > Ca2+ > Co2+ >> Cd2+, Mg2+, Fe2+ for the large K+(Ca2+) channel in neuroblastoma cells. 4. At high concentrations Pb2+, Cd2+, and Co2+ block K+(Ca2+) channels in erythrocytes by reducing the opening frequency of the channels and by reducing the single channel amplitude. The potency orders of the two blocking effects are Pb2+ > Cd2+, Co2+ >> Ca2+, and Cd2+ > Pb2+, Co2+ >> Ca2+, respectively, and are distinct from the potency orders for activation. 5. It is concluded that the different subtypes of K+(Ca2+) channels contain distinct regulatory sites involved in metal ion binding and channel opening. The K+(Ca2+) channel in erythrocytes appears to contain additional metal ion interaction sites involved in channel block.

Renal handling of Ca2+ in diabetes

Ca2+ transport in kidney has gained considerable attention in the recent past. Our laboratory has been involved in understanding the regulatory mechanisms underlying Ca2+ transport in the kidney across the renal basolateral membrane. We have shown that ANP, a cardiac hormone, mediates its biological functions by acting on its receptors in the kidney basolateral membrane. Furthermore, it has been established that ANP receptors are coupled with Ca2+ ATPase, the enzyme that participates in the vectorial translocation of Ca2+ from the tubular lumen to the plasma. It is possible that a defect in the ANP-receptor-effector system in diabetes (under certain conditions such as hypertension) may be associated with abnormal Ca2+ homeostasis and the development of nephropathy. Accordingly, future studies are needed to establish this hypothesis.

Activation of calcium influx by ATP and store depletion in primary cultures of renal proximal cells

Cytoplasmic calcium changes and calcium influx evoked by adenosine triphosphate (ATP) were investigated in primary cultures of rabbit proximal convoluted tubule cells. Extracellular ATP (50 microM) induced a biphasic increase of [Ca2+]i measured with the calcium probe fura-2. In the early phase, the mobilization of intracellular pools resulted in a transient increase of [Ca2+]i from 106 +/- 11 nM (n = 36) to 1059 +/- 115% (n = 29) of the resting level within 10 s. In the presence of external calcium, [Ca2+]i then decreased within 3 min to a sustained level (398 +/- 38%, n = 8). Measurements of fura-2 quenching by external manganese revealed that this phase was the result of an increased Ca2+ uptake, blocked by lanthanum (10 microM) and verapamil (100 microM) but not by the nifedipin (25 microM). Internal calcium store depletion by ATP induced an increased calcium influx through lanthanum- and verapamil-sensitive, nifedipin-insensitive calcium channels, located on the apical membrane of the cells. As indicated by 86Rb+ efflux measurements, ATP activated a potassium efflux that was blocked by barium and Leiurus quinquestriatus hebraeus (LQH) venom (containing charybdotoxin) indicating the involvement of Ca(2+)-sensitive K+ channels. Moreover, in the presence of the LQH venom, the internal calcium stores were not replenished after being depleted by ATP. Our results indicate that an ATP-evoked hyperpolarization of the plasma membrane leads to increased Ca2+ influx, which facilitates the replenishment of the internal stores.

Two vitamin D3-dependent calcium binding proteins increase calcium reabsorption by different mechanisms. II. Effect of CaBP 9K

The distal tubule is a major site of hormone regulated Ca2+ reabsorption. We have previously shown that in the rabbit, vitamin D depletion decreases both the Ca2+ uptake by the luminal membrane of nephron distal segments and the ATP-dependent Ca2+ transport by the basolateral membrane of these same segments. We also reported that in in vitro experiments, the vitamin D-dependent calcium binding protein (CaBP) 28K enhances Ca2+ uptake by the high-affinity Ca2+ transport system of the distal luminal membranes. The purpose of the present study was to investigate the role of CaBP 9K in Ca2+ transport by the two polar membranes of the proximal and distal nephron segments. Preloading the luminal membranes of the two series of tubules with 10 microM CaBP 9K did not modify 0.5 mM Ca2+ uptake by these membranes. In contrast, preincubation of distal tubule basolateral membranes with 10 microM CaBP 9K strongly enhanced ATP-dependent Ca2+ transport by these membranes. A similar although more modest effect was observed in proximal basolateral membranes. This effect was present only in calmodulin depleted membranes. Calmodulin also increased the ATP-dependent Ca2+ uptake by the depleted membranes. The two actions were not additive. A dose-response curve showed that the maximal CaBP 9K effect was obtained at 10(-5) M. CaBP 9K increased the Vmax of ATP-dependent uptake from 0.43 +/- 0.03 to 0.79 +/- 0.06 nmol/mg/10 sec (P < 0.005), leaving the Km unchanged. It is concluded that CaBP 9K activates the Ca2+ pump in the basolateral membrane of both the proximal and distal tubule.(ABSTRACT TRUNCATED AT 250 WORDS)

Two vitamin D3-dependent calcium binding proteins increase calcium reabsorption by different mechanisms. I. Effect of CaBP 28K

Several clearance and micropuncture studies suggest that vitamin D deficiency decreases calcium (Ca2+) reabsorption by the nephron. In confirmation of these observations, we reported recently that in vitamin D depleted rabbits, Ca2+ transport by both the luminal and the basolateral membranes of the distal tubule was significantly diminished. It is also in the distal tubule that vitamin D stimulates the synthesis of two calcium binding proteins (CaBP) with molecular weights of 28K and 9K. The purpose of this study was to investigate the role of the CaBP 28K in the regulation of Ca2+ transport by the two polar membranes of the distal nephron of the rabbit. Results were compared to those obtained with the corresponding membranes of the proximal tubule. The basolateral and luminal membranes were purified from proximal and distal tubule suspensions separately. Ca2+ uptake was measured by the rapid filtration technique. Preloading the luminal membrane vesicles from proximal and distal tubules with 3 microM CaBP 28K strongly enhanced the initial Ca2+ uptake by the two membranes. When the vesicles, preloaded with CaBP 28K and incubated with 0.5 mM Ca2+, were abruptly diluted in a medium containing 10 microM A 23187 and 2 mM EGTA, the release of Ca2+ was slower than with control vesicles, suggesting that the excess of transported Ca2+ was bound inside the vesicles. The effect of CaBP 28K was dose-dependent with a half-maximal effect at 10(-6) M and a maximal effect at 5 x 10(-6) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Cadmium ion is a non-competitive inhibitor of red cell Ca(2+)-ATPase activity

In the presence as well as in the absence of calmodulin, Cd2+ inhibits the human erythrocyte plasma membrane Ca(2+)-ATPase activity non-competitively with Ki = 2 nM, whereas ATP-dependent Ca(2+)-transport across the red cell membrane was found to be inhibited competitively by Cd2+ (Verbost, P.M., Flik, G., Pang, P.K.T., Lock, R.A.C. and Wendelaar Bonga, S.E. (1989) J. Biol. Chem. 264, 5613-5615). In this study it will be argued that Cd2+ also inhibits Ca(2+)-transport non-competitively, and that the discrepancy with previous conclusions most probably relies on use of an incorrect computer program that calculates the free concentrations of Ca2+ and Cd2+ at the experimental conditions applied for measurement of Ca2+ uptake.

Characterization of calcium transport in the luminal and the basolateral membrane from kidney cortex of hypercalcemic rats bearing the Walker 256 carcinosarcoma

The Walker 256 (W256) carcinosarcoma did not significantly modify calcium uptake by brush border membrane (BBM) vesicles in the kidney of the host rat, compared with that in membranes isolated from control animals. However, it showed a tendency to increase at near equilibrium in W256 tumor-host rats, associated with a decreased BBM protein content. ATP-dependent calcium influx by basolateral membrane (BLM) vesicles from W256 tumor-bearing rats was also increased compared with that in control BLM. This stimulation was due to a decreased Km for calcium. Passive calcium permeability or the Na+/Ca2+ exchanger were unchanged in BLM from W256 tumor-host rats compared with control BLM. Pre-stimulation of control rat cortical tubules with either 10(-7) M parathyroid hormone-related protein (PTHrP) (1-34) or 10(-4) M N6,2'-O-dibutiryl cyclic AMP before BLM isolation, did not modify the ATP-dependent calcium uptake by BLM vesicles compared with control membranes. However, our results do not rule out that the stimulated ATP-dependent calcium influx in BLM from W256 tumor-host rats could be mediated by the interaction among PTHrP and other humoral factors. Our findings suggest a possible mechanism for the increased renal calcium reabsorption in this animal model for humoral hypercalcemia of malignancy.