大鼠肾近球小管细胞胞饮体膜氢泵活性和水的渗透性转运的特征

本实验采用异硫氰酸-葡聚糖荧光素(fluorescein isothiocyanate-dextran,FITC-dextran)体内标记法,研究大鼠肾近球小管细胞胞饮体(endosome)膜上 H~+-ATP 酶的活性及水的渗透性转运。通过观察在胞饮体外加入一定量 ATP 后,胞饮体内 pH 值的时间反应曲线,从而测定 ATP-依赖的 H~+在胞饮体膜上的转运情况。胞饮体内的酸化速度及 pH 的最低值与加入的 ATP 浓度有关。在加入 ATP 前,胞饮体内的 pH 值为7.4,加入不同浓度的 ATP 后,即[ATP]为0.005,0.05,0.5,5和10mmol/L,胞饮体内 pH 最低值分别为7.30,6.99,6.68,6.38和6.39。此种由 ATP 引起的酸化反应,被0.5mmol/L N-ethylmaleimide(NEM)抑制97%,但不被钒酸盐和 oligomycin 所抑制。实验还同时观察了此种胞饮体水的渗透性转运机制。通过在胞饮体膜内外建立一个蔗糖浓度梯度。观察 FITC-dextran 荧光信号的快速动力学变化过程,从而测定由于渗透压梯度引起的水在胞饮体膜上转运的特征。在230℃时,水的渗透性通透系数(osmotic water permeability coefficient,P_f)为0.03cm/s;加入0.5mmol/L HgCl_2后,水的转运被抑制70%。此抑制反应可被5mmol/L 巯基乙醇(β-Mcrcaptoethanol)完全逆转。上述结果提示:大鼠肾近球小管胞饮体膜含有H~+-ATP 酶和水的转运通道。胞饮     

Maturation of rat proximal tubule chloride permeability

We have previously shown that neonate rabbit tubules have a lower chloride permeability but comparable mannitol permeability compared with adult proximal tubules. The surprising finding of lower chloride permeability in neonate proximals compared with adults impacts net chloride transport in this segment, which reabsorbs 60% of the filtered chloride in adults. However, this maturational difference in chloride permeability may not be applicable to other species. The present in vitro microperfusion study directly examined the chloride and mannitol permeability using in vitro perfused rat proximal tubules during postnatal maturation. Whereas there was no maturational change in mannitol permeability, chloride permeability was 6.3 +/- 1.3 x 10(-5) cm/s in neonate rat proximal convoluted tubule and 16.1 +/- 2.3 x 10(-5) cm/s in adult rat proximal convoluted tubule (P < 0.01). There was also a maturational increase in chloride permeability in the rat proximal straight tubule (5.1 +/- 0.6 x 10(-5) cm/s vs. 9.3 +/- 0.6 x 10(-5) cm/s, P < 0.01). There was no maturational change in bicarbonate-to-chloride permeabilities (P(HCO3)/P(Cl)) in the rat proximal straight tubules (PST) and proximal convoluted tubules (PCT) or in the sodium-to-chloride permeability (P(Na)/P(Cl)) in the proximal straight tubule; however, there was a significant maturational decrease in proximal convoluted tubule P(Na)/P(Cl) with postnatal development (1.31 +/- 0.12 in neonates vs. 0.75 +/- 0.06 in adults, P < 0.001). There was no difference in the transepithelial resistance measured by current injection and cable analysis in the PCT, but there was a maturational decrease in the PST (7.2 +/- 0.8 vs. 4.6 +/- 0.1 ohms x cm2, P < 0.05). These studies demonstrate there are maturational changes in the rat paracellular pathway that impact net NaCl transport during development.     

Calcium transport by sarcoplasmic reticulum of vascular smooth muscle: II. Effects of calmodulin and calmodulin inhibitors.

The role of calmodulin (CaM) in modulating calcium (Ca) uptake by sarcoplasmic reticulum (SR) of vascular smooth muscle was studied in saponin skinned strips of rat caudal artery. Exogenous CaM concentrations ranging from 0.3-1.8 microM did not statistically change the steady state MgATP-dependent Ca content, the MgATP-independent Ca content, or the oxalate-stimulated Ca influx. Calmidazolium (CDZ), W-7, and trifluoperazine (TFP) were used to examine the potential effect of an endogenous CaM pool on inward Ca transport. The IC50 of these antagonists for inhibition of Ca-CaM-stimulated phosphodiesterase activity and Ca-activated superprecipitation of canine aortic actomyosin was measured and found to be in the low micromolar range with a rank order of potency for inhibition of CDZ greater than TFP greater than W-7. In skinned tissues, micromolar concentrations of antagonists that inhibited CaM-mediated reactions in isolated enzyme systems did not reduce Ca content or oxalate-stimulated Ca influx. At higher concentrations of 100-200 microM, the MgATP-dependent Ca content was significantly reduced by TFP and W-7 but not by CDZ. The order of potency for inhibition of Ca uptake was TFP greater than W-7 greater than CDZ. The MgATP-independent Ca content was significantly decreased only by 200 microM TFP. Although none of these inhibitors significantly altered Ca efflux at concentrations up to 100 microM, Ca release was significantly stimulated by all three at 200 microM. The TFP-stimulated Ca release was partially inhibited by ruthenium red. The results indicate that neither exogenous CaM nor an endogenous CaM pool directly modulates inward Ca transport by the SR of saponin skinned caudal artery. The inhibition of Ca uptake produced by hundred micromolar concentrations of CaM antagonists fails to correlate with the order of and with the potency of inhibition measured in isolated enzyme systems. This suggests that the inhibition of Ca uptake produced by high concentrations of these antagonists may be independent of a specific interaction with CaM. The activation of Ca release by high concentrations of CaM antagonists may involve a nonspecific increase in membrane permeability as well as modulation of a membrane Ca channel.     

Loss of alpha 2-macroglobulin and epidermal growth factor surface binding induced by phenothiazines and naphthalene sulfonamides

We have found that certain naphthalenesulfonamides [e.g., N-6(-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7)] and phenothiazines [e.g., trifluoperazine (TFP)] induce a loss of cell-surface receptors for alpha 2-macroglobulin, and epidermal growth factor (EGF) in fibroblasts. The loss of alpha 2-macroglobulin receptors is independent of receptor occupancy and is rapidly reversed upon removal of these agents from the culture medium. The extent of EGF receptor loss is less than for alpha 2-macroglobulin, and the EGF receptors do not reappear at the surface when W-7 is removed. Receptor loss was measured as a change in the capacity for binding iodinated ligands; no change in affinity of binding was observed. This receptor loss could reflect inactivation of receptors or internalization. W-7 did not induce a loss of cell surface beta 2-microglobulin, a membrane protein which is excluded from coated pits and which is not internalized, indicating that the effect of W-7 was specific for membrane receptors and not a result of bulk depletion of plasma membrane. The loss of alpha 2-macroglobulin and EGF receptors occurs at concentrations which do not cause an increase in the pH of endocytic vesicles or the cytoplasm, indicating that these agents act by a mechanism distinct from the effect of other weak bases. Since both TFP and W-7 are potent inhibitors of calmodulin, we investigated the possibility that inhibition of calmodulin was responsible for the loss of receptors. Three lines of evidence suggest that calmodulin inhibition is not responsible for the inhibition of binding and endocytosis: 1) Promethazine, a phenothiazine that is a poor inhibitor of calmodulin, is nearly as effective as TFP at inhibiting endocytosis; calmidazolium, a potent inhibitor of several calmodulin functions, did not cause a loss of binding; 2) the microinjection of calmodulin into cells did not reverse the effects of W-7; using pressure microinjection, we introduced up to a 100-fold excess of calmodulin over native levels into individual gerbil fibroma cells; using rhodamine-labeled alpha 2-macroglobulin, we saw that the W-7 induced inhibition of receptor-mediated endocytosis was the same in injected and uninjected cells; 3) we injected calcineurin, a calmodulin-binding protein, into cells (1-3 pg/cell) and observed no effect on the receptor-mediated endocytosis of rhodamine-labeled alpha 2-macroglobulin. These data indicated that cell surface receptor numbers can be regulated by a cellular component that is not cytoplasmic calmodulin but that shares some drug sensitivities with calmodulin.     

Neuropeptide Y inhibits cAMP accumulation in renal proximal convoluted tubules

The effect of neuropeptide Y (NPY) on cAMP accumulation in various segments of the rabbit nephron was examined. NPY inhibited parathyroid hormone-stimulated cAMP accumulation in the proximal convoluted tubule in a concentration-dependent manner. NPY also inhibited forskolin-stimulated cAMP production in this segment of the nephron. In contrast, NPY had no effect on parathyroid hormone or forskolin-stimulated cAMP accumulation in the proximal straight tubule. Similarly, NPY had no effect on forskolin-stimulated cAMP levels along the rest of the nephron. These results are consistent with previous studies which have localized NPY receptors to the proximal convoluted tubule, and suggest that NPY via its effects on cAMP metabolism may play a role in proximal tubule transport.     

Comparison of the roles of calmodulin and protein kinase C in activation of the human neutrophil respiratory burst

The roles of calmodulin and protein kinase C in the activation of the human neutrophil respiratory burst were characterized pharmacologically. The protein kinase C inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) and N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9) did not inhibit superoxide anion generation by neutrophils stimulated for 30 minutes with N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) or 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA). However, H-7 did depress superoxide production during the first 5 minutes following stimulation. In contrast, the specific calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and the dual calmodulin antagonist/protein kinase C inhibitor trifluoperazine (TFP) were potent inhibitors of the response throughout the 30 minute incubation. Stimulation of neutrophils with submaximal doses of FMLP or PMA failed to promote inhibition of the respiratory burst by H-7 or H-9, but did stimulate a respiratory burst response which was not inhibited by TFP or W-7. These results suggest that while protein kinase C may play a role in the initiation of the respiratory burst response, propagation of the response is dependent on calmodulin-dependent processes. The inability of TFP and W-7 to inhibit superoxide anion generation in response to submaximal stimulatory doses of FMLP or PMA suggests that calmodulin-independent processes may also be involved in activation of the respiratory burst.     

ATP-dependent Ca2+ transport in the rat parotid basolateral plasma membrane is regulated by calmodulin

Calmodulin regulation of ATP-dependent Ca2+ transport activity was assessed in inverted basolateral plasma membrane vesicles (BLMV) isolated from rat parotid glands. The initial rate of Ca2+ transport in media containing 100 nM Ca2+ was stimulated by approximately 60% at maximal concentrations (300 nM) of exogenously added calmodulin (CAM). Half-maximal activation was obtained at 50 and 175 nM CAM in KCl and mannitol containing assay media, respectively. In the KCl medium, addition of 300 nM CAM increased the affinity of the BLMV Ca2+ transport activity for Ca2+ from approximately 70 nM, in the absence of added CAM, to approximately 50 nM. Vmax was consistently increased by approximately 20% under these conditions. When BLMV were treated with ethylene glycol bis(beta-aminoethylether) N,N'-tetraacetic acid (EGTA) (200 microM), the affinity of the transporter for Ca2+ decreased by 50% to approximately 150 nM, with no change in Vmax. When CAM was added to the EGTA-treated membranes, Ca2+ transport activity was comparable to that obtained when CAM was added directly to control, untreated BLMV. The CAM antagonists, trifluoperazine (TFP), W-7, and calmidazolium, inhibited Ca2+ transport in the presence of CAM. Half-maximal inhibition of transport was achieved by 12 microM TFP and 20 microM W-7. Calmidazolium (1 microM) inhibited Ca2+ transport by 75%. The inhibitory effects on ATP-dependent Ca2+ transport exerted by these agents were not due to an increase in the passive permeability of the membranes to Ca2+. Furthermore, in the absence of added CAM, the inhibitory effects of these agents on initial Ca2+ transport rate was decreased. The data presented suggest that the Ca2+-dependent interaction of CAM with the ATP-dependent Ca2+ transporter in rat parotid BLMV modifies the kinetic properties of this Ca2+ transporting mechanism.     

Inhibition of myogenesis by trifluoperazine and compound 48/80

When trifluoperazine (TFP), a calmodulin antagonist, was given to chick or rat myoblasts in cultures, formation of multinucleated myotubes was inhibited. The inhibition of cell fusion by TFP in rat cultures prevents the normal increase in the amount of acetylcholine receptors (AChR) and creatine kinase (CK), while the levels of these proteins in chick muscle cultures are hardly affected. Another calmodulin antagonist, compound 48/80, inhibits fusion at doses that correspond closely to its antagonistic effects on calmodulin. Thus, our results suggest a possible role for calmodulin in the regulation of myoblast fusion, but not on the appearance of muscle proteins.     

Na+,K+ pump and Na+-coupled ion carriers in isolated mammalian kidney epithelial cells: regulation by protein kinase C.

This review updates our current knowledge on the regulation of Na+/H+ exchanger, Na+,K+,Cl- cotransporter, Na+,Pi cotransporter, and Na+,K+ pump in isolated epithelial cells from mammalian kidney by protein kinase C (PKC). In cells derived from different tubule segments, an activator of PKC, 4beta-phorbol 12-myristate 13-acetate (PMA), inhibits apical Na+/H+ exchanger (NHE3), Na+,Pi cotransport, and basolateral Na+,K+ cotransport (NKCCl) and augments Na+,K+ pump. In PMA-treated proximal tubules, activation of Na+,K+ pump probably plays a major role in increased reabsorption of salt and osmotically obliged water. In Madin-Darby canine kidney (MDCK) cells, which are highly abundant with intercalated cells from the collecting duct, PMA completely blocks Na+,K+,Cl- cotransport and decreases the activity of Na+,Pi cotransport by 30-40%. In these cells, agonists of P2 purinoceptors inhibit Na+,K+,Cl- and Na+,Pi cotransport by 50-70% via a PKC-independent pathway. In contrast with MDCK cells, in epithelial cells derived from proximal and distal tubules of the rabbit kidney, Na+,K+,Cl- cotransport is inhibited by PMA but is insensitive to P2 receptor activation. In proximal tubules, PKC-induced inhibition of NHE3 and Na+,Pi cotransporter can be triggered by parathyroid hormone. Both PKC and cAMP signaling contribute to dopaminergic inhibition of NHE3 and Na+,K+ pump. The receptors triggering PKC-mediated activation of Na+,K+ pump remain unknown. Recent data suggest that the PKC signaling system is involved in abnormalities of dopaminergic regulation of renal ion transport in hypertension and in the development of diabetic complications. The physiological and pathophysiological implications of PKC-independent regulation of renal ion transporters by P2 purinoceptors has not yet been examined.     

Epidermal growth factor binding, stimulation of phosphorylation, and inhibition of gluconeogenesis in rat proximal tubule

Epidermal growth factor and insulin share many biological activities, including stimulation of cell proliferation, ion flux, glycolysis, fatty acid and glycogen synthesis, and activation of receptor-linked tyrosine kinase activity. In the kidney, insulin has been shown to regulate transport processes and inhibit gluconeogenesis in the proximal tubule. Since the kidney represents a major source of EGF, the present studies investigated whether proximal tubule contained EGF receptors, whether EGF receptors were localized to apical or basolateral membranes, and whether EGF receptor activation participated in the regulation of an important proximal tubule function, gluconeogenesis. Specific EGF receptors were demonstrated in the basolateral membrane of proximal tubule. Following incubation with 125I EGF, basolateral membranes demonstrated equilibrium binding at 4 degrees C and 23 degrees C. There was 78 +/- 2% specific binding (n = 13). The dissociation constant (Kd) was 1.5 x 10(-9) M and maximal binding was 44 fmol/mg protein. There was ninefold more specific binding to proximal tubule basolateral membrane than to brush border membrane. In basolateral, but not brush border membranes, EGF induced phosphorylation of the tyrosine residues of intrinsic membrane proteins, including a 170 kDa protein, corresponding to the EGF receptor. In the presence of the gluconeogenic substrates, alanine, lactate, and succinate, proximal tubule suspensions synthesized glucose. EGF inhibited glucose production in a concentration-dependent manner over a concentration range of 3 x 10(-11) to 3 x 10(-9) M. In addition, EGF inhibited angiotensin II-stimulated glucose production in the proximal tubule suspensions. EGF did not significantly increase net glucose metabolism nor decrease cellular ATP concentrations. Therefore, these studies demonstrated that rat proximal tubule contained specific receptors for EGF that were localized to the basolateral membrane and linked to tyrosine kinase activity. EGF significantly inhibited proximal tubule glucose production without significantly increasing net glucose consumption.     

The inhibitory effects of calmodulin antagonists on TSH-stimulated thyroid hormone release by mouse thyroid lobes

Calcium ions have been shown to play a mojor regulatory role in the release of various hormones from a wide variety of endocrine organs. More recently, in vitro evidence suggests that a calcium-binding protein, calmodulin, is also involved in the release of many hormones. So we examined the effects of several types of calmodulin antagonists on TSH-stimulated thyroid hormone release in vitro. Mouse thyroid lobes (one thyro-tracheal unit/tube) were incubated in Krebs-Ringer bicarbonate buffer at 37 degrees C for 4h. Free thyroxine (fT4) released in the incubation medium, thyroidal cAMP and calmodulin content were measured by RIA. TSH (5 mU/ml) and dibutyryl cAMP (DBC) (200 micrograms/ml) caused a 2-4 fold increase in thyroidal release of fT4. The stimulatory effects of TSH on fT4 release were significantly inhibited by trifluoprazine and prenylamine lactate at the concentration of 5 X 10(-5) M. More specific calmodulin antagonists, W-7 and W-13, were also shown to inhibit TSH stimulation of fT4 release at the concentration of 5 X 10(-5) M. In contrast, TSH stimulation of fT4 release was not depressed by non-specific antagonists, W-5 or W-12, at the same concentration as 5 X 10(-5) M. Further, W-13 also markedly inhibited DBC-stimulated fT4 release. Neither TSH nor PGI2 altered the thyroidal calmodulin content, dissociating with a marked increase in the cAMP concentration. These results suggest that calmodulin plays an important role in TSH-stimulated thyroid hormone release and further that this mechanism exists, at least in part, at the site subsequent to the generation of cAMP.     

Roles of ZIP8, ZIP14, and DMT1 in transport of cadmium and manganese in mouse kidney proximal tubule cells

Chronic exposure to cadmium causes preferential accumulation of cadmium in the kidney, leading to nephrotoxicity. In the process of renal cadmium accumulation, the cadmium bound to a low-molecular-weight metal-binding protein, metallothionein, has been considered to play an important role in reabsorption by epithelial cells of proximal tubules in the kidney. However, the role and mechanism of the transport of Cd(2+) ions in proximal tubule cells remain unclear. Zinc transporters such as Zrt, Irt-related protein 8 (ZIP8) and ZIP14, and divalent metal transporter 1 (DMT1) have been reported to have affinities for Cd(2+) and Mn(2+). To examine the roles of these metal transporters in the absorption of luminal Cd(2+) and Mn(2+) into proximal tubule cells, we utilized a cell culture system, in which apical and basolateral transport of metals can be separately examined. The uptake of Cd(2+) and Mn(2+) from the apical side of proximal tubule cells was inhibited by simultaneous addition of Mn(2+) and Cd(2+), respectively. The knockdown of ZIP8, ZIP14 or DMT1 by siRNA transfection significantly reduced the uptake of Cd(2+) and Mn(2+) from the apical membrane. The excretion of Cd(2+) and Mn(2+) was detected predominantly in the apical side of the proximal tubule cells. In situ hybridization of these transporters revealed that ZIP8 and ZIP14 are highly expressed in the proximal tubules of the outer stripe of the outer medulla. These results suggest that ZIP8 and ZIP14 expressed in the S3 segment of proximal tubules play significant roles in the absorption of Cd(2+) and Mn(2+) in the kidney.     

Insulin-induced myosin light-chain phosphorylation during receptor capping in IM-9 human B-lymphoblasts.

We have examined further the interaction between insulin surface receptors and the cytoskeleton of IM-9 human lymphoblasts. Using immunocytochemical techniques, we determined that actin, myosin, calmodulin and myosin light-chain kinase (MLCK) are all accumulated directly underneath insulin-receptor caps. In addition, we have now established that the concentration of intracellular Ca2+ (as measured by fura-2 fluorescence) increases just before insulin-induced receptor capping. Most importantly, we found that the binding of insulin to its receptor induces phosphorylation of myosin light chain in vivo. Furthermore, a number of drugs known to abolish the activation properties of calmodulin, such as trifluoperazine (TFP) or W-7, strongly inhibit insulin-receptor capping and myosin light-chain phosphorylation. These data imply that an actomyosin cytoskeletal contraction, regulated by Ca2+/calmodulin and MLCK, is involved in insulin-receptor capping. Biochemical analysis in vitro has revealed that IM-9 insulin receptors are physically associated with actin and myosin; and most interestingly, the binding of insulin-receptor/cytoskeletal complex significantly enhances the phosphorylation of the 20 kDa myosin light chain. This insulin-induced phosphorylation is inhibited by calmodulin antagonists (e.g. TFP and W-7), suggesting that the phosphorylation is catalysed by MLCK. Together, these results strongly suggest that MLCK-mediated myosin light-chain phosphorylation plays an important role in regulating the membrane-associated actomyosin contraction required for the collection of insulin receptors into caps.     

Sodium-sensitive, probenecid-insensitive p-aminohippuric acid uptake in cultured renal proximal tubule cells of the rabbit.

In the intact kidney, renal proximal tubule cells accumulate p-aminohippurate (PAH) via a basolateral, probenecid- and sodium-sensitive transport system. Primary cultures of rabbit proximal tubule cells retain sodium-glucose co-transport in culture, but little is known about PAH transport in this system. Purified proximal tubule cells from a rabbit were grown in culture and assessed for PAH and alpha-methyl-D-glucoside uptake capacities as well as proximal tubule marker enzyme activities. Control PAH uptake on collagen-coated filters (20 +/- 3 pmol/mg protein.min; n = 8) was not significantly different from uptake in the presence of 1 mM probenecid (19 +/- 4 pmol/mg protein.min; n = 8). Uptake from the basal side of the cell was 3.9 +/- 0.7 times greater than that from the apical side. In multi-well plate studies, the uptake was significantly reduced by removing sodium from the medium and stimulated by coating the wells with collagen. Glutarate (10 mM) had no effect on the uptake of PAH. Other differentiated proximal tubule characteristics were retained in culture, including the ability to form domes and to transport glucose by a phlorizin-sensitive system. Phlorizin-sensitive 1 mM alpha-methyl-D-glucoside uptake was 134 +/- 42 pmol/mg protein.min (n = 7; P less than 0.02). The proximal tubule marker enzymes alkaline phosphatase and gamma-glutamyltranspeptidase, increased in activity in the cultures after confluence. It was concluded that whereas some differentiated properties were retained during primary culture of rabbit proximal tubule cells, the PAH transport system was selectively lost or modified from that present in the intact kidney.     

Trifluoperazine and W-7 inhibit mating in Chlamydomonas at an early stage of gametic interaction

Gametic mating by Chlamydomonas reinhardi is inhibited in a dose-dependent and reversible manner by the calmodulin antagonists trifluoperazine (TFP) and W-7, but not by W-5, an analog of W-7 having lower affinity for calmodulin. Quantitation of the sequential steps of mating showed that TFP and W-7 both allow normal levels of flagellar agglutination but prevent all subsequent steps. Gametes agglutinate aberrantly and do not form mating pairs. Further, both of these drugs prevent the translocation of latex beads along the flagellar surface. Our observations suggest that calmodulin may play an integral role in the translocation of flagellar adhesion sites during the tip-locking stage of the Chlamydomonas mating reaction. Flagellar surface motility may be crucial to the transduction of signals during mating and may share regulatory mechanisms with other forms of surface motility.     

Serum-induced inhibition of isotonic fluid absorption by the kidney proximal tubule II. Evidence that complements is involved

The strong inhibitory effect of intraluminally perfused serum on the isotonic absorption by the rat kidney proximal tubule is abolished when serum is preincubated with inulin, zymosan, cobra venom factor, hydrazine or isolated kidney brushborder membrane. Rabbit serum genetically deficient in C6 does not have an inhibitory effect but is fully reconstituted by the addition of purified rabbit C6. Treatment of serum with anti-C6 serum also abolishes its inhibitory effect. These and previously reported data are best interpreted to indicate that complement-mediated cell lysis is the mechanism for the serum-induced inhibition of isotonic absorption.     

Calcium-dependent control of caldesmon-actin interaction by S100 protein

Caldesmon from chicken gizzard muscle has been examined for ability to interact with S100 protein using sedimentation, low-shear viscosity, and affinity chromatography. Ca2+/S100 protein, like Ca2+/calmodulin, inhibited the binding of caldesmon to F-actin in a concentration-dependent manner and the inhibition was not observed in the absence of Ca2+. Caldesmon was bound to S100 protein-Sepharose in the presence of Ca2+ and released with EGTA, indicating that there is a direct interaction between caldesmon and S100 protein. The binding of S100 protein to caldesmon also relieved actomyosin Mg2(+)-ATPase inhibition by caldesmon. The molar ratio of S100 protein to caldesmon required for half-maximal restoration was about 0.3, a value less than that in the case of calmodulin. S100 protein, however, was less effective in terms of the maximal extent of the restoration. With respect to Ca2(+)-sensitivity, the restoration profiles were monophasic with a midpoint at 3 x 10(-5) M for S100 protein and 8 x 10(-6) M for calmodulin. The restoration by S100 protein was almost wholly inhibited by TFP, but not by W-7. Taken together, our results suggest that a Ca2(+)-binding protein other than calmodulin may regulate caldesmon-dependent cellular functions.     

Expression of mRNA coding for kidney and red cell water channels in Xenopus oocytes

The existence and identity of protein water transporters in biological membranes has been uncertain. Osmotic water permeability (Pf) was measured in defolliculated Xenopus oocytes microinjected with water or mRNA from kidney cortex, kidney papilla, reticulocyte, brain, and muscle. Pf was measured by quantitative image analysis from the time course of oocyte swelling in response to an osmotic gradient. When assayed at 10 degrees C, Pf in water-injected oocytes increased from (3.6 +/- 0.9) x 10(-4) cm/s (S.D., n = 16) to 74 x 10(-4) cm/s with addition of amphotericin B, showing absence of unstirred layers. At 48-72 h after injection of 50 ng of unfractionated mRNA, Pf (in cm/s x 10(-4] was: 4.0 +/- 1.5 (rabbit brain, n = 15), 4.2 +/- 1.8 (rabbit muscle, n = 10), 18.4 +/- 6.3 (rabbit reticulocyte, n = 20), 16.1 +/- 5.6 (rat renal papilla, n = 24), 12.9 +/- 6.3 (rat renal cortex, n = 20), 14.4 +/- 6.1 (rabbit renal papilla, n = 15), and 11.8 +/- 3.4 (rabbit renal cortex, n = 8). In oocytes injected with mRNA from rat renal papilla, Pf was inhibited reversibly by 0.3 mM HgCl2 (4.1 +/- 1.6, n = 10); expressed water channels from kidney and red cell had activation energies of less than 4 kcal/mol. These results show functional oocyte expression of water channels from red cell, kidney proximal tubule (cortex), and the vasopressin-sensitive kidney collecting tubule (papilla), indicating that water channels are proteins, and providing an approach for the expression cloning of water channels.     

Evidence for the presence of functional beta-adrenoceptor along the proximal tubule of the rat kidney

Evidence for the presence of beta adrenoceptors on proximal tubules from the rat kidney has been obtained using enriched tubule suspensions prepared by Percoll centrifugation. Intact tubules demonstrated simultaneous enrichment of parathyroid hormone and isoproterenol sensitive cAMP production with no enrichment of antidiuretic hormone sensitive cAMP production. Both norepinephrine and epinephrine were less potent than isoproterenol and the stimulatory effect of catecholamines could be blocked with propranolol but not phentolamine. The stimulatory effect of norepinephrine on cellular phenylalanine uptake is blunted by co-addition of isoproterenol suggesting that the beta receptor may modulatory catecholamine stimulated transport.