Inositol 1,4,5-trisphosphate releases Ca2+ from a nonmitochondrial store site in permeabilized rat cortical kidney cells

Inositol 1,4,5-trisphosphate and its receptors

Activation of cells by many extracellular agonists leads to the production of inositol 1,4,5-trisphosphate (IP₃). IP₃ is a global messenger that easily diffuses in the cytosol. Its receptor (IP₃R) is a Ca(2+)-release channel located on intracellular membranes, especially the endoplasmic reticulum (ER). The IP₃R has an affinity for IP(3) in the low nanomolar range. A prime regulator of the IP₃R is the Ca(2+) ion itself. Cytosolic Ca(2+) is considered as a co-agonist of the IP₃R, as it strongly increases IP(3)R activity at concentrations up to about 300 nM. In contrast, at higher concentrations, cytosolic Ca(2+) inhibits the IP₃R. Also the luminal Ca(2+) sensitizes the IP₃R. In higher organisms three genes encode for an IP₃R and additional diversity exists as a result of alternative splicing mechanisms and the formation of homo- and heterotetramers. The various IP₃R isoforms have a similar structure and a similar function, but due to differences in their affinity for IP₃, their variable sensitivity to regulatory parameters, their differential interaction with associated proteins, and the variation in their subcellular localization, they participate differently in the formation of intracellular Ca(2+) signals and this affects therefore the physiological consequences of these signals.

The effect of L-type Ca2+ channel blockers on anoxia-induced increases in intracellular Ca2+ concentration in rabbit proximal tubule cells in primary culture

Ca2+ channel blockers (CCB) have been shown to be protective against ischaemic damage of the kidney, suggesting an important role for intracellular Ca2+ ([Ca2+]i) in generating cell damage. To delineate the mechanism behind this protective effect, we studied [Ca2+]i in cultured proximal tubule (PT) cells during anoxia in the absence of glycolysis and the effect of methoxyverapamil (D 600) and felodipine on [Ca2+]i during anoxia. A method was developed whereby [Ca2+]i in cultured PT cells could be measured continuously with a fura-2 imaging technique during anoxic periods up to 60 min. Complete absence of O2 was realised by inclusion of a mixture of oxygenases in an anoxic chamber. [Ca2+]i in PT cells started to rise after 10 min of anoxia and reached maximal levels at 30 min, which remained stable up to 60 min. The onset of this increase and the maximal levels reached varied markedly among individual cells. The mean values for normoxic and anoxic [Ca2+]i were 118 +/- 2 (n = 98) and 662 +/- 22 (n = 160) nM, respectively. D 600 (1 microM), but not felodipine (10 microM), significantly reduced basal [Ca2+]i in normoxic incubations. During anoxia 1 microM and 100 microM D 600 significantly decreased anoxic [Ca2+]i levels by 22 and 63% respectively. Felodipine at 10 microM was as effective as 1 microM D 600. Removal of extracellular Ca2+ and addition of 0.1 mM La3+ completely abolished anoxia-induced increases in [Ca2+]i. We conclude that anoxia induces increases in [Ca2+]i in rabbit PT cells in primary culture, which results from Ca2+ influx. Since this Ca2+ influx is partially inhibited by low doses of CCBs, L-type Ca2+ channels may be involved.

Development and characterization of rabbit proximal tubular epithelial cell lines

We have isolated rabbit kidney proximal tubular epithelial cell lines. The selection was based on their ability to form confluent monolayers on porous supports and to maintain receptor-mediated signal transduction and ion transport, characteristic of the proximal tubule. The isolation method consisted of several steps: (1) superficial cortical proximal tubule segments were microdissected and cultured on a matrix-coated porous support until cells formed a confluent monolayer; (2) primary cultures showing hormone-regulated ion transport typical for the proximal tubule were selected and co-cultured with irradiated fibroblasts; and (3) the epithelial cells surviving after several passages were expanded and passaged on porous substrates. Most of the cell lines developed in this manner were obtained by co-culture with irradiated fibroblasts producing a recombinant retrovirus encoding SV40 large T antigen and G418 resistance. However, SV40 T antigen expression was not essential for immortalization, since neither T antigen nor G418 resistance was detected in the isolated cell lines and co-culture with non-producing 3T3 cells gave similar results. One cell line (vEPT) has been characterized in some detail with respect to morphological, biochemical, and ion transport properties. This line forms confluent monolayers with apical microvilli, tight junctions, and convolutions of the basolateral plasma membrane. Once confluent, monolayers maintain conductances of 25 to 32 mS/cm2 for several weeks in culture and possess phlorizin-sensitive short-circuit current (Isc) in glucose containing media, indicative of apical Na(+)-glucose co-transport. vEPT cells also retain receptor and signaling mechanisms for angiotensin II (Ang II). Apical and basal Ang II and 5,6-epoxy-eicosatrienoic acid (5,6-EET) modulate the Isc in a manner similar to primary cultures. The cell lines share with primary cultures expression of the cytokeratins K8, K10/K11, and K19 ("nomenclature" [21]). They also retain several receptor and signal transduction mechanisms. For example, Ang II, arachidonate, bradykinin, 5,6-EET, parathyroid hormone (residues 1 through 34), and purine nucleotides increase cytosolic Ca2+, PTH elevates cAMP levels, and Ang II enhances proximal tubule-specific arachidonic acid metabolism.

Primary cultures of normal rat kidney proximal tubule epithelial cells for studies of renal cell injury

Normal rat kidney proximal tubule epithelial cell cultures were obtained by collagenase digestion of cortex and studied for 10 days. To assess the purity of the seeding suspension, we histochemically demonstrated gamma-glutamyltranspeptidase in greater than 95% of the starting material. To identify cell types in cultures, we investigated several markers. Cells stained positively for lectin Arachis hypogaea (rat proximal tubule) and negatively for Lotus tetragonolobus (rat distal tubule). Intermediate filament expression of cytokeratin confirmed the epithelial differentiation of the cultured cells. Using indirect immunofluorescence, we found that cultures were negative for vimentin and Factor VIII. Cells exhibited activities of two brush border enzymes, gamma-glutamyltranspeptidase and leucine aminopeptidase, and Na(+)-dependent glucose transport activity. Multicellular domes were evident in the Week 2 of culture. Proliferation was studied by comparing growth factor-supplemented serum-free medium to cells grown in serum; growth enhancers included insulin, hydrocortisone, transferrin, glucose, bovine albumin, and epidermal growth factor. Cells proliferate best in medium with 5 or 10% serum and in serum-free medium supplemented with insulin, hydrocortisone, transferrin, glucose, and bovine albumin. Proliferation was assessed by determining cell number (population doublings). By light microscopy, the cells were squamous with numerous mitochondria, a central nucleus, and a rather well-defined homogeneous ectoplasm. By electron microscopy, the cells were polarized with microvilli and cell junctions at the upper surface and a thin basal lamina toward the culture dish. These data show that the proximal tubule epithelial cells retain a number of functional characteristics and that they represent an excellent model for studies of normal and abnormal biology of the renal proximal tubule epithelium.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.

H+/Ca2+ exchange in rabbit renal cortical endosomes

We have examined the effect of second messengers on ATP-driven H+ transport in an H+ ATPase-bearing endosomal fraction isolated from rabbit renal cortex. cAMP (0.1 mM) had no effect on H+ transport. Acridine orange fluorescence in the presence of 0.5 mM Ca2+ (+1 mM EGTA) was 19 +/- 6% of control. Inhibition of ATP-driven H+ transport by Ca2+ was concentration dependent; 0.25 and 0.5 mM Ca2+ (+1 mM EGTA) inhibited acridine orange fluorescence by approximately 50 and approximately 80%, respectively. Ca2+ also produced a concentration-dependent increase in the rate of pH-gradient dissipation. Ca2+ did not affect ATP hydrolysis. ATP-dependent Br- uptake was virtually unchanged in the presence of 0.5 mM Ca2+ (+1 mM EGTA). These vesicles were also shown to transport Ca2+ in an ATP-dependent mode. Inositol 1,4,5-trisphosphate had no effect on ATP-dependent Ca2+ uptake. These results are consistent with the co-existence of an H+ ATPase and an H+/Ca2+ exchanger on these endosomes, the latter transport system using the H+ gradient to energize Ca2+ uptake. Attempts to demonstrate an H+/Ca2+ antiporter in the absence of ATP have been unsuccessful. Yet, when a pH gradient was established by preincubation with ATP and residual ATP was subsequently removed by hexokinase + glucose, stimulation of Ca2+ uptake could be demonstrated. A Ca2(+)-dependent increase in H+ permeability and an ATP-dependent Ca2+ uptake might have important implications for the regulation of vacuolar H+ ATPase activity as well as the homeostasis of cytosolic Ca2+ concentration.

G regulatory proteins and muscarinic receptor signal transduction in mucous acini of rat submandibular gland

The involvement of G regulatory proteins in muscarinic receptor signal transduction was examined in electrically permeabilized rat submandibular acinar cells. The guanine nucleotide analog, GTP gamma S, caused the dose dependent hydrolysis of membrane phosphatidylinositol 4,5-bisphosphate to release IP3. This response was insensitive to pertussis toxin treatment and was duplicated by NaF but not by GDP beta S. Enhanced IP3 synthesis was observed with a combination of GTP gamma S and carbachol. Exogenous IP3, as well as carbachol and GTP gamma S, provoked the release of sequestered 45Ca2+ from non-mitochondrial stores. In intact cells, carbachol significantly reduced the level of cyclic AMP induced by the beta-adrenergic agonist, isoproterenol, to 69% of its normal value. Pertussis toxin abolished this inhibitory action of carbachol on cyclic nucleotide levels. These results suggest that muscarinic receptors are coupled to two separate G regulatory proteins in submandibular mucous acini-the pertussis toxin-insensitive Gp of the phosphoinositide transduction pathway associated with elevated cytosolic calcium levels, and the pertussis toxin-sensitive Gi inhibitory protein of the adenylate cyclase complex.

Proton ATPase in rat renal cortical endocytotic vesicles

To relate ATPase activity to the ATP-driven H+-pump in rat renal endocytotic vesicles we applied an in vitro coupled optical test and a Pi-liberation assay. Endocytotic vesicles contain an ouabain-, vanadate- and oligomycin-insensitive ATPase. The ionophores for K+ and H+, valinomycin and carbonylcyanide p-chloro-methoxyphenylhydrazone (CCCP), respectively, stimulated ATPase activity, indicating its relation to the electrogenic H+-pump. This conclusion is supported by a similar distribution on a Percoll gradient of ATP-driven H+ uptake into endosomes and ionophore-stimulated ATPase activity. Coupled optical and Pi-liberation assays were then used to characterize the H+-ATPase with respect to the requirement for pH, nucleotides, anions, and mono- and divalent cations. The H+-ATPase activity was decreased by widely used blockers: N-ethylmaleimide (NEM), dicyclohexylcarbodiimide (DCCD) and diethylstilbestrol (DES). Different sensitivities to these blockers proved that alkaline phosphatase and H+-ATPase are separate entities. To investigate whether the NEM-, DCCD- and DES-sensitive ATPase activity is confined to intact endocytotic vesicles, cellular membranes from rat kidney cortex were separated on a Percoll density gradient. Surprisingly, endocytotic vesicles contain only a small fraction of the total NEM-, DCCD- and DES-sensitive ATPase activity. The majority of the blocker-sensitive ATPases belongs to membranes of as yet undefined cellular origin.

Calcium homeostasis of epithelial cells

1. Cytosolic free Ca2+ is an important regulator of ion transport processes in epithelial cells. 2. Free Ca2+ concentration is regulated by a concerted action of Ca2+ transport systems in plasma membranes and intracellular organelles. 3. These transport systems were studied in intestinal and renal cortical cells with emphasis on the transport capacities and Ca2+ affinities. 4. Ca2+ accumulation by permeabilized cells was compared to Ca2+ uptake by isolated organelles and membrane fractions. 5. Effects induced by cell or organelle isolation methods and the influence of temperature and pH on Ca2+ transport capacities were studied.

Intracellular calcium ions as regulators of renal tubular sodium transport

This review addresses the putative role of intracellular calcium ions in the regulation of sodium transport by renal tubules. Cytoplasmic calcium-ion activities in proximal tubules of Necturus are less than 10(-7) M and can be increased by lowering the electrochemical potential gradient for sodium ions across the peritubular cell membrane, or by addition of quinidine or ionomycin to peritubular fluid. Whereas lowering of the peritubular Na concentration increases cytosolic [Ca++] and [H+], ionomycin, a calcium ionophore, raises intracellular [Ca++] without decreasing pHi. The intracellular calcium-ion level is maintained by transport processes in the plasma membrane and membranes of intracellular organelles, as well as by calcium-binding proteins. Calcium ions inhibit net transport of sodium by reducing the rate of sodium entry across the luminal cell membrane. In the collecting tubule this inhibition is caused, at least in part, by an indirect reduction in the activity of the amiloride-sensitive sodium channel.

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

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