Biomedical subjects

J L Renfro

Publications and source records attributed to J L Renfro.

At least 37 records · Page 2Linked to original sources

PubMed1991

Taurine secretion in cultured winter flounder renal epithelium.

The mechanism for the secretion of taurine by the marine fish kidney was examined in flounder renal primary cultures and teased tubules. The question addressed was whether the secretion of taurine, but not beta-alanine, a related beta-amino acid, is due to differences in their basolateral uptake, intracellular concentration, or luminal efflux mechanism. Aminooxyacetate (AOA) (10 mM), an inhibitor of beta-alanine oxidation, allowed the basolateral beta-alanine accumulation (measured in teased tubules) and the beta-alanine intracellular concentration (measured in primary cultures) to be similar to that for taurine. However the addition of AOA did not promote the net secretion of beta-alanine (measured as unidirectional fluxes in the cultured epithelium mounted in an Ussing chamber). The organic anions bromcresol green (BCG) (0.1 mM) and probenecid (1 mM) inhibited the net secretion of taurine by affecting the luminal efflux step. This suggests that a BCG- and probenecid-sensitive carrier in the luminal membrane facilitates the cell-to-lumen efflux of taurine and thus achieves net secretion.

Aminooxyacetic Acid↗

PubMed1990

Streptomycin toxicity in primary cultures of flounder renal proximal tubule cells.

The aminoglycoside antibiotic streptomycin is a known nephrotoxin in vivo and a common component of cell culture media. The effects of streptomycin (100 micrograms/ml) on transepithelial electrical properties, glucose transport, glycolytic metabolism, and morphology were examined in primary proximal tubule cell cultures from winter flounder (Pseudopleuronectes americanus) kidney. Streptomycin treatment on either Days 2 to 12 or Days 8 to 13 abolished the transepithelial potential difference and short-circuit current across the monolayer but had no effect on transepithelial resistance in confluent 12 to 13-d cultures, suggesting the loss of active transepithelial transport. Consistent with these findings, mucosal-to-serosal glucose fluxes were greatly reduced in streptomycin-treated cultures and insensitive to the transport inhibitor phlorizin, indicating the absence of the apical Na-dependent glucose transport system associated with net glucose reabsorption. In addition to transport processes, antibiotic treatment also interfered with cellular energy metabolism as judged by the rapid reduction in glycolytic lactate production observed in the presence of streptomycin. Scanning and transmission electron microscopy revealed that streptomycin-treated cultures were composed of cuboidal-to-columnar shaped cells which maintained intact tight junctions similar to control cultures. However, apical microvilli, the presumed sites of mucosal transport systems, were severely reduced in number in streptomycin-treated cultures. We concluded that streptomycin, at a dose commonly used in cell culture, inhibited the expression of differentiated function by flounder proximal tubule cell cultures. These cell cultures may provide a suitable model system for examination of the mechanisms of aminoglycoside nephrotoxicity.

Animals↗

PubMed1990

Organic anion secretion by winter flounder renal proximal tubule primary monolayer cultures.

Recent models of para-aminohippuric acid (PAH) transport by renal proximal tubule have been deduced from isolated membrane vesicles and stopped flow microperfusion. The flounder proximal tubule primary monlayer cultures mounted in Ussing chambers have provided a means to examine the relationship of these models to transepithelial transport. Unidirectional transepithelial 3H-PAH fluxes were determined in 12-day-old monolayers on floating collagen gels under continuously short-circuited conditions. The kinetic values of PAH secretory flux were complex. Over the range of 0.6 microM to 1.5 mM the K1/2 was 0.4 mM and Vmax was 80 nmol/cm2/h. Reabsorptive flux did not saturate. 1 mM probenecid inhibited 95% of the secretory flux (0.07 +/- 0.18 compared with control of 1.38 +/- 0.34 nmol/cm2/h at 10 microM PAH) and had no significant effect on reabsorptive flux (probenecid, 0.07 +/- 0.04; control, 0.05 +/- 0.01 nmol/cm2/h). 0.1 mM 4-acetamido-4'-isothiocyanalostilbene-2,2'-diasulfonic acid and 1 mM benzoylpropionic acid also significantly inhibited secretory flux (95 and 78%, respectively). These inhibitors appeared to be specific in that no effects on transepithelial potential difference, resistance or phlorizin-sensitive current were seen. Removal of Na during flux measurement reduced net PAH secretion to zero, and 1 mM ouabain reduced PAH secretory flux to 10% of control within 90 min with no effect on reabsorptive (leak) flux. Clamping transepithelial voltage to +/- 10 mV had no significant effect on PAH fluxes. Addition of glutarate to flounder saline significantly inhibited PAH secretory flux starting at 0.1 mM and caused 90% inhibition at 1 mM; no stimulation of transport was noted at any glutarate concentration.

Animals↗

PubMed1989

Control of phosphate transport in flounder renal proximal tubule primary cultures.

Unidirectional mucosal-to-serosal (Jm----s) and serosal-to-mucosal (Js----m) transepithelial phosphate fluxes across monolayers of flounder (Pseudopleuronectes americanus) renal proximal tubule cells in primary culture were examined for effects of diacylglycerols, phorbol ester, A23187, forskolin, and extracellular phosphate availability. Tissues were cultured on floating collagen rafts and studied short circuited in Ussing chambers. Transepithelial electrical properties were continuously monitored and were unaffected by any of the treatments compared with paired controls. Under usual conditions (phosphate = 0.4 mM) tissues invariably displayed net phosphate reabsorption [Js----m = 2.3 +/- 0.52; Jm----a = 7.1 +/- 1.77; Jnet = 4.9 +/- 1.45 (SE) nmol.cm-2.h-1]. Acute elevation of bath phosphate concentration above 0.5 mM stimulated net secretion. Exposure to 100 microM 1,2-dihexanoyl-sn-glycerol stimulated net phosphate secretion within 30 min, the result of a fivefold increase in Js----m. Phorbol-12,13-didecanoate stimulated net phosphate secretion by increasing Js----m and decreasing Jm----s. The inactive diacylglycerol, 1,3-didecanoyl-rac-glycerol (100 microM), had no effect on phosphate fluxes. A23187 stimulated net phosphate secretion; Jm----s was reduced almost fourfold while Js----m was increased threefold. Forskolin (10 microM) stimulated net reabsorption more than threefold after a long latency (2 h). These data indicate that renal phosphate secretion and reabsorption may be regulated by several putative intracellular messengers. In addition, extracellular phosphate availability may modulate renal phosphate handling.

Animals↗

PubMed1989

Glucocorticoid inhibition of Na-SO4 transport by chick renal brush-border membranes.

To examine the effect of glucocorticoids on sulfate transport by the chick (domestic Gallus gallus) renal tubule we dosed 3-wk-old animals with 60 micrograms dexamethasone/100 g body wt at 24 and 6 h before isolation of renal brush-border (BBM) and basolateral membranes (BLM). Dexamethasone treatment significantly reduced Na-dependent sulfate transport by BBM and had no effect in paired membranes on bicarbonate, proton, or electrical gradient-driven sulfate transport. The glucocorticoid treatment had no statistically significant effect on HCO3-SO4 exchange in the BLM. Kinetic analysis of the dexamethasone effect on the Na-SO4 transport process showed that apparent Vmax was significantly decreased to almost one-half that seen in controls (from 676 to 348 pmol.mg protein-1.5 s-1). The Km in control BBM was 0.40 +/- 0.095 mM and was not significantly different in dexamethasone-treated membranes (0.53 +/- 0.094 mM). To determine whether the dexamethasone-induced decrease in Na-SO4 transport by BBM was indirectly caused by stimulation of Na-H exchange and more rapid dissipation of the initial Na gradient used to drive sulfate uptake, we examined the effect of 0.1 mM amiloride on Na-SO4 uptake by BBM. With amiloride present, dexamethasone treatment caused Vmax to significantly drop from 1,102 to 660 pmol.mg protein-1.5 s-1. Amiloride had no statistically significant effect on the Km. The extent to which amiloride increased Na-SO4 transport and blocked 22Na uptake by BBM did not appear to be related to hormone treatment. The data indicate that glucocorticoids may participate in the regulation of sulfate excretion.

Alkaline Phosphatase↗

PubMed1989

Adaptability of marine teleost renal inorganic sulfate excretion: evidence for glucocorticoid involvement.

The marine winter flounder, Pseudopleuronectes americanus, ingests seawater (SW) and excretes most of the resultant divalent ion load by renal tubule secretion. The site of secretion is generally thought to be the proximal tubule. Although sulfate clearance ratios (C ratio-sulfate clearance/polyethylene glycol clearance) have been reported as high as 12 in SW-acclimated animals, the present study shows that net secretion ceases after acclimation to 10% SW (SO4 free). Intravenous infusion of sulfate into the latter induced net secretion within 5 h (C ratio increased from 0.83 +/- 0.23 to 2.12 +/- 0.33). Increased sulfate secretion coincided with an increase in the magnitude of HCO3-SO4 exchange in brush-border membrane vesicles (BBMV) isolated from flounder renal tubules. Treatment of 10% SW-acclimated flounder with 60 micrograms dexamethasone/100 g body wt also caused an increase in the HCO3-SO4 exchange in BBMV to a level comparable to that of BBMV from SW fish. The glucocorticoid effect was further tested in flounder renal tubule primary monolayer cultures. Measurement of unidirectional sulfate fluxes (J) showed that in the presence of cortisol net secretion (serosal-to-mucosal flux) dominated (Js----m = 111 +/- 6.5, Jm----s = 9 +/- 4.3, Jnet = 102 +/- 2.2 nmol.h-1.cm-2). Removal of cortisol from the culture medium significantly reduced net sulfate secretion to one-third of control (Js----m = 39 +/- 13.9, Jm----s = 6 +/- 1.0, Jnet = 33 +/- 14.1 nmol.h-1.cm-2).

Adaptation, Physiological↗

PubMed1988

Recent applications of fish cell culture to biomedical research.

Tissues of the fishes are as amenable to the techniques of modern cell culture as mammalian tissues and organs, and yet this vast resource, comprising thousands of vertebrate species, remains largely unexplored. The model systems that have been developed demonstrate the utility of fish cells as sources of special adaptations and exaggerated physiological systems. In this review, we briefly describe several of the successful models along with recent developments in fish cell culture with the hope of stimulating increased interest in the lower vertebrates as useful complements to mammalian cell culture in biomedical research. The topics covered include epithelial ion transport, endocrinological studies, the cellular stress (heat shock) response, thermotolerance, cancer biology, and environmental toxicology.

Animals↗

PubMed1987

Sulfate transport by chick renal tubule brush-border and basolateral membranes.

Brush-border and basolateral membrane vesicles (BBMV and BLMV, respectively) were prepared from chick kidney by a calcium precipitation method and by centrifugation on an 8% Percoll self-generating gradient, respectively. In BBMV a 100-mM Na gluconate gradient, out greater than in, caused concentrative sulfate uptake approximately fivefold greater at 1 min than at 60 min (equilibrium) whether or not the membranes were short-circuited with 100 mM K gluconate, in = out, plus 20 micrograms valinomycin/mg protein. A 48-mM HCO3- gradient, in greater than out, stimulated a 2.5-fold higher uptake at 1 min than at 60 min, and short circuiting as above had no effect on the magnitude of this response. Imposition of a H+ gradient (pH 5.4 out vs. pH 7.4 in) caused concentrative uptake fourfold higher at 1 min than at equilibrium. Short circuiting as above or addition of 0.1 mM carbonyl cyanide m-chlorophenylhydrazone (CCCP) significantly inhibited the pH gradient effect. Creation of an inside positive electrical potential with 100 mM K gluconate, out greater than in, plus valinomycin, also caused concentrative sulfate uptake. The K gradient in the absence of valinomycin had no effect on sulfate uptake (compared with isosmotic mannitol). Based on inhibitor/competitor effects, these are distinct sulfate transport processes. In chick BLMV, imposition of an HCO3- gradient, in greater than out, produced concentrative sulfate uptake; however, neither Na+ nor H+ gradients had significant effects at 15 s. 4-Acetamido-4'-isothiocyanostilbene 2,2'-disulfonic acid disodium at 0.1 mM was an effective inhibitor of BLMV bicarbonate-sulfate exchange; however, neither Cl-, SCN-, nor CCCP inhibited.

Animals↗

PubMed1986

Primary culture of flounder renal tubule cells: transepithelial transport.

Renal proximal tubule cells from the winter flounder (Pseudopleuronectes americanus) were maintained in a functionally differentiated state for up to 16 days in primary culture on floating collagen gels. The cells were confluent after 7-8 days in culture, contracted the collagen gels, and exhibited ciliary activity. Electron microscopy indicated that the cultures were composed of continuous sheets of columnar epithelial cells that had established structural polarity. When mounted in Ussing chambers, the cultures exhibited a small mucosa-negative potential difference (0.6 +/- 0.10 mV) and a low transepithelial resistance (23 +/- 2.3 omega X cm2). Short-circuit current averaged 24 microA/cm2. The cultured epithelium was four times more permeable to Na than to Cl and actively secreted sulfate and p-aminohippuric acid and reabsorbed hexoses. Glucose reabsorption was rheogenic and occurred via a high-affinity (Km = 0.16 mM), low-capacity (Vmax = 5 microA/cm2), phlorizin-sensitive transport system. We concluded that the cultured cells express many of the differentiated properties of the intact flounder proximal tubule and thus provide a suitable model system for studying renal transport processes.

Acids↗

PubMed1985

Peritubular uptake and brush border transport of 28Mg by flounder renal tubules.

The uptake of 28Mg by isolated renal tubules of winter flounder, Pseudopleuronectes americanus, was studied by compartmental analysis. Two phases of uptake were seen in steady-state conditions. The slow-exchanging compartment was 46% of the total Mg content, and uptake into this compartment was saturable (Km = 0.36 mM) and inhibited by 10 mM CaCl2, dinitrophenol, and ouabain but not by furosemide, Na-free medium, or cytochalasin B. The fast-exchanging compartment was 5% of the total Mg, and uptake into this compartment showed sigmoid saturation kinetics (K 1/2 = 1.62 mM). The fast uptake rate was inhibited by dinitrophenol, ouabain, Na-free medium, 10 mM CaCl2, and furosemide but stimulated by cytochalasin B. 28Mg efflux from brush border membrane vesicles (BBMV) was stimulated by both an inside positive electrical potential generated by 100 mM KCl (out greater than in, in the presence of valinomycin) and by 100 mM NaCl (out greater than in). The latter effect was still present when the membrane electrical potential was short-circuited with 100 mM KCl (in = out) in the presence of valinomycin. Uptake of Mg by BBMV was stimulated by reversal of the aforementioned gradients. Both uptake and efflux were stimulated by A23187. BBMV uptake was inhibited by 10 mM CaCl2 and unaffected by furosemide. The relationship of electrical gradient-driven and Na gradient-driven Mg transport to the Mg secretory function of these tubules is discussed.

Animals↗

PubMed1984

Parathyroid hormone effect on chicken renal brush-border membrane phosphate transport.

Renal clearance studies showed that parathyroidectomy (PTX) stimulated renal PO4 reabsorption within 90 min in 3-wk-old chicks. Subsequent parathyroid hormone (PTH) infusion caused net PO4 secretion. Kidneys were taken for brush-border membrane (BBM) preparations 3 h after PTX or sham operations and 1 h after PTH or saline injections, thus yielding four categories of PTH status: PTX/saline, sham/saline, PTX/PTH, and sham/PTH. Efficacy of PTX and PTH was confirmed by examination of serum Ca concentrations. A 100 mM NaSCN gradient, out greater than in, caused concentrative PO4 uptake by BBM from sham/saline animals. Concentrative uptake was not produced by 100 mM KSCN, out greater than in; pH 7.4in vs. pH 5.4out; or 100 mM NaC1, out = in. Removal of endogenous PTH (PTX/saline) significantly stimulated Na-dependent PO4 uptake compared with sham/saline. PTH infusion significantly depressed Na-dependent PO4 uptake in PTX/PTH and sham/PTH groups compared with sham/saline and PTX/saline. Na-dependent, concentrative glucose uptake was present and unchanged by PTH. Kinetic analysis, based on 5-s uptakes, showed that both apparent affinity and maximal velocity (Vmax) for Na-dependent PO4 transport were decreased by PTH treatment. The data suggest that PTH influences avian renal PO4 excretion at least in part through a Na-dependent PO4 transport system in proximal tubule BBM.

Animals↗

PubMed1983

Renal sulfate transport at the basolateral membrane is mediated by anion exchange.

The transport of sulfate was studied in basolateral membrane (BLM) vesicles isolated from rat kidney cortex by centrifugation on a Percoll self-generating gradient. In contrast to sulfate transport at the luminal membrane, sulfate uptake by BLM vesicles was not sodium dependent. However, imposition of an inside greater than outside bicarbonate gradient stimulated BLM sulfate uptake nearly 10-fold and produced a transient overshoot of about 4-fold. This process appeared to become saturated at high concentrations of either bicarbonate or sulfate. Sulfate itself, thiosulfate, and hydroxyl, but not chloride or thiocyanate, were able to substitute for bicarbonate. None of these anions was as effective as bicarbonate. The sulfate/bicarbonate exchange was unaltered by manipulation of membrane potential, suggesting that it was electroneutral. Both bicarbonate stimulation and overshoot could be prevented by known inhibitors of anion transport, including mercuric chloride, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, and phloretin. Bicarbonate-stimulated sulfate uptake also was inhibited by thiosulfate, probenecid, and acetazolamide. Thus, rat kidney BLM vesicles showed carrier-mediated anion exchange. Its properties indicate that this carrier may participate in both reabsorptive and secretory sulfate transport.

Animals↗

PubMed1983

Esophageal desalination of seawater in flounder: role of active sodium transport.

The esophagus of the flounder, Pseudopleuronectes americanus, was studied to determine how salinity of ingested seawater (SW) is decreased before fluid absorption in the intestine. Drinking rate was 2.5 ml X h-1 X kg-1. Stomach fluid osmolality was 45% that of seawater, and intestinal fluid was isosmotic to plasma. Esophagus and stomach were nearly impermeable to 28Mg; thus Mg concentrations were accurate indicators of fluid addition and NaCl removal between pharynx and stomach. Measurements of water and ion fluxes across isolated esophageal epithelium mounted in Ussing chambers and bathed by Ringer solution showed that the tritiated water flux was lower in esophagus than in intestine and that 22Na flux ratio was 1.4 (Jm leads to s/Js leads to m) regardless of acclimation medium (100 or 10% SW). Potential difference was zero, and electrical resistance averaged 90 omega X cm2. Mucosal-to-serosal Na transport was inhibited by 0.1 mM amiloride, 0.1 mM ouabain, and Cl-free medium, whereas 1.0 mM furosemide had no effect. Net esophageal Na absorption (mucosal-to-serosal) averaged 10.0 mumol X h-1 X cm-2 with mucosa exposed to SW and was inhibited 46% by 0.1 mM ouabain. Taken together the above observations suggest a role for both passive and active esophageal Na transport in SW desalination.

Animals↗

PubMed1983

Sulfate transport by flounder renal tubule brush border: presence of anion exchange.

Transport of sulfate into brush border membrane (BBM) vesicles from renal tubules of southern flounder, Paralichthys lethostigma, was investigated. Imposition of a 21 mM HCO-3 gradient, in greater than out, produced concentrative sulfate uptake (overshoot) fourfold higher than equilibrium. Gradient conditions were necessary to produce this response; no overshoot was produced by the mere presence of HCO-3. Generation of potassium diffusion potentials with KCl gradients and valinomycin or short-circuiting of possible other ionic diffusion potentials had no effect on sulfate uptake. The rate-limiting step in HCO-3:SO2-4 exchange is, therefore, an electroneutral process. Although HCO-3 was the most effective counterion for sulfate, SCN-, Cl-, and S2O2-3 were also capable of promoting concentrative sulfate uptake. Sulfate uptake in the absence of counterion and bicarbonate-dependent sulfate uptake were both shown to be saturable processes. HCO-3-driven sulfate uptake could be inhibited by disulfonic stilbene, HgCl2, probenecid, chlorophenol red, and bromcresol green. Phloridzin, p-aminohippuric acid, 2,4-dinitrophenol, isethionic acid, and citric acid had no significant effect on HCO-3:SO2-4 exchange. The role of this sulfate:anion exchanger in active renal tubule sulfate secretion is discussed.

Adenosine Triphosphatases↗

PubMed1982

Effect of Na+ and ATP on peritubular Ca transport by the marine teleost renal tubule.

Ca uptake rates and efflux rate coefficients were determined in isolated renal tubules of the winter flounder, Pseudopleuronectes americanus. Na-free medium and 10(-4) M ouabain depressed while 0.2 mM dinitrophenol stimulated Ca efflux. The effects on Ca movement appeared to be associated with the adenosine triphosphate (ATP) concentration of the tubular fluid space (TFS), rather than the bath-to-tissue Na gradient. Elevation of the ATP concentration of TFS by incubation of tissue in external ATP more than doubled Ca uptake rate. Inhibition of Ca uptake by lanthanum and stimulation by Ca ionophore A23187 suggested that ATP altered plasma membrane Ca transport. Incubation in external phosphate had no effect. Plasma membrane vesicles (PMV) prepared from flounder tubules showed saturable Ca uptake inhibited by Mg, unaffected by Na gradients, and stimulated by intravesicular but not extravesicular ATP. ATP appeared to stimulate PMV Ca uptake by increasing membrane Ca binding. We concluded that Ca uptake across the peritubular membranes was not directly linked to the serosal Na gradient but was greatly influenced by the intracellular ATP concentration.

Adenosine Triphosphate↗

PubMed1982

H+-dependent sulfate secretion in the marine teleost renal tubule.

Renal sulfate secretion was studied in the southern flounder Paralichthys lethostigma. In the intact animal sulfate clearance averaged 12 times the glomerular filtration rate and about 93% of the excreted sulfate was secreted. The secretory maximum was 150 mumol . kg body wt-1 . h-1 at 5 mM plasma sulfate. DIDS inhibited sulfate secretion in vivo. In vitro sulfate uptake by renal tubule fragments was saturable (Km = 0.92 mM) and could be inhibited by DIDS, SITS, and antimycin A. Na-free medium and ouabain also inhibited sulfate uptake, suggesting involvement of the bath-to-cell Na gradient in sulfate secretion. However, no Na effect on sulfate uptake could be demonstrated in brush border membrane (BBM) or basolateral membrane (BLM) vesicles. Instead, concentrative sulfate uptake (i.e., overshoot) by BLM but not BBM vesicles could be produced by an out greater than in proton gradient. Generation of an inside-positive membrane potential (valinomycin in the presence of an out greater than in potassium gradient) slightly stimulated sulfate uptake by BLM and had no effect on BBM, indicating that the rate-limiting step for sulfate uptake at both membranes was an electroneutral process. H+ gradient-dependent sulfate uptake by BLM was inhibited by the proton ionophore FCCP, by the anion transport inhibitor DIDS, and by HgCl2. A tentative model of sulfate secretion is proposed.

Animals↗

PubMed1980

Relationship between renal fluid and Mg secretion in a glomerular marine teleost.

Renal tubular fluid secretion has been studied in seawater-acclimated winter flounder, Pseudopleuronectes americanus. Although this animal has a filtration kidney and a relatively high glomerular filtration rate (GFR = 1.35 ml . kg-1 . h-1), apparently more than half of the final urine volume was secreted fluid, and net fluid secretion was frequently observed. The dominant divalent ion excreted in the urine was Mg, of which 98% was secreted. A strong correlation (r = 0.98) was seen between secreted Mg and the rate of tubular fluid secretion. The calculated concentration of Mg in secreted fluid was 169 mM, a figure that was substantiated by values obtained for urine Mg concentration when urine flow was due almost entirely to tubular fluid secretion. Alterations in urine flow rate reflected changes in the rate of Mg secretion; however, no correlation was seen between GFR and secreted Mg, which may indicate independence of renal portal blood flow and glomerular blood flow. No relationship between Mg secretion and sodium reabsorption was apparent. These observations support the hypothesis that Mg secretion together with accompanying anions (Cl and SO4) accounts entirely for tubular fluid secretion in the winter flounder.

Animals↗

PubMed1978

Calcium transport across peritubular surface of the marine teleost renal tubule.

Isolated renal tubules of seawater-acclimated flounder (Pseudopleuronectes americanus) readily accumulated Ca and Mg when exposed to increased concentrations of these ions. Evidence based on the ability of the tubules to maintain high K levels indicated that the Ca was neither free in the cytosol nor taken up by mitochondria. Two phases of Ca uptake were seen. The fast phase was kinetically complex and not affected by Mg, dinitrophenol, cytochalasin B, or Na-free medium, but was stimulated by ouabain. The slow phase showed Michaelis-Menten kinetics and was inhibited by Mg and dinitrophenol, but was stimulated by Na-free medium, cytochalasin B, and ouabain. The slow phase was also more responsive to changes in bath Ca activity and was thought to be involved with the powerful divalent cation secretory system of these tubules. No correlation between divalent cation uptake and fluid content of the tubules was seen.

Adaptation, Physiological↗