Subcuticular skin closure using a 'blunt' needle

Recognising the desirability of avoiding the use of sharp needles, especially for wound closure, this study was undertaken to determine whether it was technically feasible to perform a subcuticular skin closure with a 'blunt' Ethiguard needle (Ethicon Ltd). A total of 108 skin incisions was closed in 40 patients. In each case it was found that the needle successfully penetrated the subcuticular layers allowing the use of a standard subcuticular closure technique with the advantage of reduced risk of skin or glove puncture.

Genomic structure, expression and evolution of the alfalfa aspartate aminotransferase genes

Genomic clones encoding two isozymes of aspartate aminotransferase (AAT) were isolated from an alfalfa genomic library and their DNA sequences were determined. The AAT1 gene contains 12 exons that encode a cytosolic protein expressed at similar levels in roots, stems and nodules. In nodules, the amount of AAT1 mRNA was similar at all stages of development, and was slightly reduced in nodules incapable of fixing nitrogen. The AAT1 mRNA is polyadenylated at multiple sites differing by more than 250 bp. The AAT2 gene contains 11 exons, with 5 introns located in positions identical to those found in animal AAT genes, and encodes a plastid-localized isozyme. The AAT2 mRNA is polyadenylated at a very limited range of sites. The transit peptide of AAT2 is encoded by the first two and part of the third exon. AAT2 mRNA is much more abundant in nodules than in other organs, and increases dramatically during the course of nodule development. Unlike AAT1, expression of AAT2 is significantly reduced in nodules incapable of fixing nitrogen. Phylogenetic analysis of deduced AAT proteins revealed 4 separate but related groups of AAT proteins; the animal cytosolic AATs, the plant cytosolic AATs, the plant plastid AATs, and the mitochondrial AATs.

Potassium-induced chloride secretion across the frog retinal pigment epithelium

In the intact eye, a transition from light to dark increases K concentration ([K]o) from approximately 2 to 5 mM in the extracellular (subretinal) space between the photoreceptors and the retinal pigment epithelium (RPE) apical membrane. In control (HCO3/CO2) Ringer solution, 36Cl was actively absorbed across isolated bullfrog RPE (retina to choroid) at a rate of 0.31 +/- 0.02 (SE) mu eq.cm-2.h-1 (n = 15). Elevating apical [K]o from 2 to 5 mM reversed active 36Cl transport to secretion (choroid to retina), with a rate of 0.76 +/- 0.17 mu eq.cm-2.h-1. This reversal was completely inhibited by 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) in either the apical or basal bath. In open circuit, elevating [K]o induced a similar reversal of net 36Cl flux and inhibited fluid absorption by approximately 25%. Apical Ba2+ (1 mM), decreased CO2 (5 to 1%), or increased apical bath HCO3 concentration ([HCO3]o) also caused a DIDS-inhibitable reversal of active 36Cl flux. A 10-fold reduction of apical bath Na or [HCO3]o significantly inhibited [K]o, Ba2+, and low CO2-induced Cl secretion. All of these results can be understood in terms of an intracellular pH-dependent stimulation of the basolateral membrane Cl-HCO3 exchanger.

Acidification stimulates chloride and fluid absorption across frog retinal pigment epithelium

Radioactive tracers and a modified capacitance-probe technique were used to characterize the mechanisms that mediate Cl and fluid absorption across the bullfrog retinal pigment epithelium (RPE)-choroid. In control (HCO3/CO2) Ringer solution, 36Cl was actively absorbed (retina to choroid) at a mean rate of 0.34 mu eq.cm-2.h-1 (n = 34) and accounted for approximately 25% of the short-circuit current. Apical bumetanide (100 microM) or basal 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS; 1 mM) inhibited active Cl transport by 70 and 62%, respectively. Active Cl absorption was doubled, either by removing HCO3 from the bathing media or by elevating CO2 from 5 to 13%, and the increased flux was inhibited by apical bumetanide or basal DIDS. Open-circuit measurements of fluid absorption rate (Jv) and the net fluxes of 36Cl, 22Na, and 86Rb (K substitute) indicated that CO2-induced acidification stimulated NaCl and fluid absorption across the RPE. During acidification, bumetanide produced a twofold larger inhibition of Jv compared with control. Stimulation of net Cl absorption was most likely caused by inhibition of the the basolateral membrane intracellular pH-dependent Cl-HCO3 exchanger.

pHi-dependent Cl-HCO3 exchange at the basolateral membrane of frog retinal pigment epithelium

Intracellular pH (pHi) measurements in frog retinal pigment epithelium using the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein demonstrate that the basolateral membrane contains a pHi-sensitive Cl-HCO3 exchanger. In control Ringer solution, the removal of Cl from the basal bath alkalinized the cells by 0.07 +/- 0.03 (SD) pH units (n = 39) with an initial rate of 0.022 +/- 0.0013 pH units/min. This effect was blocked by 0.5 mM basal 4,4'-diisothiocyanostilbene-2,2'- disulfonic acid or the removal of HCO3 from both the apical and basal baths. The rate of the exchange is reduced by acidification and increased by alkalinization. Increasing apical bath K concentration ([K]o) from 2 to 5 mM approximates the [K]o change in the subretinal space of the intact eye following a transition from light to dark. This [K]o change alkalinized the cells by increasing the rate of the apical membrane Na-HCO3 cotransporter. In 5 mM apical [K]o, the initial rate of the 0 Cl-induced alkalinization was significantly increased to 304 +/- 13% (n = 4) of control (2 mM [K]o). These mechanisms regulate pHi and could also buffer changes in subretinal pH.

K+ and Cl- transport mechanisms in bovine pigment epithelium that could modulate subretinal space volume and composition

1. Conventional and ion-selective double-barrelled microelectrodes were used in an in vitro bovine retinal pigment epithelium (RPE)-choroid preparation to measure the changes in membrane voltage, resistance and intracellular K+ and Cl- activities produced by small, physiological changes in extracellular potassium ([K+]o). 2. In the intact eye, light-induced changes in [K+]o occur in the extracellular (or subretinal) space that separates the neural retina and the RPE apical membrane. These [K+]o changes can be approximated in vitro by decreasing apical bath [K+]o from 5 to 2 mM. 3. This in vitro change in [K+]o simultaneously decreased intracellular Cl- and K+ activities (aCli and aKi) by 25 +/- 6 mM (n = 8) and 19 +/- 7 mM (n = 4) (mean +/- S.D.), respectively. In control Ringer solution (5 mM [K+]o) aCli and aKi were 65 +/- 10 mM (n = 28) and 65 +/- 8 mM (n = 6), respectively. 4. The [K+]o-induced decreases in aCli and aKi were both significantly inhibited, either by blocking the apical membrane K+ conductance with Ba2+ or the basolateral membrane Cl- conductance with DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulphonic acid). 5. Transepithelial current pulses were used to determine the relative basolateral membrane Cl- conductance, TClBAS, was approximately 0.6 (n = 3), and the relative apical membrane K+ conductance, TKAP, was approximately 0.7 (n = 2). Step changes in basal bath [K+]o were used to estimate the relative basolateral membrane K+ conductance, TKBAS, was approximately 0.34 (n = 3). 6. These data show that the apical membrane K+ conductance and the basolateral membrane Cl- conductance are electrically coupled. In vivo, this coupling could have significant functional importance by modulating the relative hydration of the subretinal space, regulating RPE cell volume, and buffering the chemical composition of the subretinal space.

Lactate transport in freshly isolated human fetal retinal pigment epithelium

PURPOSE

To study transport mechanisms for small monocarboxylic acids in the apical and basolateral membranes of freshly isolated, human fetal retinal pigment epithelium.

METHODS

The epithelium was mounted in a small Ussing chamber that allowed separate perfusion of both the apical and basal compartments and simultaneous measurements of intracellular pH, transepithelial potential, and tissue resistance. Intracellular pH was measured using a pH-sensitive dye, 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein.

RESULTS

When 10-100 mM lactate or pyruvate was added to the apical bath the cells acidified by 0.10-0.25 pH units. There were no differences between the initial rates of intracellular acidification produced by L-lactate and D-lactate. These rates could be described as Michaelis-Menten functions of the concentrations of lactate and pyruvate. The Km values were 42 +/- 12 mM for L-lactate and 34 +/- 8 mM for pyruvate. The rates of acidification caused by 50 mM L-lactate were reversibly reduced by 44% or 35% after apical administration of probenecid (2 mM) or alpha-cyano-4-hydroxycinnamate (2 mM), and irreversibly reduced by 78% after apical administration of the sulfhydryl-reagent mersalyl acid (2 mM). The intracellular acidifications caused by apical pyruvate (50 mM) were completely and reversibly inhibited by 50 mM apical L-lactate. Addition of 50 to 100 mM lactate to the basal bath caused intracellular alkalinizations, which could be inhibited by Na+ removal in the basal bath or by 2 mM alpha-cyano-4-hydroxycinnamate in the apical bath.

Altered fluid transport across airway epithelium in cystic fibrosis

In cystic fibrosis (CF), absence or dysfunction of a phosphorylation-regulated chloride channel [CF transmembrane conductance regulator (CFTR)] leads to the loss or reduction of chloride secretion into the airways. Active sodium absorption is also increased in CF, and both of these ion transport changes could alter fluid transport across the airways. Under baseline conditions, cultured human airway epithelia from normal individuals absorbed fluid, and this absorption was increased in epithelia from patients with CF. In normal and CF epithelial cultures fluid absorption was inhibited by amiloride. Adenosine 3',5'-monophosphate stimulated fluid secretion in normal epithelial cultures but not in cultures from individuals with CF. In contrast, fluid secretion induced by nucleotide triphosphates (uridine triphosphate or adenosine triphosphate) was unaltered in cultures of epithelia from patients with CF, suggesting an approach to the treatment of CF.

Molecular characterization of NADH-dependent glutamate synthase from alfalfa nodules

Alfalfa NADH-dependent glutamate synthase (NADH-GOGAT), together with glutamine synthetase, plays a central role in the assimilation of symbiotically fixed nitrogen into amino acids in root nodules. Antibodies previously raised against purified NADH-GOGAT were employed to screen a cDNA library prepared using RNA isolated from nodules of 20-day-old alfalfa plants. A 7.2-kb cDNA clone was obtained that contained the entire protein coding region of NADH-GOGAT. Analysis of this cDNA and determination of the amino-terminal amino acids of the purified protein revealed that NADH-GOGAT is synthesized as a 2194-amino acid protein that includes a 101-amino acid presequence. The deduced amino acid sequence shares significant identity with maize ferredoxin-dependent GOGAT, and with both large and small subunits of Escherichia coli NADPH-GOGAT. DNA gel blot analysis of alfalfa genomic DNA suggests the presence of a single NADH-GOGAT gene or a small gene family. The expression of NADH-GOGAT mRNA, enzyme protein, and enzyme activity was developmentally regulated in root nodules. A dramatic increase in gene expression occurred coincidentally with the onset of nitrogen fixation in the bacteroid, and was absent in both ineffective plants that were nodulated with effective Rhizobium meliloti and effective plants that had been nodulated with ineffective R. meliloti strains. Maximum NADH-GOGAT expression, therefore, appears to require an effective, nitrogen-fixing symbiosis.

Na-dependent pHi regulatory mechanisms in native human retinal pigment epithelium

This study provides the first information about pHi regulatory mechanisms in human retinal pigment epithelium (RPE). The experiments were carried out on fresh explant tissues from adult donor and fetal eyes, and pHi was measured using fluorescence microscopy techniques and the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. In adult donor RPE, the resting pHi is 7.30 +/- 0.14 (mean +/- standard deviation; n = 6) in HCO3 Ringer's solution. In HCO3 Ringer's solution, apical Na removal caused rapid cell acidification with an initial rate of 0.40 +/- 0.10 pH U/min (n = 4). This Na-dependent acidification was partially inhibited by apical amiloride (n = 1) and DIDS (n = 1). In HCO3 Ringer's solution, pHi recovery from an acid load (NH4 prepulse) also was blocked by apical Na removal. In nominally HCO3-free Ringer's solution, apical amiloride (1 mmol/l) acidified the cells. These results suggest that the apical membrane of adult human RPE contains an Na/H exchanger and possibly a Na-dependent, DIDS-inhibitable pH regulatory mechanism, perhaps a NaHCO3 cotransporter. For the fetal RPE, the resting pHi was 7.16 +/- 0.10 (n = 9) and 7.19 +/- 0.10 (n = 20) in HCO3 and HCO3-free Ringer's solution, respectively. In HCO3 and HCO3-free Ringer's solution, apical amiloride (1 mmol/l) acidified the cells and the removal of apical Na caused cell acidification with an initial rate of 0.30 +/- 0.08 (n = 32) and 0.58 +/- 0.29 (n = 6) pH U/min, respectively. The pHi recovery from an acid load also was blocked by apical amiloride and apical Na removal. These results suggest that the apical membrane Na/H exchanger is the dominant acid extrusion mechanism in human fetal RPE.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion transport mechanisms in native human retinal pigment epithelium

Electrophysiologic techniques were used to characterize the electrical properties and the ion transport mechanisms at the apical and basolateral membranes of the human retinal pigment epithelium (RPE). These experiments used fresh native tissue from adult donor and fetal eyes. In the upper range, adult donor RPE had an apical membrane resting potential (VA) of approximately equal to -60 mV and a transepithelial potential (TEP) and resistance (Rt) of 3.5 mV and 148 omega.cm2, respectively. The means were at least 50% of these values. In RPE from fetuses of gestational age 19-23 wk, VA was -56 +/- 4 mV, TEP was 2.2 +/- 1.5 mV, Rt was 206 +/- 151 omega.cm2, and the ratio of apical to basolateral resistance was 0.70 +/- 0.50 (mean +/- standard deviation; n = 15). The apical membrane of the adult donor and fetal RPE contains a large relative K+ conductance (TK > 0.3) that is barium blockable. In fetal RPE, there is evidence for separate K+ and Cl- conductive mechanisms at the basolateral membrane. However, the evidence for the Cl- conductance is indirect. The fetal RPE apical membrane, but not the basolateral membrane, contains a ouabain-sensitive mechanism that exhibits two distinct phases of apical depolarization. The first, rapid phase suggests that the pump is electrogenic. The apical membrane of fetal RPE contains a bumetanide-sensitive mechanism and a receptor activated by nanomolar amounts of epinephrine. In fetal RPE, step changes in apical [K+]o between 5 and 2 mmol/l produced a delayed basolateral membrane hyperpolarization that in situ generates the fast oscillation trough of the electroretinogram.

Aspartate aminotransferase in effective and ineffective alfalfa nodules : cloning of a cDNA and determination of enzyme activity, protein, and mRNA levels

Aspartate aminotransferase (AAT) is a key plant enzyme affecting nitrogen and carbon metabolism, particularly in legume root nodules and leaves of C(4) species. To ascertain the molecular genetic characteristics and biochemical regulation of AAT, we have isolated a cDNA encoding the nodule-enhanced AAT (AAT-2) of alfalfa (Medicago sativa L.) by screening a root nodule cDNA expression library with antibodies. Complementation of an Escherichia coli AAT mutant with the alfalfa nodule AAT-2 cDNA verified the identity of the clone. The deduced amino acid sequence of alfalfa AAT-2 is 53 and 47% identical to animal mitochondrial and cytosolic AATs, respectively. The deduced molecular mass of AAT-2 is 50,959 daltons, whereas the mass of purified AAT-2 is about 40 kilodaltons as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the protein's N-terminal domain (amino acids 1-59) contains many of the characteristics of plastid-targeting peptides. We postulate that AAT-2 is localized to the plastid. Southern blot analysis suggests that AAT-2 is encoded by a small, multigene family. The expression of AAT-2 mRNA in nodules is severalfold greater than that in either leaves or roots. Northern and western blots showed that expression of AAT activity during effective nodule development is accompanied by a sevenfold increase in AAT-2 mRNA and a comparable increase in enzyme protein. By contrast, plant-controlled ineffective nodules express AAT-2 mRNA at much lower levels and have little to no AAT-2 enzyme protein. Expression of root nodule AAT-2 appears to be regulated by at least two events: the first is independent of nitrogenase activity; the second is associated with nodule effectiveness.

Alpha-1-adrenergic modulation of K and Cl transport in bovine retinal pigment epithelium

Intracellular microelectrode techniques were used to characterize the electrical responses of the bovine retinal pigment epithelium (RPE)-choroid to epinephrine (EP) and several other catecholamines that are putative paracrine signals between the neural retina and the RPE. Nanomolar amounts of EP or norepinephrine (NEP), added to the apical bath, caused a series of conductance and voltage changes, first at the basolateral or choroid-facing membrane and then at the apical or retina-facing membrane. The relative potency of several adrenergic agonists and antagonists indicates that EP modulation of RPE transport begins with the activation of apical alpha-1-adrenergic receptors. The membrane-permeable calcium (Ca2+) buffer, amyl-BAPTA (1,2-bis(o-aminophenoxy)-ethane-N,N,N',N' tetraacetic acid) inhibited the EP-induced voltage and conductance changes by approximately 50-80%, implicating [Ca2+]i as a second messenger. This conclusion is supported by experiments using the Ca2+ ionophore A23187, which mimics the effects of EP. The basolateral membrane voltage response to EP was blocked by lowering cell Cl, by the presence of DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid) in the basal bath, and by current clamping VB to the Cl equilibrium potential. In the latter experiments the EP-induced conductance changes were unaltered, indicating that EP increases basolateral membrane Cl conductance independent of voltage. The EP-induced change in basolateral Cl conductance was followed by a secondary decrease in apical membrane K conductance (approximately 50%) as measured by delta [K]o-induced diffusion potentials. Decreasing apical K from 5 to 2 mM in the presence of EP mimicked the effect of light on RPE apical and basolateral membrane voltage. These results indicate that EP may be an important paracrine signal that provides exquisite control of RPE physiology.

Epinephrine stimulates fluid absorption across bovine retinal pigment epithelium

The authors used a modified capacitive probe technique to simultaneously assess the effect of apical epinephrine on fluid transport rate (Jv), transepithelial potential (TEP), and transepithelial resistance (Rt) across bovine retinal pigment epithelium (RPE). In control Ringer, the RPE absorbed fluid at a rate of 1.42 +/- 0.34 microliters/cm2.hr (mean +/- SEM; 22 tissues). Tissues with the highest TEP (8-9 mV) and Rt (160-220 omega.cm2) had maximum fluid absorption rates (3-4 microliters/cm2.hr). Apical epinephrine (100 nM) stimulated Jv by a factor of 3, from 0.70 +/- 0.18 microliter/cm2.hr to 2.17 +/- 0.24 microliters/cm2.hr and TEP from 4.6 +/- 0.4 mV to 7.0 +/- 0.6 mV (n = 6). The epinephrine-induced transport changes were inhibited by apical bumetanide (0.1 mM). The alpha-1 adrenergic antagonist prazosin (1 microM) completely blocked the epinephrine-induced stimulation of Jv and TEP. In contrast, the beta adrenergic antagonist propranolol (1 microM) had no effect on epinephrine-induced transport changes. These results, coupled with previous studies on bovine RPE, suggest that the mechanisms underlying the epinephrine-induced stimulation of fluid absorption include an apical membrane alpha-1 adrenergic receptor, a bumetanide-inhibitable apical membrane Na-K-2Cl cotransporter, and a basolateral membrane Cl conductance.

pHi regulation in frog retinal pigment epithelium: two apical membrane mechanisms

This study demonstrates that the apical membrane of frog retinal pigment epithelium (RPE) contains two intracellular pH (pHi) regulatory mechanisms, an electrogenic Na-HCO3 cotransporter blocked by DIDS and an amiloride-inhibitable Na-H antiporter. pHi was studied using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). In these cells resting pHi equals 7.26 +/- 0.09 (n = 58). After an acid load (NH4Cl prepulse), pHi recovery required apical extracellular Na concentration ([Na]o) in HCO3 or HCO3-free Ringer. In HCO3 Ringer recovery was completely blocked by 1 mM apical DIDS (n = 5) but was not affected by absence of Cl. In HCO3-free Ringer, recovery was completely blocked by 1 mM apical amiloride (n = 3). At resting pHi, the intrinsic pH-buffering capacity of the cell is approximately 7.1 mM/pH and rises monotonically as pHi decreases. In HCO3 Ringer, the initial rate of acidification caused by apical Na removal, 0.39 +/- 0.03 pH/min (n = 26), was 80-90% inhibited by apical DIDS (n = 5) and 16% inhibited by 1 mM apical amiloride (n = 7), but not affected by absence of Cl. In HCO3 Ringer, initial rates of acidification induced by apical DIDS or amiloride were 0.11 +/- 0.06 (n = 5) and 0.03 +/- 0.02 pH/min (n = 7), respectively. These results indicate that the Na-HCO3 cotransporter accounts for 80-90% of the acid extrusion from frog RPE cells. Increasing apical [K]o from 2 to 5 mM approximates the in vivo apical [K]o changes during a light-dark transition and alkalinizes the cells. [K]o-induced alkalinization had an initial rate of 0.11 +/- 0.02 pH/min (n = 16), which was approximately 75% inhibited by apical DIDS (to 0.04 +/- 0.01 pH/min, n = 7) and completely blocked by HCO3/CO2 removal from both bathing solutions. [K]o-induced pHi changes alter RPE transport mechanisms and may affect RPE-photoreceptor interactions.

Apical and basal membrane ion transport mechanisms in bovine retinal pigment epithelium

1. Intracellular voltage recordings using conventional and double-barrelled chloride-selective microelectrodes have been used to identify several transport mechanisms at the apical and basolateral membranes of the isolated bovine retinal pigment epithelium (RPE)-choroid preparation. Intracellular recordings were obtained from two cell populations, melanotic (pigmented) and amelanotic (non-pigmented). The electrical properties of these two populations are practically identical. For melanotic cells the average apical resting membrane potential (VA) is -61 +/- 2 mV (mean +/- S.E.M., n = 49 cells, thirty-three eyes). For these cells the ratio of apical to basolateral membrane resistance (a) was 0.22 +/- 0.02. The mean transepithelial voltage and resistance were 6 +/- 1 mV and 138 +/- 7 omega cm2, respectively. 2. The apical membrane, which faces the distal retina, contains a Ba(2+)-inhibitable K+ conductance and a ouabain-inhibitable, electrogenic Na(+)-K+ pump. In addition it contains a bumetanide-sensitive mechanism, the putative Na(+)-K(+)-Cl- cotransporter. The basolateral membrane contains a DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid)-inhibitable chloride channel. The relative conductances of the apical and basolateral membranes to K+ and Cl- are TK approximately 0.9 and TCl approximately 0.7, respectively. 3. The ouabain-induced fast phase of apical membrane depolarization (0-30 s) was used to calculate the equivalent resistances of the apical (RA) and basolateral (RB) cell membranes, as well as the paracellular or shunt resistance (RS). They are: 3190 +/- 400, 17920 +/- 2730 and 2550 +/- 200 omega (mean +/- S.E.M., n = 9 tissues), respectively. From these data the equivalent electromotive forces (EMF) at the apical (EA) and basolateral (EB) membranes were also calculated. They are: -69 +/- 5.0 and -24 +/- 5.0 mV, respectively. 4. Intracellular Cl- activity (aiCl) was measured using double-barreled ion-selective microelectrodes. In the steady state aiCl = 61 +/- 4.0 mM and the Nernst potential ECl = -13.5 +/- 1.5 mV (mean +/- S.E.M., n = 4). 5. In the intact eye or in retina, RPE-choroid preparations it has been shown that the transition between light and dark alters the K+ concentration in the extracellular (or subretinal) space between the photoreceptors and the apical membrane of the RPE. These light-induced changes in subretinal [K+]o were qualitatively simulated in vitro by altering apical K+ between 5 and 2 mM. This produced a sequence of voltage changes at the apical and basolateral membranes that had three operationally distinct phases. Phase 1 is generated by the combination of an apical membrane K+ diffusion potential and inhibition of the electrogenic Na(+)-K+ pump.(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium-dependent volume regulation in retinal pigment epithelium is mediated by Na,K,Cl cotransport

Changes in retinal pigment epithelial (RPE) cell volume were measured by monitoring changes in intracellular tetramethylammonium (TMA) using double-barreled K-resin microelectrodes. Hyperosmotic addition of 25 or 50 mM mannitol to the Ringer of the apical bath resulted in a rapid (approximately 30 s) osmometric cell shrinkage. The initial cell shrinkage was followed by a much slower (minutes) secondary shrinkage that is probably due to loss of cell solute. When apical [K+] was elevated from 2 to 5 mM during or before a hyperosmotic pulse, the RPE cell regulated its volume by reswelling towards control within 3-10 min. This change in apical [K+] is very similar to the increase in subretinal [K+]o that occurs after a transition from light to dark in the intact vertebrate eye. The K-dependent regulatory volume increase (RVI) was inhibited by apical Na removal, Cl reduction, or the presence of bumetanide. These results strongly suggest that a Na(K),Cl cotransport mechanism at the apical membrane mediates RVI in the bullfrog RPE. A unique aspect of this cotransporter is that it also functions at a lower rate under steady-state conditions. The transport requirements for Na, K, and Cl, the inhibition of RVI by bumetanide, and thermodynamic calculations indicate that this mechanism transports Na, K, and Cl in the ratio of 1:1:2.

Aspartate Aminotransferase in Alfalfa Root Nodules : III. Genotypic and Tissue Expression of Aspartate Aminotransferase in Alfalfa and Other Species

Aspartate aminotransferase (AAT) plays an important role in nitrogen metabolism in all plants and is particularly important in the assimilation of fixed N derived from the legume-Rhizoblum symbiosis. Two isozymes of AAT (AAT-1 and AAT-2) occur in alfalfa (Medicago sativa L.). Antibodies against alfalfa nodule AAT-2 do not recognize AAT-1, and these antibodies were used to study AAT-2 expression in different tissues and genotypes of alfalfa and also in other legume and nonlegume species. Rocket immunoelectrophoresis indicated that nodules of 38-day-old alfalfa plants contained about eight times more AAT-2 than did nodules of 7-day-old plants, confirming the nodule-enhanced nature of this isozyme. AAT-2 was estimated to make up 16, 15, 5, and 8 milligrams per gram of total soluble protein in mature nodules, roots, stems, and leaves, respectively, of effective N(2)-fixing alfalfa. The concentration of AAT-2 in nodules of ineffective non-N(2)-fixing alafalfa genotypes was about 70% less than that of effective nodules. Western blots of soluble protein from nodules of nine legume species indicated that a 40-kilodalton polypeptide that reacts strongly with AAT-2 antibodies is conserved in legumes. Nodule AAT-2 immunoprecipitation data suggested that amide- and ureide-type legumes may differ in expression and regulation of the enzyme. In addition, Western blotting and immunoprecipitations of AAT activity demonstrated that antibodies against alfalfa AAT-2 are highly cross-reactive with AAT enzyme protein in leaves of soybean (Glycine max L.), wheat (Triticum aestivum L.), and maize (Zea mays L.) and in roots of maize, but not with AAT in soybean and wheat roots. Results from this study indicate that AAT-2 is structurally conserved and localized in similar tissues among diverse species.