Identification and characterization of alkenyl hydrolase (lysoplasmalogenase) in microsomes and identification of a plasmalogen-active phospholipase A2 in cytosol of small intestinal epithelium.

A lysoplasmalogenase (EC 3.3.2.2; EC 3.3.2.5) that liberates free aldehyde from 1-alk-1'-enyl-sn-glycero-3-phospho-ethanolamine or -choline (lysoplasmalogen) was identified and characterized in rat gastrointestinal tract epithelial cells. Glycerophosphoethanolamine was produced in the reaction in equimolar amounts with the free aldehyde. The microsomal membrane associated enzyme was present throughout the length of the small intestines, with the highest activity in the jejunum and proximal ileum. The rate of alkenyl ether bond hydrolysis was dependent on the concentrations of microsomal protein and substrate, and was linear with respect to time. The enzyme hydrolyzed both ethanolamine- and choline-lysoplasmalogens with similar affinities; the Km values were 40 and 66 microM, respectively. The enzyme had no activity with 1-alk-1'-enyl-2-acyl-sn-glycero-3-phospho-ethanolamine or -choline (intact plasmalogen), thus indicating enzyme specificity for a free hydroxyl group at the sn-2 position. The specific activities were 70 nmol/min/mg protein and 57 nmol/min/mg protein, respectively, for ethanolamine- and choline-lysoplasmalogen. The pH optimum was between 6.8 and 7.4. The enzyme required no known cofactors and was not affected by low mM levels of Ca2+, Mg2+, EDTA, or EGTA. The detergents, Triton X-100, deoxycholate, and octyl glucoside inhibited the enzyme. The chemical and physical properties of the lysoplasmalogenase were very similar to those of the enzyme in liver and brain microsomes. In developmental studies the specific activities of the small intestinal and liver enzymes increased markedly, 11.1- and 3.4-fold, respectively, in the first approximately 40 days of postnatal life. A plasmalogen-active phospholipase A2 activity was identified in the cytosol of the small intestines (3.3 nmol/min/mg protein) and liver (0.3 nmol/min/mg protein) using a novel coupled enzyme assay with microsomal lysoplasmalogenase as the coupling enzyme.

Inhibition of Na+-dependent Ca2+ efflux from heart mitochondria by amiloride analogues.

The Na+-induced release of accumulated Ca2+ from heart mitochondria is inhibited by amiloride, benzamil and several other amiloride analogues. These drugs do not affect uptake or release of Ca2+ mediated by the ruthenium red-sensitive uniporter and their effects, like those of diltiazem and other Ca2+-antagonists, appear to be localized principally at the Na+/Ca2+ antiporter of the mitochondrion. Benzamil inhibits Na+/Ca2+ antiport non-competitively with respect to [Na+] with a Ki of 167 microM. In the presence of 1.5 mM Pi the Ki for benzamil inhibition of this reaction is decreased to 87 microM.

Inosine and guanosine preserve neuronal and glial cell viability in mouse spinal cord cultures during chemical hypoxia.

Murine spinal cord primary mixed cultures were treated with the respiratory inhibitor, rotenone, to mimic hypoxic conditions. Under these conditions neurons rapidly underwent oncosis (necrosis) with a complete loss in viability occurring within 260 min; however, astrocytes, which accounted for most of the cell population, died more slowly with 50% viability occurring at 565 min. Inosine preserved both total cell and neuronal viability in a concentration-dependent manner. The time of inosine addition relative to hypoxic insult was critical with the most effective protection occurring when inosine was added just prior to or within 5 min after insult. Inosine was ineffective when added 30 min after hypoxic insult. The effect of guanosine was similar to that of inosine. Treatment of cultures with BCX-34, a purine nucleoside phosphorylase inhibitor, prevented protection by inosine or guanosine, suggesting involvement of a purine nucleoside phosphorylase in the nucleoside protective effect.

H+-dependent efflux of Ca2+ from heart mitochondria.

A rapid loss of accumulated Ca2+ is produced by addition of H+ to isolated heart mitochondria. The H+-dependent Ca+ efflux requires that either (a) the NAD(P)H pool of the mitochondrion be oxidized, or (b) the endogenous adenine nucleotides be depleted. The loss of Ca2+ is accompanied by swelling and loss of endogenous Mg2+. The rate of H+-dependent Ca2+ efflux depends on the amount of Ca2+ and Pi taken up and the extent of the pH drop imposed. In the absence of ruthenium red the H+-induced Ca2+-efflux is partially offset by a spontaneous re-accumulation of released Ca2+. The H+-induced Ca2+ efflux is inhibited when the Pi transporter is blocked with N-ethylmaleimide, is strongly opposed by oligomycin and exogenous adenine nucleotides (particularly ADP), and inhibited by nupercaine. The H+-dependent Ca2+ efflux is decreased markedly when Na+ replaces the K+ of the suspending medium or when the exogenous K+/H+ exchanger nigericin is present. These results suggest that the H+-dependent loss of accumulated Ca2+ results from relatively nonspecific changes in membrane permeability and is not a reflection of a Ca2+/H+ exchange reaction.

Responses of the normal rat kidney to sequential ischemic events.

UNLABELLED: This study was undertaken to help define how one episode of renal ischemia, insufficient to cause acute renal failure, influences the susceptibility of the kidney to a second more severe ischemic event. Female Sprague-Dawley rats underwent either 15 min of bilateral renal artery occlusion (RAO) or sham RAO. They were subjected 30 min, 3.5 h, or 24 h later to 25 min of RAO. Renal function (GFR, BUN, creatinine), histology, and adenine nucleotide concentrations were compared before and after the 25-min ischemic event. Only the rats with a 30-min hiatus between the 15- and 25-min bouts of RAO had significantly worse renal failure than controls subjected to a single 25-min ischemic event. Three findings were noted only in the rats with increased susceptibility: tubular cell swelling and luminal membrane injury prior to 25 min of RAO and a relative failure of ATP formation immediately following 25 min of RAO. Susceptibility to 25 min of RAO did not correlate with preischemia ATP content. CONCLUSION: prior mild ischemic injury transiently lowers renal resistance to a second ischemic event. Normal resistance is rapidly restored once improvements in prior cell membrane injury, cell volume regulation, and cellular energetics occur. However, resistance to additional ischemia can be normal despite persisting depressions in renal ATP content.

Adenosine, inosine, and guanosine protect glial cells during glucose deprivation and mitochondrial inhibition: correlation between protection and ATP preservation.

The purpose of this study was to determine the mechanism by which adenosine, inosine, and guanosine delay cell death in glial cells (ROC-1) that are subjected to glucose deprivation and mitochondrial respiratory chain inhibition with amobarbital (GDMI). ROC-1 cells are hybrid cells formed by fusion of a rat oligodendrocyte and a rat C6 glioma cell. Under GDMI, ATP was depleted rapidly from ROC-1 cells, followed on a much larger time scale by a loss of cell viability. Restoration of ATP synthesis during this interlude between ATP depletion and cell death prevented further loss of viability. Moreover, the addition of adenosine, inosine, or guanosine immediately before the amobarbital retarded the decline in ATP and preserved cell viability. The protective effects on ATP and viability were dependent on nucleoside concentration between 50 and 1,500 microM. Furthermore, protection required nucleoside transport into the cell and the continued presence of nucleoside during GDMI. A significant positive correlation between ATP content at 16 min and cell viability at 350 min after the onset of GDMI was established (r = 0.98). Modest increases in cellular lactate levels were observed during GDMI (1.2 nmol/mg/min lactate produced); however, incubation with 1,500 microM inosine or guanosine increased lactate accumulation sixfold. The protective effects of inosine and guanosine on cell viability and ATP were >90% blocked after treatment with 50 microM BCX-34, a nucleoside phosphorylase inhibitor. Accordingly, lactate levels also were lower in BCX-34-treated cells incubated with inosine or guanosine. We conclude that under GDMI, the ribose moiety of inosine and guanosine is converted to phosphorylated glycolytic intermediates via the pentose phosphate pathway, and its subsequent catabolism in glycolysis provides the ATP necessary for maintaining plasmalemmal integrity.

Ruthenium red-sensitive and -insensitive release of Ca2+ from uncoupled heart mitochondria.

The uncoupler-induced release of accumulated Ca2+ from heart mitochondria can be separated into two components, one sensitive and one insensitive to ruthenium red. In mitochondria maintaining reduced NAD(P)H pools and adequate levels of endogenous adenine nucleotides, the release of Ca2+ following addition of an uncoupler is virtually all inhibited by ruthenium red and can be presumed to occur via reversal of the Ca2+ uniporter. When ruthenium red is added to block efflux via this pathway, high rates of Ca2+ efflux can still be induced by an uncoupler, provided either NADH is oxidized or mitochondrial adenine nucleotide pools are depleted by prior treatment. This ruthenium red-insensitive Ca2+-efflux pathway is dependent on the level of Ca2+ accumulated and is accompanied by swelling of the mitochondria and loss of endogenous Mg2+. Loss of Ca2+ by this relatively nonspecific pathway is strongly inhibited by Sr2+ and by nupercaine, as well as by oligomycin and exogenous adenine nucleotides. The loss of Ca2+ from uncoupled heart mitochondria occurs via a combination of these two mechanisms except under conditions chosen specifically to limit efflux to one or the other pathway.

Responses of the ischemic acute renal failure kidney to additional ischemic events.

To test whether the ischemic acute renal failure (IARF) kidney has increased susceptibility to additional ischemic events, IARF was induced in female Sprague-Dawley rats [40 min of bilateral renal artery occlusion (RAO)] and either 18 or 48 hr later, at the height of morphologic injury, they were rechallenged with either 25 or 40 min of RAO. Changes in renal function (GFR, blood flow), morphology, and adenine nucleotide (AN) concentrations in response to these second ischemic challenges were compared to those of normal kidneys subjected to a single ischemic event. In additional experiments, rates of recovery from IARF were compared between rats subjected to one or two bouts of RAO (40 min, 24 hr apart). IARF kidneys retained a significantly greater percent of their baseline GFR and had comparable or higher absolute GFRs after 25 or 40 min of RAO than control rats. IARF rats showed no significant exacerbation of their underlying morphologic injury by superimposing a second ischemic event. IARF kidneys (24 hr post RAO) had normal AN concentrations, and by 30 min of reflow from a second 40 min of RAO, they re-established their AN energy charge and retained AN pools as well as control kidneys. A second 40-min bout of RAO did not significantly prolong recovery rates from the first 40-min ischemic event. In additional experiments, intraperitoneal injection of normal urine or solute matched artificial urine (urea, creatinine, NaCl) into normal rats to mimic the degree of azotemia seen in the IARF rats induced significant and comparable protection against 40 min of RAO. We conclude that the IARF kidney, at or near the height of its functional and morphologic injury, does not have increased susceptibility to additional ischemic insults. Rather a modicum of protection appears to exist, possibly due to renal-failure-induced increments in solute loads per nephron.

K+/H+ antiport in heart mitochondria.

Heart mitochondria depleted of endogenous divalent cations by treatment with A23187 and EDTA swell in (a) K+ acetate or (b) K+ nitrate when an uncoupler is present. These mitochondria also exchange matrix 42K+ with external K+, Na+, or Li+ in a reaction that does not require respiration and is insensitive to uncouplers. Untreated control mitochondria do not swell in either medium nor do they show the passive cation exchange. Both the swelling and the exchange reactions are inhibited by Mg2+ and by quinine and other lipophilic amines. Swelling and exchange are both strongly activated at alkaline pH, and the exchange reaction is also increased markedly by hypotonic conditions. All of these properties correspond to those reported for a respiration-dependent extrusion of K+ from Mg2+-depleted mitochondria, a reaction attributed to a latent Mg2+- and H+-sensitive K+/H+ antiport. The swelling reactions are strongly inhibited by dicyclohexylcarbodiimide reacted under hypotonic conditions, but the exchange reaction is not sensitive to this reagent. Heart mitochondria depleted of Mg2+ show marked increases in their permeability to H+, to anions, and possibly to cations, and the permeability to each of these components is further increased at alkaline pH. This generalized increase in membrane permeability makes it likely that K+/H+ antiport is not the only pathway available for K+ movement in these mitochondria. It is concluded that the swelling, 42K+ exchange, and K+ extrusion data are all consistent with the presence of the putative K+/H+ antiport but that definitive evidence for the participation of such a component in these reactions is still lacking.