The fraction of phosphatidylinositol that activates the (Na+ + K+)-ATPase in rabbit kidney microsomes is clearly associated with the enzyme protein

Compartmental study of rat renal phospholipid metabolism

Phospholipid content and metabolism were studied in rat renal papillary, medullary and cortical slices. The highest concentration of phospholipids was found in cortex and the lowest in papilla samples (ratio cortex/medulla, 1.3; cortex/papilla, 3.7). The profile of the various phospholipids was different depending on the zone. The most important difference was the relative concentrations of sphingomyelin (CerPCho) and phosphatidylinositol (PtdIns) with ratios for PtdIns/CerPCho of 5.0, 3.3 and 2.5 in papilla, medulla, and cortex, respectively. In the three zones, PtdIns showed the highest specific activity for [2-14C]glycerol and [1-14C]arachidonic acid incorporation. By contrast, a higher amount of [1-14C]palmitic acid was incorporated into phosphatidylcholine than into any other phospholipid. The various radioactive precursors were only poorly incorporated into phosphatidylethanolamine. No radioactivity was associated with phosphatidylserine. The papilla possesses the most active phospholipid metabolism of all the pathways studied.

Mastoparan inhibits rat renal NaK-ATPase activity

Rat renal NaK-ATPase was inhibited by mastoparan in a dose dependent fashion. This inhibition reached completion within 30 seconds. Due to mastoparan's rapid effects on NaK-ATPase activity, this inhibition does not appear to involve either a decrease in the rate of synthesis or an increase in their degradation of NaK-ATPase since these processes require a latency period of at least several minutes. In addition, the phosphoenzyme intermediate formed in the presence of mastoparan was greater than that formed in its absence further indicating that inhibition of NaK-ATPase by mastoparan is not due to a decrease in the number of NaK-ATPase. A possible mechanism for the inhibition is that mastoparan stabilizes the phosphoenzyme intermediate and reduces the Vmax of the enzyme by decreasing the rate of turnover of existing enzyme sites. Neomycin, an inhibitor of inositol phospholipid metabolism, was also found to attenuate the inhibition of Na,K-ATPase by mastoparan, suggesting that the mechanism of this inhibition may involve degradation of the phosphatidylinositol "pool".

Inhibition of alpha-aminoisobutyric acid uptake by mastoparan in Madin-Darby Canine Kidney cells

Mastoparan, a wasp venom, was found to inhibit Na(+)-dependent net alpha-aminoisobutyric acid (AIB) uptake in Madin Darby Canine Kidney (MDCK) cells. Mastoparan also produced a significant increase in AIB efflux when compared to controls. Pretreatment of MDCK cells with 2 mM neomycin attenuated mastoparan's inhibition of net AIB uptake and completely suppressed mastoparan-mediated increases in AIB efflux when compared to controls. These data suggest that mastoparan's inhibition of net AIB uptake involves more than a single basic mechanism.

Cardiotoxin from cobra venom increases the level of phosphatidylinositol 4-monophosphate and phosphatidylinositol kinase activity in two cell lines

In rat basophilic leukemia-2H3 (RBL-2H3) and Madin-Darby canine kidney (MDCK) cells, cardiotoxin from cobra venom induced a marked decrease in the level of [3H] phosphatidylinositol and a corresponding increase in the level of [3H]phosphatidylinositol 4-monophosphate over the course of 20 min as demonstrated in cells that had been labeled to equilibrium with [3H]inositol. The effect was dependent on the concentration (5-30 micrograms/ml) of the toxin. In plasma membrane-enriched fractions isolated from the two cell lines, the cardiotoxin enhanced the endogenous activity of phosphatidylinositol kinase especially at temperatures above 14 degrees C. In RBL-2H3 cells, cardiotoxin also induced release of substantial amounts of histamine and lactate dehydrogenase. The release of histamine, but not of lactate dehydrogenase, was totally dependent on external calcium and this release probably represented an exocytotic response of the cells to cardiotoxin. Although, initially, treatment with the toxin did not impair antigen-induced hydrolysis of inositol phospholipids or prevent the antigen-induced rise in the concentration of cytosol Ca2+, prolonged exposure to the toxin did result in a progressive loss of responsiveness of RBL-2H3 cells to antigen.

Reduced Na+ + K+-ATPase activity in peripheral nerve of streptozotocin-diabetic rats: a role for protein kinase C?

Six weeks after induction of diabetes, the rate of ouabain-sensitive 86Rb+ accumulation, a parameter which reflects Na+ + K+-ATPase pumping activity, was significantly reduced in endoneurial preparations of sciatic nerve from untreated diabetic rats compared with that in control rats (Trial, 1, 0.19 +/- 0.09 versus 0.48 +/- 0.13 pmol/min per mg wet weight of tissue, p less than 0.001; Trial 2, 0.27 +/- 0.16 versus 0.47 +/- 0.18, p less than 0.01). This decrease in ouabain-sensitive 86Rb+ uptake was not observed in nerves from diabetic rats maintained on sorbinil (an aldose reductase inhibitor) or myo-inositol diets. Protein kinase C activity was demonstrated in the soluble fraction of a sciatic nerve homogenate by assaying for lipid-activated, Ca+-dependent phosphorylation of calf thymus histone. No significant difference in the time course of kinase C activity was observed between cytosol fractions of nerve homogenates from control and diabetic rats (control, 6.22 +/- 0.97 pmol 32P incorporated/mg cytosol protein in 50 min; diabetic, 5.32 +/- 0.71). Three low molecular weight neural proteins (each with Mr less than 29,000) were identified as substrates for protein kinase C.

Effect of diabetes and Sorbinil treatment on phospholipid metabolism in rat glomeruli

To explore the hypothesis that changes in membrane phospholipids accompany tissue myo-inositol depletion and reduced (Na+ + K+)-ATPase activity in diabetes, we examined phospholipid concentrations in glomeruli isolated from control and streptozotocin-diabetic rats and the effect of diabetes on myo-[3H]inositol incorporation in vitro into glomerular phosphatidylinositol. Since the aldose reductase inhibitor, Sorbinil, prevents the fall in myo-inositol and the decrease in (Na+ + K+)-ATPase activity associated with diabetes, phospholipid and phosphatidylinositol content were also examined in glomeruli isolated from Sorbinil-treated diabetic rats. Total phospholipids (microgram phosphorus/mg dry weight) did not differ in the three groups of animals. The concentration of phosphatidylcholine was elevated in preparations from diabetic rats, both untreated and Sorbinil-treated. Phosphatidylethanolamine was reduced in glomeruli from Sorbinil-treated rats. Neither acute experimental diabetes nor Sorbinil treatment produced detectable changes in the glomerular concentration of phosphatidylinositol. In vitro incubations with glomeruli isolated from control and diabetic animals resulted in increased levels of incorporation of myo-[3H]inositol into phospholipids of diabetic glomeruli. The specific activity of [3H]phosphatidylinositol in glomeruli from diabetic rats was significantly greater than that in control samples. The findings do not support the postulate invoking correspondent changes in myo-inositol and phosphatidylinositol contents as contributory to diminished glomerular (Na+ + K+)-ATPase activity in diabetes, but are compatible with depletion of glomerular intracellular myo-inositol in diabetes.

Aldose reductase, glomerular metabolism, and diabetic nephropathy

To explore a possible link between diabetic nephropathy and the enhanced activity of the polyol pathway known to occur in diabetes, we examined several pertinent metabolic parameters in glomeruli isolated from control and streptozotocin-diabetic rats and assessed whether changes observed in diabetic glomeruli could be prevented by the oral administration of the aldose reductase inhibitor sorbinil. We found that glomerular polyol content is significantly increased in diabetes, whereas glomerular myo-inositol content is significantly reduced. The sorbitol accumulation and myo-inositol depletion were both completely prevented by sorbinil, which was given throughout the duration of diabetes. Activity of the membrane-bound sodium/potassium adenosine triphosphatase (Na-K-ATPase) was decreased in diabetic samples; this change was also completely prevented by sorbinil. Erythrocyte deformability is another factor that has been implicated in the pathogenesis of microangiopathic complications. The ability of red blood cells to undergo an adaptation in shape that permits passage through the smallest vessels is impaired in diabetes. Using blood from control, diabetic, and sorbinil-treated diabetic rats, we found that erythrocyte deformability was decreased in diabetic samples and that sorbinil treatment significantly improved this parameter. Thus, if the glomerular consequences of sorbitol accumulation, myo-inositol depletion, reduced Na-K-ATPase activity, and decreased erythrocyte deformability are pathogenetically implicated in diabetic nephropathy, the ability of sorbinil to impact on these changes suggests that it could favorably impact on the nephropathic process.

Binding of an aldose reductase inhibitor to renal glomeruli

The aldose reductase inhibitor Sorbinil affects several membrane-associated complexes including Na/K-ATPase activity, transport processes, and impulse propagation. These considerations, coupled with the drug's aromatic nature, suggested the possibility of direct interaction with cell membranes. In the present study, binding of [3H]-Sorbinil to isolated glomeruli was demonstrated. Binding is dose-dependent and saturable, and can be inhibited by increasing concentrations of unlabeled Sorbinil. These results may help explain the compound's diverse effects on membrane-associated processes.

Inhibitory action of phosphatidylinositol on synaptosomal (Na+ + K+)-ATPase activity

Phosphatidylinositol and several other phospholipids were tested for their ability to influence the (Na+ + K+)-ATPase activity of the cortical synaptic membrane from rats at various levels of free Ca2+. Phosphatidylinositol, but not phosphatidylethanolamine, phosphatidylcholine nor phosphatidylserine, markedly inhibited this enzyme activity, when the free Ca2+ concentration in the incubation media was less than 2.5 X 10(-6) M. This result suggests that phosphatidylinositol may play a role in the depolarization and/or the release of neurotransmitters or intracellular substances in the brain.

Phospholipid metabolism in rat kidney cortical tubules. I. Effect of renal substrates

Renal phospholipid metabolism was studied in rat cortical tubule suspensions by combining net measurements with precursor incorporation studies in order to be able to calculate the turnover of the different phospholipid species. Net amounts of tubular phosphatidylcholine (PC), phosphatidylinositol (PI) and phosphatidylethanolamine (PE) were 96.7 +/- 3.9, 29.2 +/- 2.9 and 74.1 +/- 6.0, mumol per g protein, respectively. Incubation of tubules in the absence or presence of renal substrates did not change net phospholipid contents. The predominant fatty acids were palmitate (47%) in PC and stearate in PI (56%) and PE (41%). Highest [14C]palmitate and [14C]glycerol incorporation rates were found in PC. In contrast, 32P-labeling was highest in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate exceeding PI-labeling by a factor of 2.4 and 5.2, respectively. In contrast, [3H]inositol incorporation into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was only 20% compared to that into PI. Addition of renal substrates like lactate, glutamine, glycerol and fatty acids increased precursor incorporation. The stimulating effects of gluconeogenic precursors and fatty acids were more than additive. Comparison of rates of [14C]glycerol, [32P]- and [3H]inositol incorporation suggests that de novo phospholipid biosynthesis is represented by these measurements, while the fatty acid fraction in addition turns over by exchange processes. According to our conclusions PE, PC, PI and phosphoinositides turn over with a half-life of 139, 16, 15 and 0.7 h, respectively, under optimal in vitro conditions.

Non-uniform distribution of phospholipids in (Na+ + K+)-ATPase-rich membranes from Torpedo marmorata electric organ evidenced by spin-spin interactions between spin-labeled phospholipids

(Na+ + K+)-ATPase membranes from Torpedo marmorata electric organ were labeled with high concentrations of paramagnetic phospholipid analogs (up to 5 mol of spin labels for 100 mol of endogeneous phospholipids). The mobile lipid bilayer component of the complex resonance spectra obtained showed low-field linewidth broadening due to spin-spin interactions; this was used as a measure of probe concentration in this compartment. Our results show that in the lipid shell surrounding the protein, there is a considerable enrichment in negatively charged phospholipids over neutral ones.

Changes in renal phospholipid fatty acids in diabetes mellitus: correlation with changes in adenylate cyclase activity

Male Sprague-Dawley rats made diabetic with alloxan (37.5 mg/kg) or streptozotocin (65 mg/kg) were killed after 3-6 weeks of disease; renal tissues were studied for phospholipid content and for fatty acid composition of the phospholipids. No consistent change was noted in total phospholipid content nor in the proportion of various phospholipids in diabetics. However, diabetic animals showed a consistent reduction of arachidonic acid content in phosphatidylcholine (PC) and phosphatidylethanolamine in whole renal cortex, plasma membranes purified from renal cortex, and in isolated glomeruli. Associated with the fall in arachidonic acid was a rise in linoleic acid in the samples studied. Insulin therapy returned the fatty acid profiles to normal. These results are similar to patterns observed in other diabetic tissues and suggest that diabetes is associated with generalized changes in cell membranes. That these structural changes may have functional significance is suggested by demonstrated alterations in the temperature-dependence of adenylate cyclase in renal plasma membranes of diabetic animals. Adenylate cyclase is thought to be intimately associated with PC in plasma membranes, a phospholipid showing significant changes in fatty acid content in diabetes (unsaturation index 165 +/- 2 for normals, 147 +/- 5 for diabetics). Na+,K+-ATPase which is thought to be primarily associated in vivo with phosphatidylinositol (PI), shows no change in apparent energy of activation in diabetes. The fatty acid content of PI is minimally altered in diabetes, and the unsaturation index is unchanged.

Sympathetic denervation impairs agonist-stimulated phosphatidylinositol metabolism in rat parotid glands

Substance P, muscarinic and alpha-adrenoceptor agonists stimulated the incorporation of [3H]inositol into phosphatidylinositol in rat parotid gland slices. Surgical denervation of the sympathetic input to the rat parotid gland by superior cervical ganglionectomy produced marked reductions in these responses. The stimulated incorporation of radiolabelled precursors into phosphatidylinositol is a measure of its resynthesis after receptor-mediated breakdown of inositol phospholipids. We therefore examined the enzymic site of the lesion induced by sympathetic denervation using parotid gland slices labelled with either [3H]inositol or [32P]phosphate and stimulated with substance P. Receptor-activated phospholipase C attack upon [3H]inositol phospholipids was assayed by measuring the formation of [3H]inositol 1-phosphate in the presence of 10 mM-Li+ to inhibit further breakdown. It was not affected by denervation. Substance P elicited a rapid breakdown of phosphatidylinositol 4,5-bisphosphate and this response was reduced in the denervated gland. The second step in stimulated phosphatidylinositol turnover, phosphorylation of diacylglycerol to phosphatidate was not affected by denervation. Sympathetic denervation appears to induce a specific enzymic lesion in the parotid gland that impairs receptor-stimulated resynthesis of phosphatidylinositol from phosphatidate. This change in membrane lipid metabolism may be related to a number of the effects of sympathetic denervation, such as agonist supersensitivity, reduced gland cell proliferation and induction of new surface receptors.

Influence of cationic amphiphilic drugs on the characteristics of ouabain-binding to cardiac Na+/K+-ATPase

The influence of 12 cationic amphiphilic compounds on the equilibrium and kinetic characteristics of the binding of tritium-labelled ouabain to the lipoprotein Na+/K+-ATPase present in a crude membrane suspension of guinea pig myocardium was investigated. The drugs, e.g. local anaesthetic, antiarrhythmic and psychotropic agents, inhibited specific binding of ouabain in a concentration-dependent manner by reducing its affinity without affecting the number of binding sites. In the presence of chlorpromazine, propranolol and dibucaine, the decreased affinity of ouabain was due to both a diminished association rate and an increased dissociation rate, while in the presence of the weakly potent procaine only the association rate of ouabain was found to be reduced. The different potency of the catamphiphilic drugs was well correlated to the degree of their hydrophobicity. Evidence is presented that the protonized form of the drugs is the effective one. Concerning the mode of action, the catamphiphilic drugs are proposed to interact with the phospholipid part of the lipoprotein Na+/K+-ATPase, thereby indirectly altering the conformation of the embedded protein moiety and thus reducing the proper fit between ouabain and its receptor.

Hepatic plasma membranes from genetically obese (ob/ob) mice: studies on fluorescence polarization, phospholipid composition and 5'-nucleotidase activity

Hepatic plasma membrane lipids of lean (+/?) and obese (ob/ob) mice have been investigated using 1,6-diphenylhexatriene (DPH). Arrhenius plots of DPH fluorescence polarization in membranes showed the breakpoint in obese mice was reduced from 21 to 15 degrees C, whereas the breakpoint of 5'-nucleotidase activity was raised from 23 to 32 degrees C. Arrhenius break temperatures of DPH polarization and 5'-nucleotidase activity responded differently to housing mice at 34 degrees C and triiodothyronine (T3) treatment. Studies of DPH polarization in liposomes and phospholipid fatty acid composition suggested that differences in sphingomyelin acyl composition determine Arrhenius characteristics of hepatic 5'-nucleotidase in lean and obese mice.

Minireview. Phosphatidylinositol specific phospholipases C

The role of phosphatidylinositol-specific phospholipase C (PIase C) in a) the enigmatic phosphatidylinositol (PI) turnover and b) in our understanding of membrane enzyme-PI interactions is the subject matter of this article. PIase C is present in both procaryotes and eukaryotes. This enzyme is considered to be involved in the cells PI breakdown which occurs in response to several external stimuli. Recent information on the physical properties, Ca2+ requirement, cellular localization and modulation of the activity of PIase C of mammalian systems can help to evaluate the PI turnover from a new angle. Existing evidence suggests that Ca2+-dependent PI breakdown is probably mediated through the cytosolic and particulate PIase C while a Ca2+ independent pathway is catalyzed by a lysosomal enzyme. Apparently PI turnover may be operating through more than one mechanism. The association of this phenomenon with a membrane receptor event linked with "Ca2+ gating" may have to be reconsidered. Modulation of the PIase C activity by unsaturated amphiphiles or the presence of this enzyme in different physico-chemical forms could be a potential regulatory feature. Hydrolysis of membrane PI of a number of cells and tissues by the bacterial PIase C has been shown to cause substantial release of acetylcholinesterase, alkaline phosphatase and 5'-nucleotidase in free, soluble form. Other membrane enzymes, e.g., alkaline phosphodiesterase I, L-leucyl-beta naphthyl amidase and Ca2+ or Mg2+ ATPase are not affected. These results indicate a specific interaction between PI and certain enzymes in membranes. The chemical nature of this linkage, whether it is covalent or non-covalent, has also been explored and has provided intriguing insight into this phenomenon. New findings also indicate that hydrolysis of PI by PIase C also can cause modifications in membrane-enzyme activities, e.g., adenylate cyclase.