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Biomedical subjects

B Sacktor

Publications and source records attributed to B Sacktor.

At least 91 records · Page 5Linked to original sources
PubMed1981

Sodium gradient-dependent phosphate transport in renal brush border membrane vesicles. Effect of an intravesicular greater than extravesicular proton gradient.

A H+ gradient (intravesicular greater than extravesicular), in the absence of a Na+ gradient (extravesicular greater than intravesicular) stimulated phosphate uptake by renal brush border membrane vesicles and provided the driving force to effect the transient accumulation of phosphate against its concentration gradient. The H+ gradient-dependent uptake of phosphate had an absolute requirement for Na+. The rates of uptake and peak accumulation were functions of the delta pH and the concentration of H+ in the intravesicular medium. The H+ gradient-energized Na+-phosphate cotransport system was not affected by valinomycin- or carbonyl cyanide p-fluoromethoxyphenylhydrazone-induced ion diffusion potentials. Therefore, it was independent of the membrane potential, i.e. an electroneutral process. Amiloride, which inhibited the H+-Na+ exchange reaction and prevented the efflux of H+ from the intravesicular medium, enhanced the uptake of phosphate. A model is proposed by which the H+ gradient mediates the uphill transport of phosphate. It is suggested that a similar process may operate in more physiologically intact preparations and may provide one mechanism by which acid-base balance regulates renal phosphate transport.

Animals↗
PubMed1981

Sodium gradient- and sodium plus potassium gradient-dependent L-glutamate uptake in renal basolateral membrane vesicles.

A membrane preparation enriched in the basolateral segment of the plasma membrane was isolated from the rat renal cortex by a procedure that included separation of particulates on a self-generating Percoll gradient. The uptake of L-glutamate by the basolateral membrane vesicles was studied. A Na+ gradient (Na+]o greater than [Na+]i) stimulated the uptake of L-glutamate and provided the driving force for the uphill transport of the acidic amino acid, suggesting a Na+-L-glutamate cotransport system in the basolateral membrane. A K+ gradient ([K+]i greater than [K+]o) increased the uptake additionally. This effect was specific for K+(Rb+). The action of the K+ gradient in enhancing the uptake of L-glutamate had an absolute requirement for Na+. In the presence of Na+, but in the absence of a Na+ gradient. i.e., [Na+]o = [Na+]i, the K+ gradient also energized the concentrative uptake of L-glutamate. This effect of the K+ gradient was not attributable to an alteration in membrane potential. The finding of a concentrative uptake system for L-glutamate energized by both Na+ ([Na+]o greater than [Na+]i and K+ ([K+]o) gradients in the basolateral membrane, combined with previous reports of an ion gradient-dependent uphill transport system for this amino acid in the brush border membrane, suggests a mechanism by which L-glutamate is accumulated intracellularly in the renal proximal tubule to extraordinarily high concentrations.

Animals↗
PubMed1981

Ca2+-dependent and Ca2+-independent degradation of phosphatidylinositol in rabbit vas deferens.

The effects of Ca2+ and acetylcholine on the degradation and synthesis of phosphatidylinositol in rabbit vas deferens was studied in vitro by a pulse--chase technique and by measuring the content of the phospholipid in the tissue. Ca2+-dependent degradation of phosphatidylinositol was found in slices and homogenates prelabelled with myo-[2-3H]inositol. The phosphatidylinositol content of the slices also decreased by a Ca2+-dependent mechanism. On the other hand, removal of intracellular Ca2+ with the ionophore A23187 and EGTA increased the amount of phosphatidylinositol. These results indicate that the intracellular Ca2+ concentration has an important role in regulating the phosphatidylinositol content of the tissue. Increasing the extracellular K+ concentration, which causes an increase in plasma-membrane Ca2+ permeability, did not enhance phosphatidylinositol breakdown nor decrease its tissue content. However, phosphatidylinositol synthesis was clearly inhibited. After stimulation of the smooth muscle with acetylcholine, degradation of phosphatidylinositol was enhanced. Furthermore, the content of phosphatidylinositol in the tissue also decreased. These phenomena were evident even in the absence of Ca2+. The acetylcholine-induced degradation of phosphatidylinositol was blocked by the muscarinic antagonist atropine, but not by the nicotinic antagonist (+)-tubocurarine. The acetylcholine-induced decrease in the phosphatidylinositol content of the tissue led to the compensatory synthesis of phosphatidylinositol. Synthesis was separated from degradation in the same tissue. Compensatory synthesis was inhibited by acetylcholine. The degradation of phosphatidylinositol induced by acetylcholine was not inhibited by 8-bromoguanosine 3':5'-cyclic monophosphate, indicating that the degradative process was not mediated by an increase in the cyclic nucleotide.

Acetylcholine↗
PubMed1981

Stimulation of the efflux of L-glutamate from renal brush-border membrane vesicles by extravesicular potassium.

The rate of efflux of L-glutamate from renal brush-border membrane vesicles was enhanced by Na+ and by extravesicular L-glutamate, but not by D-glutamate nor analogs of L-glutamate that do not share the Na+-L-glutamate co-transport system. These results suggest that efflux was mediated by the Na+-L-glutamate carrier. The efflux of L-glutamate was increased by extravesicular K+ or Rb+ but not by Li+, choline+, or Tris+. These findings, together with previous results showing that intravesicular K+ or Rb+ increased L-glutamate uptake and that a K+ gradient energized the concentrative uptake of the acidic amino acid in the absence of other gradients, provide evidence consistent with the hypothesis that the co-transport of Na+-L-glutamate is coupled to the transmembrane flux of K+.

Animals↗
PubMed1981

Regulation of hormone(PTH and PGE1)-stimulated adenylate cyclase by renal cytosolic factors.

Cytosolic factors in a 50--75% (NH4)2SO4 fraction of the 105 000 x g supernatant of the renal cortex modulated adenylate cyclase activity in membrane preparations enriched in renal tubular cell basal--lateral membranes. The crude factor preparation had no effect on basal activity but it contained components that augmented the stimulated of the enzyme by NaF, parathyroid hormone (PTH), prostaglandin E1 (PGE1), and inhibited the activation of the enzyme by GMP--PNP. The factor(s) potentiating the stimulation by the hormones was partially purified (13-fold) by DEAE-cellulose and Sephadex G-75 chromatography. During purification, the component(s) that increased hormone-stimulated adenylate cyclase was separated from those affecting the activity in the presence of NaF and GMP--PNP. The factor(s) enhanced the PTH- and PGE1-stimulated enzyme at all concentrations of hormone, suggesting that the affinity for the hormone was not affected. The factor(s) was heat-stable. Partial proteolysis with chymotrypsin greatly reduced the ability of the factor(s) to enhance hormonal responsive adenylate cyclase. However, the factor(s) was resistant to trypsin digestion. The effect of the factor was not due to GTP, nor was GTP necessary for its action. Ca2+ was not needed for the enhancing activity of the factor(s). These findings suggest the presence in the cytosol of the kidney cortex of a protein(s) that regulates the response of renal adenylate cyclase to hormones. The relationship between this kidney cytosolic factor and those reported in other tissues remains to be established.

Adenylyl Cyclases↗
PubMed1981

Inhibition by Ca2+ of the incorporation of myo-inositol into phosphatidylinositol.

The incorporation of myo-[2-3H]inositol into phosphatidylinositol of the aorta and the vas deferens was measured and the effects of Ca2+ and other divalent cations were determined. When incubated in normal Krebs-Ringer buffer, only negligible radioactivity was incorporated into aorta slices. Mn2+ increased the incorporation greatly. The enhanced incorporation was attributable to an increase in CDP-diglyceride:inositol transferase activity, rather than the myo-inositol exchange reaction. Transferase activity was increased 20-fold by 1 mM Mn2+, in the presence of 20 mM Mg2+. The Mn2+-stimulated activity was strongly inhibited by Ca2+. In the absence of Mn2+, but presence of 20 mM Mg2+, transferase activity was inhibited 80% by 0.01 mM Ca2+. Removal of endogenous Ca2+ from the tissue by ionophore A23187 and EGTA increased the incorporation of myo-[2-3H]inositol into phosphatidylinositol. These findings indicate that Ca2+ inhibited the synthesis of phosphatidylinositol. The proposed action of cholinergic and alpha-adrenergic agonists in enhancing the degradation and turnover of phosphatidyl-inositol and in provoking the influx of Ca2+ should be unfavorable to the recovery of cellular phosphatidylinositol content.

Animals↗
PubMed1981

In vitro effects of parathyroid hormone on kidney cortical slices: cAMP responses and concomitant inhibition of the Na+ gradient-dependent uptake of phosphate by brush border membrane vesicles isolated from the renal slices.

Mouse renal cortical slices were incubated with parathyroid hormone (30 U/ml) for 2 min. Brush border membrane vesicles isolated from the treated slices had a decreased Na+ gradient-dependent uptake of phosphate. Concomitantly, the hormone elicited the activation of adenylate cyclase, the increase in tissue level of cAMP, and the enhancement of cAMP-dependent protein kinase.

Adenylyl Cyclases↗
PubMed1980

myo-Inositol transport in renal brush border vesicles and it inhibition by D-glucose.

We examined the mechanism of myo-inositol uptake by rabbit renal proximal tubule brush border membrane vesicles and characterized the relationship between the transports of myo-inositol and D-glucose. A 100 mM Na+ electrochemical gradient (extravesicular medium > intravesicular medium) stimulated the initial rate of myo-inositol uptake 20- to 60-fold. Other cation gradients were ineffective. The Na+ myo-inositol co-transport system was shown to be electrogenic. The Na+ electrochemical gradient-dependent uptake of myo-inositol saturated at about 1 mM myo-inositol, with an apparent Km of 94 micro M at an initial 100 mM Na+ gradient. D-Glucose was an inhibitor of the Na+ gradient-dependent uptake of myo-inositol. D-Glucose, but not L-glucose, elicited accelerative exchange diffusion of myo-inositol. myo-Inositol did not significantly inhibit the Na+ gradient-dependent transport of D-glucose. We suggest that D-glucose inhibits myo-inositol uptake by dissipating the membrane potential and sharing the myo-inositol carrier. The inhibition of myo-inositol transport across the brush border membrane by D-glucose explains how glycosuria could produce inosituria in patients with diabetes mellitus.

Animals↗
PubMed1980

Gentamicin uptake by renal tubule brush border membrane vesicles.

The uptake of [3H]gentamicin by isolated rabbit renal proximal tubule brush border membrane vesicles was studied. Uptake was biphasic, with an initial rapid uptake followed by a prolonged slower phase. Approximately half of the total uptake represented binding; the other half represented transport into an intravesicular space. Scatchard analysis indicated the presence of two binding sites, differing in affinity (8 X 10(3) and 0.9 X 10(3) M-1) and number of sites per milligram of protein (1.2 and 3.7 nmol/mg of membrane protein, respectively). [3H]Gentamicin uptake was not affected by a Na+ electrochemical gradient, a valinomycin-generated (inside negative) K+ diffusion potential or the presence of phlorizin and D- of L-glucose. These findings indicate that the mechanism of uptake of the aminoglycoside was distinct from those of sugar and amino acids. Uptake of [3H]gentamicin was inhibited and reversed by the unlabeled aminoglycoside and by spermine. Spermine, on a molar basis, was as effective as gentamicin. These results suggest that gentamicin and spermine may have a common polyamine transport system and demonstrate the feasibility of further investigations to prevent aminoglycoside accumulation and possibly subsequent nephrotoxicity.

Animals↗