Similarities between sodium channels in excitable membranes and in epithelia

Summary The inhibitory effects of the pyrazine derivative, amiloride, on sodium transport in an amphibian epithelium has been studied as a function of pH. It is concluded that the charged (guanidinium) group interacts with a negatively charged acid grouping in the membrane. Similarities between sodium channels in excitable membranes and epithelia are highlighted.     

Role of lysine side chains in metallothionein

Summary pK_a-Values of lysine residues of mammalian metallothionein were determined by chemical titration measurements of -CH₂ lysine resonances in the¹H-NMR spectra. They are about 0.5 pH-unit higher than the average pK_a-value of a metal-free derivative, suggesting interaction of the positively charged residues with the two three-fold negatively charged metal-thiolate clusters of the metal-containing form. Deprotonation of the lysines leads to circular dichroism changes attributable to an electrostatically induced structural transition of the protein.To whom correspondence concerning this paper should be addressed.     

Triple arginines in the cytoplasmic tail of endomannosidase are not essential for type II membrane topology and Golgi localization

Endomannosidase is a Golgi-localized endoglycosidase, which provides an alternate glucosidase-independent pathway of glucose trimming. Using a protease protection assay we demonstrated that Golgi-endomannosidase is a type II membrane protein. The first 25 amino acids of this protein, containing the cytoplasmic tail and the transmembrane domain, were sufficient for Golgi retention of fused reporter proteins alpha1-antitrypsin or green fluorescent protein. However, shortening or deletion of the transmembrane domain prevented Golgi localization, while lengthening it partially reduced Golgi retention of the enzyme. Substitution of the highly conserved positively charged amino acids within the cytoplasmic tail had neither an effect on type II topology nor on the inherent Golgi localization of the enzyme. In contrast, cytoplasmic tail-deleted rat endomannosidase possessed an inverted topology resulting in endoplasmic reticulum mislocalization. Thus, proper topology rather than the presence of positively charged amino acids in the cytoplasmic tail is critical for Golgi localization of rat endomannosidase.     

Interaction between bradykinin and voltage-sensitive sodium channels in myelinated nerve fibers

The effects of externally and internally applied bradykinin on the excitability of single myelinated nerve fibers were studied. External bradykinin (10 microM) slightly prolongs the action potential of a single myelinated nerve fiber; hence, when the fibers are stimulated by long-lasting pulses, this raises the frequency of repetitive firing in sensory fibers and evokes repetitive activity in motor fibers. Under voltage-clamp conditions, sodium channel inactivation is slowed, while sodium channel activation remains unaffected. Prolonged depolarization of the membrane leads to a maintained sodium current. The voltage dependence of the steady-state sodium current inactivation (h infinity) is shifted in the depolarized direction by approximately 10 mV. Internally applied bradykinin produces a frequency-dependent block of the sodium current. The phenomena described here imply that more than one site on the sodium channel is modified by bradykinin.     

Peptide YY enhances NaCl and water absorption in the rat colon in vivo

Peptide YY (PYY) is thought to possess paracrine and endocrine functions. The highest concentrations of this peptide are in the colonic mucosa. The effect of PYY on electrolyte and water transport in the rat colon was studied in vivo. Under urethane anesthesia, rat colonic loops were perfused at a constant rate with physiological buffer solution containing phenol red as a nonabsorbable volume marker, and net movements of water, sodium, chloride and potassium in the perfused colon were determined every 10 min. Intravenous administration of PYY produced a dose-dependent increase in the net absorption of sodium chloride and water, as well as a decrease in the net secretion of potassium. PYY inhibited the reduction in net absorption of sodium chloride and water evoked by vasoactive intestinal peptide (VIP), but did not affect the VIP-evoked increase in net potassium secretion.These findings suggest that PYY acts as an enhancer of sodium chloride and water absorption and as an antagonist to VIP-induced secretion in the colon.     

Peptide YY enhances NaCl and water absorption in the rat colon in vivo.

Peptide YY (PYY) is thought to possess paracrine and endocrine functions. The highest concentrations of this peptide are in the colonic mucosa. The effect of PYY on electrolyte and water transport in the rat colon was studied in vivo. Under urethane anesthesia, rat colonic loops were perfused at a constant rate with physiological buffer solution containing phenol red as a nonabsorbable volume marker, and net movements of water, sodium, chloride and potassium in the perfused colon were determined every 10 min. Intravenous administration of PYY produced a dose-dependent increase in the net absorption of sodium chloride and water, as well as a decrease in the net secretion of potassium. PYY inhibited the reduction in net absorption of sodium chloride and water evoked by vasoactive intestinal peptide (VIP), but did not affect the VIP-evoked increase in net potassium secretion. These findings suggest that PYY acts as an enhancer of sodium chloride and water absorption and as an antagonist to VIP-induced secretion in the colon.     

Role of extracellular matrix molecules in the development of the sodium current in quail mesencephalic neural crest cells

The occurrence of the voltage-dependent sodium current has been studied in developing neurons from quail mesencephalic neural crest on different substrates, using the whole-cell patch clamp technique. Explants from 9–12 somite embryos were cultured on dishes coated with type I collagen, fibronectin, laminin or on plastic dishes in a chemically defined medium. After 18 h of culture the sodium current was observed in 70% of the neurons tested, and at 24 h some of these neurons were able to generate an action potential. After 18–25 h cells grown on fibronectinor collagen I-coated dishes showed a significantly higher occurrence of the sodium current (83% and 84% respectively) as compared to cells grown on uncoated plastic dishes (51%). Moreover, in the presence of fibronectin, the current density of the sodium current was more than doubled in comparison with cells grown on other substrates.     

Sodium pumps and galactose transfer in the short-circuited small intestine

Summary In a short-circuited preparation of rat jejunum, there are two sodium pumps, one electrogenic and the other neutral. When energy sources are limited, the total sodium transfer is limited. In the presence of a non-metabolized actively transferred hexose, the electrogenic pump is preferentially used. The neutral sodium pump is only able to function when additional energy is available.     

Sodium pumps and galactose transfer in the short-circuited small intestine.

In a short-circuited preparation of rat jejunum, there are two sodium pumps, one electrogenic and the other neutral. When energy sources are limited, the total sodium transfer is limited. In the presence of a non-metabolized actively transferred hexose, the electrogenic pump is preferentially used. The neutral sodium pump is only able to function when additional energy is available.     

Interaction between bradykinin and voltage-sensitive sodium channels in myelinated nerve fibers

Summary The effects of externally and internally applied bradykinin on the excitability of single myelinated nerve fibers were studied. External bradykinin (10 M) slightly prolongs the action potential of a single myelinated nerve fiber; hence, when the fibers are stimulated by long-lasting pulses, this raises the frequency of repetitive firing in sensory fibers and evokes repetitive activity in motor fibers. Under voltage-clamp conditions, sodium channel inactivation is slowed, while sodium channel activation remains unaffected. Prolonged depolarization of th membrane leads to a maintained sodium current. The voltage dependence of the steady-state sodium current inactivation (h ) is shifted in the depolarized direction by 10 mV. Internally applied bradykinin produces a frequency-dependent block of the sodium current. The phenomena described here imply that more than one site on the sodium channel is modified by bradykinin.     

Fluorescence response of acridine orange to changes in pH gradients across liposome membranes

The influence that changes in proton distribution have on the fluorescence of acridine orange was examined using negatively charged liposomes. Our results indicate that at least two mechanisms are involved: distribution of the probe between the internal aqueous phase of the liposomes and the outside medium, and binding of the probe to the liposome membranes.     

The effects of two neutral polymers on the geometry and deformability of the human erythrocyte.

Polyvinylpyrrolidone and dextran decrease cellular deformability. Changes in volume do not wholly account for the changes which imply a stiffening of the plasma membrane. The effects differ from those induced by charged macromolecules.     

Fluorescence response of acridine orange to changes in pH gradients across liposome membranes

Summary The influence that changes in proton distribution have on the fluorescence of acridine orange was examined using negatively charged liposomes. Our results indicate that at least two mechanisms are involved: distribution of the probe between the internal aqueous phase of the liposomes and the outside medium, and binding of the probe to the liposome membranes.     

硼氢化钠作为能量载体的新型能量转换技术(英文)

硼氢化钠作为能量载体的新型能量转换技术,它由直接硼氢化钠燃料电池、硼氯化钠水解制氢和硼氢化钠再生技术构成。通过总结了近年来在直接硼氢化钠燃料电池、硼氢化钠水解制氢和硼氢化钠再生技术上的进展,发现硼氢化钠作为能量载体的新型能量转换技术的雏形已日渐形成,本文总结和讨论了该项技术的优点和面临的挑战。     

Renal effects of the inhibitor of thromboxane A2-synthetase OKY-046

Acute renal failure (ARF) was associated with increased urinary thromboxane (TXA2) excretion and lessened excretion of sodium (UNaV) and fractional excretion of sodium (FENa%). The inhibitor of thromboxane A2-synthetase OKY-046 enhanced sodium excretion and fractional excretion of sodium in normal and saline loaded animals whereas it partially prevented the reduction in sodium excretion and creatinine clearance and significantly increased fractional excretion of sodium in glycerol treated rats suggesting a partial protection against the development of acute renal failure.     

Variations in cardiac noradrenaline content during sodium loading in hypertension prone and resistant rats

Summary The cardiac catecholamine content of Sabra rats and their 2 genetically derived substrains, hypertension prone and resistant rats, was studied by high pressure liquid chromatography and electrochemical detection. Both in the control period and after sodium and DOCA administration the cardiac noradrenaline level is higher in hypertension resistant rats than in Sabra rats, and also higher than in hypertension prone rats. This finding suggests that a reduction of the cardiac sympathetic nervous tone is involved in the genetic resistance to sodium.     

Can blocking the Na/K exchange pump lead to a reduction in intracellular sodium?

It has been assumed that a rise in intracellular sodium should follow inhibition of the Na/K exchange pump. However, under certain conditions a reduction in intracellular sodium following pump blockage is possible. Many results postulating 'stimulation' of the Na/K exchange pump by low doses of the cardiac glycosides can be explained in this manner.     

Decreased cyclic GMP levels in rat cecum mucosa during adaptive stimulation of Na-K-ATPase.

In rat cecal mucosa, Na-K-ATPase specific activity and sodium and fluid absorption were increased by giving polyethylene glycol administration with the drinking water. Whereas cyclic AMP levels were unchanged, cyclic GMP was reduced by about 50%. This finding suggests a regulatory role of cyclic GMP in intestinal sodium and fluid absorption.     

Regulation of the cardiac sodium pump

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.     

Molecular biology of water and salt regulation in the kidney

The kidney plays a central role in the regulation of the salt and water balance, which depends upon an array of solute and water transporters in the renal tubules and upon vascular elements in the various regions of the kidney. Many recent studies have improved our understanding of this process. In this review, we summarize the current data on the molecules involved in sodium and water transport in the renal tubules, focusing in particular on aquaporins and renal sodium transporters and channels.