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S Hilden

Publications and source records attributed to S Hilden.

8 recordsLinked to original sources
PubMed1982

Potential-dependent D-glucose uptake by renal brush border membrane vesicles in the absence of sodium.

The uptake of D-glucose by renal brush border membrane vesicles was studied in the absence of Na+. Uptake of the sugar was membrane potential dependent (inside negative), inhibited by phlorizin, sugar and stereospecific, accelerated by exchange diffusion, saturable, and temperature dependent. The binding of phlorizin in the absence of Na+ was also increased by a membrane potential (inside negative). Thus, the properties of this membrane potential-dependent, Na+-independent sugar transport system resembled those described for the Na+-D-glucose cotransport system. In the absence of Na+ but in the presence of a valinomycin-induced K+ diffusion potential the apparent Km for D-glucose was 43 mM. This contrasted with an apparent Km of 1.8 mM for the Na+ chemical gradient system. Therefore, the Na+-independent uptake system represented a low-affinity transport mechanism. It is suggested that the same carrier mediated the Na+-independent and Na+-dependent transport systems. A hypothetical model for the membrane potential-dependent stimulation of D-glucose uptake in the absence of Na+ is proposed.

Animals↗
PubMed1975

Active potassium transport coupled to active sodium transport in vesicles reconstituted from purified sodium and potassium ion-activated adenosine triphosphatase from the rectal gland of Squalus acanthias.

Vesicles containing a purified shark rectal gland (sodium + potassium)-activated adenosine triphosphatase-(NaK ATPase) were prepared by dialyzing for 2 days egg lecithin, cholate, and the NaK ATPase purified from the rectal gland of Squalus acanthias. These vesicles were capable of both Na+ and K+ transport. Studies of K+ transport were made by measuring the ATP-stimulated transport outward of 42K+ or 86Rb+. Vesicles were preloaded with isotope by equilibration at 4 degrees for 1 to 3 days. Transport of 42K+ or 86Rb+ was initiated by addition of MgATP to the vesicles. The ATP-dependent exit of either isotope was the same. Experiments are presented which show that this loss of isotope was not due to changes in ion binding but rather due to a loss in the amount of ion trapped in the vesicular volume. The transport of K+ was dependent on external Mg2+. CTP was almost as effective as ATP in stimulating K+ transport, while UTP was relatively ineffective. These effects of nucleotides parallel their effects on Na+ accumulation and their effectiveness as substrates for the enzyme. Potassium transport was inhibited by ouabain and required the presence of Na+. The following asymmetries were seen: (a) addition of external Mg2+ supported K+ transport; (b) ouabain inhibited K+ transport only if it was present inside the vesicles; (c) addition of external Na+ to the vesicles stimulated K+ transport. External Li+ was ineffective as a Na+ substitute. The specific requirement of external Na+ for K+ transport indicates that K+ exit is coupled to Na+ entry. Changes in the internal vesicular ion concentrations were studied with vesicles prepared in 20 mM NaCl and 50 mM KCl. After 1 hour of transport at 25 degrees, a typical Na+ concentration in the vesicles in the presence of ATP was 72 mM. A typical K+ concentration in the vesicles was 10 mM as measured with 42K+ or 6 mM as measured with 86Rb+. The following relationships have been calculated for Na+ transport, K+ transport and ATP hydrolysis: Na+/ATP = 1.42, K+/ATP =1.04, and Na+/K+ = 1.43. The ratio of 2.8 Na+ transported in to 2 K+ transported out is very close to the value reported for the red cell membrane. Potassium-potassium exchange similar to that observed in the red cell membrane and attributed to the Na+-K+ pump (stimulated by ATP and orthophosphate and inhibited by ouabain) was observed when vesicles were prepared in the absence of Na+. The results reported in this paper prove that the shark rectal gland NaK ATPase, which is 90 to 95% pure, is the isolated pump for the coupled transports of Na+ and K+.

Adenosine Triphosphatases↗