Isolation of vesicles containing insulin-responsive, intracellular glucose transporters from 3T3-L1 adipocytes

Insulin stimulation of glucose transport in fat and muscle cells occurs, at least in part, by the translocation of glucose transporters from intracellular membranes to the plasma membrane. In this report, we describe the isolation and partial characterization of vesicles containing translocatable intracellular transporters from 3T3-L1 adipocytes. The glucose transporter content of light microsomes in a 44,000 X g cell supernatant was found to decrease by 50% in response to insulin treatment of the adipocytes. A procedure was developed for the purification of transporter-containing vesicles from this supernatant by immunoadsorption onto Staphylococcus aureus cells coated with anti-transporter antibodies. The vesicles are about 50 nm in diameter and have a distinct polypeptide composition. After insulin treatment the number of transporter-containing vesicles decreased by about 50%, as determined both by microscopic analysis of vesicle number and by the relative abundance of vesicle polypeptides.     

The identity of the current carriers in canine lingual epithelium in vitro

Ion transport across the lingual epithelium has been implicated as an early event in gustatory transduction. The fluxes of isotopically labelled Na+ and Cl- were measured across isolated canine dorsal lingual epithelium under short-circuit conditions. The epithelium actively absorbs Na+ and to a lesser extent actively secretes Cl-. Under symmetrical conditions with Krebs-Henseleit buffer on both sides, (1) Na+ absorption accounts for 46% of the short-circuit current (Isc); (2) there are two transcellular Na+ pathways, one amiloride-sensitive and one amiloride-insensitive; (3) ouabain, added to the serosal solution, inhibits both Isc and active Na+ absorption. When hyperosmotic (0.25 M) NaCl is placed in the mucosal bath, both Isc and Na+ absorption increase; net Na+ absorption is at least as much as Isc. Ion substitution studies indicate that the tissue may transport a variety of larger ions, though not as effectively as Na+ and Cl-. Thus we have shown that the lingual epithelium, like other epithelia of the gastrointestinal tract, actively transports ions. However, it is unusual both in its response to hyperosmotic solutions and in the variety of ions that support a transepithelial short-circuit current. Since sodium ion transport under hyperosmotic conditions has been shown to correlate well with the gustatory neural response, the variety of ions transported may likewise indicate a wider role for transport in taste transduction.     

Molecular Regulation of Renal Phosphate Transport

Received: 18 April 1996/Revised: 26 June 1996     

Anonymous nuclear DNA markers in the American oyster and their implications for the heterozygote deficiency phenomenon in marine bivalves

A puzzling population-genetic phenomenon widely reported in allozyme surveys of marine bivalves is the occurrence of heterozygote deficits relative to Hardy-Weinberg expectations. Possible explanations for this pattern are categorized with respect to whether the effects should be confined to protein-level assays or are genomically pervasive and expected to be registered in both protein- and DNA-level assays. Anonymous nuclear DNA markers from the American oyster were employed to reexamine the phenomenon. In assays based on the polymerase chain reaction (PCR), two DNA-level processes were encountered that can lead to artifactual genotypic scorings: (a) differential amplification of alleles at a target locus and (b) amplification from multiple paralogous loci. We describe symptoms of these complications and prescribe methods that should generally help to ameliorate them. When artifactual scorings at two anonymous DNA loci in the American oyster were corrected, Hardy-Weinberg deviations registered in preliminary population assays decreased to nonsignificant values. Implications of these findings for the heterozygote-deficit phenomenon in marine bivalves, and for the general development and use of PCR-based assays, are discussed.      

Identification of a cDNA/Protein Leading to an Increased P i -uptake in Xenopus laevis Oocytes

In a previous report we documented an increased Na⁺-dependent transport of inorganic phosphate (P _i ) in Xenopus laevis oocytes injected with mRNA isolated from rabbit duodenum (Yagci et al., Pfluegers Arch. 422:211–216, 1992; ref 24). In the present study we have used expression cloning in oocytes to search for the cDNA/mRNA involved in this effect. The identified cDNA (provisionally named PiUS; for P _i -uptake stimulator) lead to a 3-4-fold stimulation of Na⁺-dependent P _i -uptake (10ng cRNA injected, 3–5 days of expression). Na⁺-independent uptake of P _i was also affected but transport of sulphate and l-arginine (in the presence or absence of sodium) remained unchanged. The apparent K _m -values for the induced Na⁺-dependent uptake were 0.26 ± 0.04 mm for P _i and 14.8 ± 3.0 mm for Na⁺. The 1796 bp cDNA codes for a protein of 425 amino acids. Hydropathy analysis suggests a lack of transmembrane segments. In vitro translation resulted in a protein of 60 kDa and provided no evidence of glycosylation. In Northern blots a mRNA of ∼2 kb was recognized in various tissues including different intestinal segments, kidney cortex, kidney medulla, liver and heart. Homology searches showed no similarity to proteins involved in membrane transport and its control. In conclusion, we have cloned from a rabbit small intestinal cDNA library a novel cDNA encoding a protein stimulating P _i -uptake into Xenopus laevis oocytes, but which is not a P _i -transporter itself. Received: 31 July 1996/Revised: 16 October 1996     

Basolateral plasma membrane localization of ouabain-sensitive sodium transport sites in the secretory epithelium of the avian salt gland

The distribution of Na+ pump sites (Na+-K+-ATPase) in the secretory epithelium of the avian salt gland was demonstrated by freeze-dry autoradiographic analysis of [(3)H] ouabain binding sites. Kinetic studies indicated that near saturation of tissue binding sites occurred when slices of salt glands from salt-stressed ducks were exposed to 2.2 μM ouabain (containing 5 μCi/ml [(3)H]ouabain) for 90 min. Washing with label-free Ringer's solution for 90 min extracted only 10% of the inhibitor, an amount which corresponded to ouabain present in the tissue spaces labeled by [(14)C]insulin. Increasing the KCl concentration of the incubation medium reduced the rate of ouabain binding but not the maximal amount bound. In contrast to the low level of ouabain binding to salt glands of ducks maintained on a freshwater regimen, exposure to a salt water diet led to a more than threefold increase in binding within 9-11 days. This increase paralleled the similar increment in Na+-K+-ATPase activity described previously. [(3)H]ouabain binding sites were localized autoradiographically to the folded basolateral plasma membrane of the principal secretory cells. The luminal surfaces of these cells were unlabeled. Mitotically active peripheral cells were also unlabeled. The cell-specific pattern of [(3)H]ouabain binding to principal secretory cells and the membrane-specific localization of binding sites to the nonluminal surfaces of these cells were identical to the distribution of Na+-K+-ATPase as reflected by the cytochemical localization of ouabain-sensitive and K+-dependent nitrophenyl phosphatase activity. The relationship between the nonluminal localization of Na+-K+-ATPase and the possible role of the enzyme n NaCl secretion is considered in the light of physiological data on electrolyte transport in salt glands and other secretory epithelia.     

Creatine phosphokinase activity in dysgenic (mdg/mdg) mouse muscle

The specific activity of creatine phosphokinase (CPK) was measured in the muscle of mdg/mdg and control embryos of 14-18 days' gestation. CPK specific activity values were similar in mutant and normal embryos at the earliest stages examined (14-15 days). However, after 15 1/2 days, the specific activity of the enzyme in the mdg/mdg embryos was approximately 50% lower than in the controls. The dysgenic and normal muscle extracts exhibited comparable stability after storage at -85 C. CPK activity levels in the muscle of adult heterozygotes (+/mdg) and wild-type (+/+) controls were found to be statistically identical. The findings suggest that the mdg mutation does not have a primary or direct effect on CPK activity.     

Ultracentrifugation in polyethylene tubing: An efficient method to concentrate protein solutions on the microliter scale

An ultracentrifugation procedure is described to concentrate protein solutions on the microliter scale. Protein solutions were centrifuged in U-shaped lengths of polyethylene tubing at 160 000 × g for 15 h and thereafter fractionated by cutting the tubing. The method can be performed in a conventional ultracentrifuge and needs no special equipment.     

The surface membrane of the small intestinal epithelial cell. I. Localization of adenyl cyclase.

The subcellular distribution of adenyl cyclase was investigated in small intestinal epithelial cells. Enterocytes were isolated, disrupted and the resulting membranes fractionated by differential and sucrose gradient centrifugation. Separation of luminal (brush border) and contra-luminal (basolateral) plasma membrane was achieved on a discontinuous sucrose gradient. The activity of adenyl cyclase was followed during fractionation in relation to other enzymes, notably those considered as markers for luminal and contraluminal plasma membrane. The luminal membrane was identified by the membrane-bound enzymes sucrase and alkaline phosphatase and the basolateral region by (Na+ + K+)-ATPase. Enrichment of the former two enzymes in purified luminal plasma membrane was 8-fold over cells and that of (Na+ + K+)-ATPase in purified bisolateral plasma membranes was 13-fold. F--activated adenyl cyclase co-purified with (Na+ + K+)-ATPase, suggesting a common localization on the plasma membrane. The distribution of K+-stimulated phosphatase and 5'-nucleotidase also followed (Na+ + K+)-ATPase during fractionation.