The Na(+)-K(+)-ATPase alpha2-subunit isoform modulates contractility in the perinatal mouse diaphragm

This study uses genetically altered mice to examine the contribution of the Na⁺-K⁺-ATPase ₂ catalytic subunit to resting potential, excitability, and contractility of the perinatal diaphragm. The ₂ protein is reduced by 38% in ₂-heterozygous and absent in ₂-knockout mice, and ₁-isoform is upregulated 1.9-fold in ₂-knockout. Resting potentials are depolarized by 0.8–4.0 mV in heterozygous and knockout mice. Action potential threshold, overshoot, and duration are normal. Spontaneous firing, a developmental function, is impaired in knockout diaphragm, but this does not compromise its ability to fire evoked action potential trains, the dominant mode of activation near birth. Maximum tetanic force, rate of activation, force-frequency and force-voltage relationships, and onset and magnitude of fatigue are not changed. The major phenotypic consequence of reduced ₂ content is that relaxation from contraction is 1.7-fold faster. This finding reveals a distinct cellular role of the ₂-isoform at a step after membrane excitation, which cannot be restored simply by increasing ₁ content. Na⁺/Ca²⁺ exchanger expression decreases in parallel with ₂-isoform, suggesting that Ca²⁺ extrusion is affected by the altered ₂ genotype. There are no major compensatory changes in expression of sarcoplasmic reticulum Ca²⁺-ATPase, phospholamban, or plasma membrane Ca²⁺-ATPase. These results demonstrate that the Na⁺-K⁺-ATPase ₁-isoform alone is able to maintain equilibrium K⁺ and Na⁺ gradients and to substitute for ₂-isoform in most cellular functions related to excitability and force. They further indicate that the ₂-isoform contributes significantly less at rest than expected from its proportional content but can modulate contractility during muscle contraction. Na⁺-K⁺-ATPase ₂ catalytic subunit; heterozygous mice; knockout mice; resting potential     

Complex mitogenic requirements of Na(+)-K(+)-Cl(-)-cotransport-deficient BALB/c-3T3 cells

The ability of 12-O-tetradecanoylphorbol-13-acetate (TPA) to stimulate mitogenesis in BALB/c-3T3 cells and in a Na+K+Cl(-)-cotransport-defective variant subclone was investigated. This transport variant had previously been reported to be TPA mitogenically nonresponsive (O'Brien and Prettyman: Journal of Cellular Physiology 130:377-381, 1987) since the addition of TPA to the spent medium of density-arrested cultures stimulated DNA synthesis in the parent but not the variant cell line. We now report that the addition of TPA plus insulin, either directly to the spent medium or together with fresh medium, stimulated DNA synthesis in both the parent and variant cells to approximately the same extent. The parent and transport-deficient cells differed, however, in their sensitivity to the co-mitogenic effects of insulin or insulin-like growth factors.     

Vascular smooth muscle expresses a truncated Na+, K(+)-ATPase alpha-1 subunit isoform.

By regulating transmembrane Na+ and K+ concentrations and membrane potential, the Na+,K(+)-ATPase plays an important role in regulating cardiac, skeletal, and smooth muscle function. A high degree of amino acid sequence and structural identity characterizes the three Mr 100,000 Na+,K(+)-ATPase alpha subunit isoforms expressed in cardiac and skeletal muscle. Strikingly, vascular smooth muscle utilizes alternative RNA processing of the alpha-1 gene to express a structurally distinct Mr approximately 65,000 isoform, alpha 1-T (truncated). Analysis of both its mRNA and protein structure reveals that alpha-1-T represents a major, evolutionarily conserved, truncated Na+,K(+)-ATPase isoform expressed in vascular smooth muscle. This demonstrates an unexpected complexity in the regulation of vascular smooth muscle Na+,K(+)-ATPase gene expression and suggests that a structurally novel, truncated alpha subunit may play a role in vascular smooth muscle active ion transport.     

Soy pinitol acts partly as an insulin sensitizer or insulin mediator in 3T3-L1 preadipocytes

The blood glucose-lowering property of pinitol is mediated via the insulin signaling pathway. This study was carried out to evaluate the effects of soy pinitol on adipogenesis in a 3T3-L1 cell line; 3T3-L1 preadipocytes were treated with pinitol (0-1 mM) together with insulin for 9 days. The regulation of lipid metabolism was assessed by oil-red-O staining of intracellular lipids and real-time PCR of adipogenesis-related factors. The inhibition of cell proliferation was estimated by MTT assay. Pinitol treatment did not inhibit lipid accumulation, nor did it affect expression of adipogenesis-related factors, including ADD1, aP2 and FAS, in a dose-dependent manner. Expression of adiponectin, GLUT4, IRS, C/EBPalpha and PPARgamma mRNAs, however, increased in cells treated with 0.5 mM and/or 1 mM pinitol. Pinitol treatment did not affect the inhibition of cell growth and proliferation in a dose-dependent manner. Accordingly, we suggest that pinitol is nontoxic to this cell line, and that it enhances adipogenesis by acting as an insulin sensitizer or insulin mediator via the upregulation of adiponectin, GLUT4, IRS, C/EBPalpha and PPARgamma in 3T3-L1 preadipocytes.     

Organelle relationships in cultured 3T3-L1 preadipocytes

In differentiating 3T3-L1 cells, lipid spheres, the endoplasmic reticulum (ER), microperoxisomes, and mitochondria form "constellations" that may reflect the interplay of lipid metabolizing enzymes in these organelles. ER cisternae are also situated very close to "rosettes,"plasmalemmal specializations found in mature adipocytes in vivo. As in hepatocytes and absorptive cells of the intestine, this spatial relationship of ER and plasmalemma suggests a role for rosettes in the uptake of exogenous lipid precursors. The morphological differentiation of 3T3-L1 preadipocytes includes the loss of "stress fibers" and the appearance of microfilament like structures that encase, in a complex manner, the cytosolic lipid spheres that appear during differentiation. Other features described for the first time in 3T3-L1 preadipocytes include: (a) the presence of an extensive acid phosphatase (AcPase) positive GERL from which coated vesicles apparently arise (these coated vesicles display AcPase activity and are much smaller and far more numerous than the coated vesicles that seem to arise from the plasmalemmal coated pits); (b) the abundance of AcPase-positive autophagic vacuoles; and (c) a high level of alpha- naphthyl-acetate-esterase activity which, by light microscopy cytochemistry, appears to be localized in the cytosol.     

Adipocyte conversion of cultured 3T3-L1 preadipocytes by bezafibrate

Transient exposure of cultured 3T3-L1 preadipocytes to hypolipidemic fibrate drugs results in extensive adipocyte conversion. Adipocyte conversion in culture was characterized by an increase in neutral lipids content and in adipocyte marker enzymes like hormone-sensitive lipase and glycerol-3-phosphate dehydrogenase. Adipocyte conversion in culture was also accompanied by induction of cyanide-insensitive peroxisomal palmitoyl-CoA oxidation. The conversion pattern exerted by fibrate drugs in 3T3-L1 cells was similar to that reported previously for primary cultured epididymal preadipocytes (R. Brandes, R. Arad and J. Bar-Tana, Biochim. Biophys. Acta, 877, 314-321 (1986)), and seems to refute clonal selection in the conversion sequel initiated by fibrate drugs in primary cultured preadipocytes.