An insulin-sensitive cation channel controls [Na+]i via [Ca2+]o-regulated Na+ and Ca2+ entry

Opposing actions of cGMP and calcium on the conductance of the F(0) subunit c pore

Subunit c of ATP synthase can be purified from neuronal plasma membrane and from the inner mitochondrial membrane. In the latter location the hydrophobic 75 amino acid protein is one component of the F(1) F(0) ATP synthase complex but in the former it is alone as a pore that is capable of generating spontaneous electrical oscillations. Pure mammalian subunit c when reconstituted in lipid bilayers and voltage clamped, yields a voltage sensitive pore that conducts a cation current regulated by calcium. The current is here found to be activated by cGMP with a K(M) ranging from 14 nM to 19 microM depending on calcium and temperature. It is sensitively inhibited by a number of ligands. The K(I) for calcium ranges from 100 nM to 100 microM depending on cGMP and temperature. DCCD inhibits with a K(app) of 100 nM. The polyamine nicotine inhibits at 84 nM. The pore has properties that would allow it to deliver sodium or calcium through the cell membrane in a controlled manner while maintaining membrane polarization.

Regulation of the Na+/K+-ATPase by insulin: why and how?

The sodium-potassium ATPase (Na+/K+-ATPase or Na+/K+-pump) is an enzyme present at the surface of all eukaryotic cells, which actively extrudes Na+ from cells in exchange for K+ at a ratio of 3:2, respectively. Its activity also provides the driving force for secondary active transport of solutes such as amino acids, phosphate, vitamins and, in epithelial cells, glucose. The enzyme consists of two subunits (alpha and beta) each expressed in several isoforms. Many hormones regulate Na+/K+-ATPase activity and in this review we will focus on the effects of insulin. The possible mechanisms whereby insulin controls Na+/K+-ATPase activity are discussed. These are tissue- and isoform-specific, and include reversible covalent modification of catalytic subunits, activation by a rise in intracellular Na+ concentration, altered Na+ sensitivity and changes in subunit gene or protein expression. Given the recent escalation in knowledge of insulin-stimulated signal transduction systems, it is pertinent to ask which intracellular signalling pathways are utilized by insulin in controlling Na+/K+-ATPase activity. Evidence for and against a role for the phosphatidylinositol-3-kinase and mitogen activated protein kinase arms of the insulin-stimulated intracellular signalling networks is suggested. Finally, the clinical relevance of Na+/K+-ATPase control by insulin in diabetes and related disorders is addressed.

Regulation of the Na+/K+-ATPase by insulin: why and how?

The sodium-potassium ATPase (Na+/K+-ATPase or Na+/K+-pump) is an enzyme present at the surface of all eukaryotic cells, which actively extrudes Na+ from cells in exchange for K+ at a ratio of 3:2, respectively. Its activity also provides the driving force for secondary active transport of solutes such as amino acids, phosphate, vitamins and, in epithelial cells, glucose. The enzyme consists of two subunits (alpha and beta) each expressed in several isoforms. Many hormones regulate Na+/K+-ATPase activity and in this review we will focus on the effects of insulin. The possible mechanisms whereby insulin controls Na+/K+-ATPase activity are discussed. These are tissue- and isoform-specific, and include reversible covalent modification of catalytic subunits, activation by a rise in intracellular Na+ concentration, altered Na+ sensitivity and changes in subunit gene or protein expression. Given the recent escalation in knowledge of insulin-stimulated signal transduction systems, it is pertinent to ask which intracellular signalling pathways are utilized by insulin in controlling Na+/K+-ATPase activity. Evidence for and against a role for the phosphatidylinositol-3-kinase and mitogen activated protein kinase arms of the insulin-stimulated intracellular signalling networks is suggested. Finally, the clinical relevance of Na+/K+-ATPase control by insulin in diabetes and related disorders is addressed.

Expression of a single gene produces both forms of skeletal muscle cyclic nucleotide-gated channels

Cyclic nucleotide-gated cation channels in skeletal muscle are responsible for insulin-activated sodium entry into this tissue (J. E. M. McGeoch and G. Guidotti. J. Biol. Chem. 267: 832-841, 1992). These channels have previously been isolated from rabbit skeletal muscle by 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP) affinity chromatography, which separates them into two populations differing in nucleotide affinity [L. C. Santy and G. Guidotti. Am. J. Physiol. 271 (Endocrinol. Metab. 34): E1051-E1060, 1996]. In this study, a polymerase chain reaction approach was used to identify skeletal muscle cyclic nucleotide-gated channel cDNAs. Rabbit skeletal muscle expresses the same cyclic nucleotide-gated channel as rabbit aorta (M. Biel, W. Altenhofen, R. Hullin, J. Ludwig, M. Freichel, V. Flockerzi, N. Dascal, U. B. Kaupp, and F. Hofmann. FEBS Lett. 329: 134-138, 1993). The entire cDNA for this gene was cloned from rabbit skeletal muscle and an antiserum to this protein produced. Expression of this cDNA produces a 63-kDa protein with cyclic nucleotide-gated channel activity. A similarly sized immunoreactive protein is present in sarcolemma. Purification of the expressed channels reveals that this single gene produces both native skeletal muscle channel populations.

The alpha2L111R,N122D isoform of the Na,K-ATPase expressed in HeLa cells does not undergo an adipocyte-like increase in activity in response to insulin

In the rat adipocyte, insulin increases potassium uptake by a preferential activation of the alpha2 isoform of the Na,K-ATPase. The question under consideration here is whether expression of the alpha2 isoform is sufficient to replicate its differential activation by insulin. Accordingly, we compared the effect of insulin on the activity of the ouabain resistant rat alpha1 and alpha2RD (alpha2L111R,N122D) isoforms in HeLa cells. In HeLa cells, in contrast to the rat adipocyte, insulin produces an increase of equal magnitude in the rate of 86Rb+/K+ uptake by the ouabain resistant rat alpha1 and rat alpha2RD subunits. We conclude that the mechanism of insulin activation of the alpha2RD isoform in HeLa cells differs from that of the wild type alpha2 isoform in the rat adipocyte.

Insulin-induced translocation of Na+-K+-ATPase subunits to the plasma membrane is muscle fiber type specific

We have previously shown that an acute insulin treatment induces redistribution of the alpha 2- and beta 1- isoforms of the Na+-K+-ATPase from intracellular membranes to plasma membranes detected on subcellular fractionation of mixed muscles and immunoblotting with isoform-specific antibodies (H. S. Hundal et al. J. Biol. Chem. 267: 5040-5043, 1992). In the present study we give both biochemical and morphological evidence that this insulin effect is operative in muscles composed mostly of oxidative (red) fibers but not in muscles composed mostly of glycolytic (white) fibers. The redistribution of the Na+-K+-ATPase alpha 2- and beta 1-isoforms after insulin injection was detected in membranes isolated from and muscles (soleus, red gastrocnemius, red rectus femoris, and red vastus lateralis) but not in membranes from white muscles (white gastrocnemius, tensor fasciae latae, white rectus femoris, and white vastus lateralis). After insulin injection, the potassium-dependent 3-O-methylfluorescein phosphatase activity of the enzyme was higher by 22% in the plasma membrane-enriched fraction and lower by 15% in the internal membrane fraction isolated from red but not from white muscles. Quantitative immunoelectron microscopy of ultrathin muscle cryosections showed that in vivo insulin stimulation augmented the density of Na+-K+-ATPase alpha 2- and beta 1- isoforms at the plasma membrane of soleus muscle by 80 and 124%, respectively, with no change in white gastrocnemius muscle. The effect of insulin to increase the content of Na+-K+-ATPase alpha 2- and beta 1-subunits in isolated plasma membranes was still observed when glycemia was prevented from dropping by using hyperinsulinemic-euglycemic clamps. We conclude that the insulin-induced redistribution of the alpha 2- and beta 1-isoforms of the Na+-K+-ATPase from an intracellular pool to the plasma membrane in restricted to oxidative fiber-type skeletal muscles. This may be related to the selective expression of beta 1-subunits in these fibers and implies that the beta 2-subunit, typical of glycolytic muscles, does not sustain translocation of alpha 2 beta 2-complexes.