Physiological actions of taurine

The alterations in myocardial energy substrate metabolism that occur in heart failure, and the causes and consequences of these abnormalities, are poorly understood. There is evidence to suggest th...

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Myocardial Substrate Metabolism in the Normal and Failing Heart

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

https://www.physiology.org/doi/pdf/10.1152/physrev.1999.79.3.763

Sodium/Calcium Exchange: Its Physiological Implications

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)2D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1α-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype.

The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)2D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1α-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

Vitamin D and Human Health: Lessons from Vitamin D Receptor Null Mice

Local increases in intracellular calcium ion concentration ([Ca2+]i) resulting from activation of the ryanodine-sensitive calcium-release channel in the sarcoplasmic reticulum (SR) of smooth muscle cause arterial dilation. Ryanodine-sensitive, spontaneous local increases in [Ca2+]i (Ca2+ sparks) from the SR were observed just under the surface membrane of single smooth muscle cells from myogenic cerebral arteries. Ryanodine and thapsigargin inhibited Ca2+ sparks and Ca(2+)-dependent potassium (KCa) currents, suggesting that Ca2+ sparks activate KCa channels. Furthermore, KCa channels activated by Ca2+ sparks appeared to hyperpolarize and dilate pressurized myogenic arteries because ryanodine and thapsigargin depolarized and constricted these arteries to an extent similar to that produced by blockers of KCa channels. Ca2+ sparks indirectly cause vasodilation through activation of KCa channels, but have little direct effect on spatially averaged [Ca2+]i, which regulates contraction.

https://science.sciencemag.org/content/sci/270/5236/633.full.pdf

Relaxation of arterial smooth muscle by calcium sparks

The Na(+)-Ca2+ exchanger of the cardiac sarcolemma can rapidly transport Ca2+ during excitation-contraction coupling. To begin molecular studies of this transporter, polyclonal antibodies were used to identify a complementary DNA (cDNA) clone encoding the Na(+)-Ca2+ exchanger protein. The cDNA hybridizes with a 7-kilobase RNA on a Northern blot and has an open reading frame of 970 amino acids. Hydropathy analysis suggests that the protein has multiple transmembrane helices, and a small region of the sequence is similar to that of the Na(+)- and K(+)-dependent adenosine triphosphatase. Polyclonal antibodies to a synthetic peptide from the deduced amino acid sequence react with sarcolemmal proteins of 70, 120, and 160 kilodaltons on immunoblots. RNA, synthesized from the cDNA clone, induces expression of Na(+)-Ca2+ exchange activity when injected into Xenopus oocytes.

https://science.sciencemag.org/content/sci/250/4980/562.full.pdf

Molecular cloning and functional expression of the cardiac sarcolemmal Na(+)-Ca2+ exchanger

▪ Abstract Plasma membrane Na+-Ca2+ exchange is an essential component of Ca2+ signaling pathways in several tissues. Activity is especially high in the heart where the exchanger is an important regulator of contractility. An expanding exchanger superfamily includes three mammalian Na+-Ca2+ exchanger genes and a number of alternative splicing products. New information indicates that the exchanger protein has nine transmembrane segments. The exchanger, which transports Na+ and Ca2+, is also regulated by these substrates. Some molecular information is available on regulation by Na+ and Ca2+ and by PIP2 and phosphorylation. Altered expression of the exchanger in pathophysiological states may contribute to various cardiac phenotypes. Use of transgenic approaches is beginning to improve our knowledge of exchanger function.

Sodium-calcium exchange: a molecular perspective.

The gene coding for the Na+/Ca2+ exchanger NCX1 is characterized by a cluster of six exons (A, B, C, D, E, and F) coding for a variable region in the COOH terminus of the large intracellular loop of the protein. Alternative splicing of these exons generates multiple tissue-specific variants of NCX1. Using reverse transcriptase-polymerase chain reaction, we analyzed eight previously described and four new splicing isoforms of NCX1 in a wide variety of tissues and cells. Exons A and B are mutually exclusive, as shown in earlier studies, and splicing isoforms containing exon A are preferentially expressed in heart, brain, and skeletal muscle, whereas splicing variants with exon B are found in all rat tissues except heart. The second and third isoforms of the Na+/Ca2+ exchanger, NCX2 and NCX3, show a deletion of 37 amino acids in the intracellular loop corresponding to parts of the variable region of NCX1. We identified three splicing isoforms of NCX3 in brain and skeletal muscle by reverse transcriptase-polymerase chain reaction. These splice variants are generated by including either of two alternative exons equivalent to the NCX1 exon A or B and by including or excluding a sequence equivalent to the NCX1 exon C. We did not detect any alternative splicing of NCX2. We examined selected tissues from neonatal and adult rats and found developmental regulation for NCX1 and NCX3 splicing isoforms in skeletal muscle. Specific isoform patterns were also detected for NCX1 and NCX3 in cultured cortical neurons, astrocytes, and oligodendrocytes. We suggest a new terminology to distinguish the different splice variants of individual NCX isoforms.

Kenneth D. Philipson

Papers

Molecular cloning and functional expression of the cardiac sarcolemmal Na(+)-Ca2+ exchanger

Sodium-calcium exchange: a molecular perspective.

Tissue specificity and alternative splicing of the Na+/Ca2+ exchanger isoforms NCX1, NCX2, and NCX3 in rat

Cloning of the NCX2 isoform of the plasma membrane Na+-Ca2+ exchanger

Cloning of a Third Mammalian Na+-Ca2+ Exchanger, NCX3