Antiporters

Microscopical and functional aspects of calcium-transport and deposition in terrestrial isopods

Terrestrial isopods (Crustacea) are excellent model organisms to study epithelial calcium-transport and the regulation of biomineralization processes. They molt frequently and resorb cuticular CaCO(3) before the molt to prevent excessive loss of Ca(2+) ions when the old cuticle is shed. The resorbed mineral is stored in CaCO(3) deposits within the ecdysial gap of the first four anterior sternites. After the molt, the deposits are quickly resorbed to mineralise the posterior part of the new cuticle. The deposits contain numerous small spherules composed of an organic matrix and amorphous CaCO(3), which has a high solubility and, therefore, facilitates quick mobilization of Ca(2+) and HCO(3)(-) ions. During the formation and resorption of the deposits large amounts of Ca(2+), HCO(3)(-) and H(+) are transported across the anterior sternal epithelial cells. Within the last years, various light and electron microscopical techniques have been used to characterize the CaCO(3) deposits and the cellular mechanisms involved in biomineralization. The work on the CaCO(3) deposits includes studies on the ultrastructure of the deposits, the sequence of events during deposit formation and dissolution, and the mineral composition of the sternal deposits. The differentiation of the anterior sternal epithelial cells and the mechanisms of epithelial ion transport required for the mineralization and demineralisation of the deposits was studied using various analytical light and electron microscopical techniques including polarized light microscopy, immunocytochemistry, electron microprobe analysis, electron energy loss spectroscopy and electron spectroscopic imaging. Comparative analysis of deposit morphology and the differentiation of the sternal epithelia provide information on the evolution of CaCO(3) deposit formation in relation to the degree of adaptation to terrestrial environments.

Sodium/calcium exchange: its physiological implications

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.

Immunocytochemical localization of Na+,K(+)-ATPase in the calcium-transporting sternal epithelium of the terrestrial isopod Porcellio scaber L. (Crustacea)

Terrestrial isopods store large amounts of calcium carbonate between the epithelium and the old cuticle of the first four anterior sternites before molt. During the formation of these sternal CaCO3 deposits, large amounts of calcium are transported across the anterior sternal epithelium from the base to the apical side of the integument, and in the reverse direction during resorption of the deposit. A monoclonal antibody against the avian alpha-subunit of Na+,K(+)-ATPase was used to localize Na+,K(+)-ATPase in the anterior and the posterior sternal epithelium of Porcellio scaber. Semithin cryosections 0.5 micron thick were used for immunofluorescence microscopy and ultrathin cryosections for immunogold electron microscopy. The Na+,K(+)-ATPase was localized in the basolateral plasma membrane of the posterior and anterior sternal epithelium. The apical plasma membrane, including cytoplasmic extensions into the newly secreted cuticle, was virtually devoid of the enzyme. This pattern of immunolocalization was not affected by the direction of transepithelial calcium transport associated with the deposition and resorption phases of the molt cycle.

Intracellular pH regulation in the embryonic chicken lens epithelium

1. The intracellular pH (pHi) of embryonic lens epithelia was measured by the emission ratio technique using the fluorescent pH probe carboxy-seminaphthorhodafluor-1 (Snarf-1). 2. In artificial aqueous humour solutions (AAH) containing HCO3-, pHi was 7.45, a value more alkaline than that of the bathing medium (pH = 7.3). In HCO3- -free AAH, pHi was 7.29. 3. Acetazolamide, an inhibitor of carbonic anhydrase, had no effect on resting pHi. 4. The pHi could be manipulated experimentally by changing the external pH (pHo) of HEPES-buffered AAH, the addition or withdrawal of CO2-HCO3-, or by perfusion with the weak bases NH4Cl and procaine. 5. The pHi change induced by withdrawal of 5 mM-procaine was used to calculate a value for the intrinsic cytoplasmic buffering capacity (beta i) of 16.5 mM. 6. The addition of amiloride (1 mM) or treatment with low-Na+ AAH solutions led to a decrease in pHi of 0.23 over the 10 min exposure. In addition, these treatments inhibited pHi recovery from NH4(+)-induced acidosis. These observations are consistent with the presence of amiloride-sensitive N(+)-H+ antiport. 7. Addition of exogenous antiport activity in the form of 50 microM-monensin caused an increase in pHi of 0.24. 8. In HCO3(-)-containing media, replacing Cl- by gluconate or isothionate led to an immediate, reversible increase in pHi which could be completely inhibited by 2 mM-4-acetamido-4'-isothiocyanato-stillbene-2,2'-disulphonic acid (SITS). This indicates the presence of Cl(-)-HCO3- exchange in this tissue. 9. Under HCO3(-)-free conditions, replacement of Cl- by gluconate or isothionate caused a small transient acidification followed, 5 min later, by a large sustained alkalinization. The delayed increase in pHi could be completely blocked by 1 mM-amiloride and may reflect volume-sensitive stimulation of the Na(+)-H+ antiporter as cell volume (estimated by cell height measurements) was shown to decrease significantly during this period.