Perturbation of the pump-leak balance for Na(+) and K(+) in malaria-infected erythrocytes

In humanerythrocytes infected with the mature form of the malaria parasitePlasmodium falciparum, the cytosolic concentration ofNa⁺ is increased and that of K⁺ is decreased.In this study, the membrane transport changes underlying thisperturbation were investigated using a combination of⁸⁶Rb⁺, ⁴³K⁺, and²²Na⁺ flux measurements and a semiquantitativehemolysis technique. From >15 h postinvasion, there appeared in theinfected erythrocyte membrane new permeation pathways (NPP) that causeda significant increase in the basal ion permeability of theerythrocyte membrane and that were inhibited by furosemide (0.1 mM). The NPP showed the selectivity sequenceCs⁺ > Rb⁺ > K⁺ > Na⁺, with the K⁺-to-Na⁺permeability ratio estimated as 2.3. From 18 to 36 h postinvasion, the activity of the erythrocyte Na⁺/K⁺ pumpincreased in response to increased cytosolic Na⁺ (aconsequence of the increased leakage of Na⁺ via the NPP)but underwent a progressive decrease in the latter 12 h of theparasite's occupancy of the erythrocyte (36-48 h postinvasion). Incorporation of the measured ion transport rates into a mathematical model of the human erythrocyte indicates that the induction of the NPP,together with the impairment of the Na⁺/K⁺pump, accounts for the altered Na⁺ and K⁺levels in the host cell cytosol, as well as predicting an initial decrease, followed by a lytic increase in the volume of the host erythrocyte.

     

Ca2+ transients activate calcineurin/NFATc1 and initiate fast-to-slow transformation in a primary skeletal muscle culture

The calcineurin-mediated signal transduction via nuclear factor of activated T cells (NFATc1) is involved in upregulating slow myosin heavy chain (MHC) gene expression during fast-to-slow transformation of skeletal muscle cells. This study aims to investigate the Ca²⁺ signal necessary to activate the calcineurin-NFATc1 cascade in skeletal muscle. Electrostimulation of primary myocytes from rabbit for 24 h induced a distinct fast-to-slow transformation at the MHC mRNA level and a full activation of the calcineurin-NFATc1 pathway, although resting Ca²⁺ concentration ([Ca²⁺]_i) remained unaltered at 70 nM. During activation, the calcium transients of these myocytes reach a peak concentration of 500 nM. Although 70 nM [Ca²⁺]_i does not activate calcineurin-NFAT, we show by the use of Ca²⁺ ionophore that the system is fully activated when [Ca²⁺]_i is 150 nM in a sustained manner. We conclude that the calcineurin signal transduction pathway and the slow MHC gene in cultured skeletal muscle cells are activated by repetition of the rapid high-amplitude calcium transients that are associated with excitation-contraction coupling rather than by a sustained elevation of resting Ca²⁺ concentration. muscle plasticity; NFATc1; resting calcium concentration     

Comparison of radioactive and stable isotope tracer techniques for measuring photosynthesis: 13C and 14C uptake by marine phytoplankton

The stable carbon isotope ¹³C has been used in the open oceanto estimate the inorganic carbon uptake by phytoplankton andthis technique has been compared with the ¹³C tracer method.An overall correlation coefficient of 0.806 and a regressionslope of 1.29 were calculated from 50 sample pairs gatheredduring three cruises in widely different oceanic areas rangingin production rates from 0.01 to 6 mgC m^–3 h^–1.However, significant differences between the two methods wereapparent for cruises located in nutrient-depleted areas. Possibleexplanations lie either in a volume effect, the high silicatecontent of the ¹⁴C solution which could stimulate the ¹⁴C uptakeor in errors associated with the particulate carbon measurementswhich are necessary to convert specific uptake rates to absoluteuptake rates and to yield compatible units for the comparison,in laboratory cultures the ¹⁴C technique overestimated the netparticulate carbon increase by — 16%. ⁺Present address: Laboratoire marin CNRS-IFREMER, L'Houmeau,17137 Nieul-sur-Mer, France. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

Predicted changes in concentrations of free and bound ATP and ADP during intracellular Ca2+ signaling

High Ca²⁺ concentrationscan develop near Ca²⁺ sourcesduring intracellular signaling and might lead to localized regulationof Ca²⁺-dependent processes. Byshifting the amount of Ca²⁺ andother cations associated with ATP, local highCa²⁺ concentrations might alsoalter the substrate available for membrane-associated and cytoplasmicenzymes. To study this, simultaneous equations were solved over a rangeof Ca²⁺ andMg²⁺ concentrations to determinethe general effects of Ca²⁺ on theconcentrations of free and Ca²⁺-and Mg²⁺-bound forms of ATP. Toobtain a more specific picture of the changes that might occur insmooth muscle cells, mathematical models ofCa²⁺ diffusion and regulation wereused to predict the magnitude and time course of near-membraneCa²⁺ transients and their effectson the free and bound forms of ATP near the membrane. The results ofthis work indicate that changes in freeCa²⁺ concentration over the rangeof 50 nM-100 µM would result in significant changes in free ATPconcentration, MgATP concentration, and the CaATP-to-MgATPconcentration ratio.

     

Modeling cell volume regulation in nonexcitable cells: the roles of the Na+ pump and of cotransport systems

The purpose of this study is to contribute to understanding therole ofNa⁺-K⁺-ATPaseand of ionic cotransporters in the regulation of cell volume, byemploying a model that describes the rates of change of theintracellular concentrations ofNa⁺,K⁺, andCl, of the cell volume, andof the membrane potential. In most previous models of dynamic cellularphenomena,Na⁺-K⁺-ATPaseis incorporated via phenomenological formulations; the enzyme isincorporated here via an explicit kinetic scheme. Another feature ofthe present model is the capability to perform short-term cell volumeregulation mediated by cotransporters of KCl and NaCl. The model isemployed to perform numerical simulations for a "typical" nonpolarized animal cell. Basically, the results are consistent withthe view that the Na⁺ pump mainlyplays a long-term role in the maintenance of the electrochemicalgradients of Na⁺ andK⁺ and that short-term cell volumeregulation is achieved via passive transport, exemplified in this caseby the cotransport of KCl and NaCl.

     

A quantitative study of dual action of nickel ions on the taste response to calcium ions of single fibers of the frog glossopharyngeal nerve: inhibition and enhancement by nickel ions

Unitary discharges from single water fibers of the frog glossopharyngealnerve, caused by stimulation with 0.02–5 mM CaSO₄, wererecorded from fungiform papillae with a suction electrode. NiSO₄at concentrations of 0.2–2 mM, namely, at concentrationsthat are barely effective in producing impulses, had a dualaction on the Ca²⁺ response: NiSO₄ caused both inhibition andenhancement of the Ca²⁺ response. In the present study, thisdual action of Ni²⁺ ions on the Ca²⁺ response was investigatedin detail. Single water fibers yielded a saturation type ofconcentration-response curve for CaSO₄, which suggested thatsulfateions do not affect the Ca²⁺ response. Thus, sulfateswere used as test salts in the present study. At low concentrationsof Ca²⁺ ions, Ni²⁺ ions inhibited the Ca²⁺ response, but athigher concentrations of Co²⁺ ions they enhanced it. The resultscan be explained quantitatively by the hypothesis that Ni²⁺ions inhibit the Ca²⁺ response by competing with Ca²⁺ ions forthe Ca²⁺ receptor (X_ca) that is responsible for the Ca²⁺ responseand that Ni²⁺ ions enhance the Ca²⁺ response by acting on amembrane element that interacts with X_ca. Double-reciprocalplots of the data indicate that the enhancing action of Ni²⁺ions is saturated at 1–2 mM Ni²⁺ ions and that Ni²⁺ ionsat these concentrations increase the maximal response of theCa²⁺ response by 182%. Dissociation constants for the Ca-X_cacomplex and the Ni-X_ca, complex were 4.2 x 10^–5 M and7.6 x 10^–5 M, respectively. The analysis suggests thatNi²⁺ ions enhance the Ca²⁺ response by affecting the Ca-X_cacomplex without altering the affinity of X_ca, for Ca²⁺ ions.     

Regulation of Na(+)-K(+)-Cl(-) cotransporter in primary astrocytes by dibutyryl cAMP and high [K(+)](o)

In this study, we examined theNa⁺-K⁺-Cl cotransporter activityand expression in rat cortical astrocyte differentiation. Astrocyte differentiation was induced by dibutyryl cAMP (DBcAMP, 0.25 mM) for7 days, and cells changed from a polygonal to process-bearing morphology. Basal activity of the cotransporter was significantly increased in DBcAMP-treated astrocytes (P < 0.05).Expression of an ~161-kDa cotransporter protein was increased by 91%in the DBcAMP-treated astrocytes. Moreover, the specific[³H]bumetanide binding was increased by 67% in theDBcAMP-treated astrocytes. Inhibition of protein synthesis bycyclohexamide (2-3 µg/ml) significantly attenuated theDBcAMP-mediated upregulation of the cotransporter activity andexpression. The Na⁺-K⁺-Clcotransporter in astrocytes has been suggested to play a role inK⁺ uptake. In 75 mM extracellular K⁺concentration, the cotransporter-mediated K⁺ influx wasstimulated by 147% in nontreated cells and 79% in DBcAMP-treatedcells (P < 0.05). To study whether this highK⁺-induced stimulation of the cotransporter is attributedto membrane depolarization and Ca²⁺ influx, the role of theL-type voltage-dependent Ca²⁺ channel was investigated. Thehigh-K⁺-mediated stimulation of the cotransporter activitywas abolished in the presence of either 0.5 or 1.0 µM of the L-typechannel blocker nifedipine or Ca²⁺-free HEPES buffer. Arise in intracellular free Ca²⁺ in astrocytes was observedin high K⁺. These results provide the first evidence thatthe Na⁺-K⁺-Cl cotransporterprotein expression can be regulated selectively when intracellular cAMPis elevated. The study also demonstrates that the cotransporter inastrocytes is stimulated by high K⁺ in aCa²⁺-dependent manner.

     

A link between plant diversity, elevated CO2 and soil nitrate

Interactive effects of reductions in plant species diversity and increases in atmospheric CO₂ were investigated in a long-term study in nutrient-poor calcareous grassland. Throughout the experiment, soil nitrate was persistently increased at low plant species diversity, and CO₂ enrichment reduced soil [NO₃^-] at all levels of plant species diversity. In our study, soil [NO₃^-] was unrelated to root length density, microbial biomass N, community legume contents, and experimental plant communities differed only little in total N pools. However, potential nitrification revealed exactly the same treatment effects as soil [NO₃^-], providing circumstantial evidence that nitrification rates drove the observed changes in [NO₃^-]. One possible explanation for plant diversity effects on nitrification lies in spatial and temporal interspecific differences in plant N uptake, which would more often allow accumulation of NH₄⁺ in part of the soil profile at low diversity than in more species-rich plant communities. Consequently, nitrification rates and soil [NO₃^-] would increase. Elevated CO₂ increased soil water contents, which may have improved NO₃^- diffusion to the root surface thereby reducing soil [NO₃^-]. Higher soil moisture at elevated CO₂ might also reduce nitrification rates due to less aerobic conditions. The accordance of the diversity effect on soil [NO₃^-] with previous experiments suggests that increased soil [NO₃^-] at low species diversity is a fairly general phenomenon, although the mechanisms causing high [NO₃^-] may vary. In contrast, experimental evidence for effects of CO₂ enrichment on soil [NO₃^-] is ambiguous, and the antagonistic interaction of plant species reductions and elevated CO₂ we have observed is thus probably less universal.     

Soil microbial community and bacterial functional diversity at Machu Picchu, King George Island, Antarctica

The objective of this study was to evaluate the potential contribution of the soil microbial community in the vicinity of two plant covers, Sanionia uncinata and Deschampsia antarctica, at Machu Picchu Station, King George Island, Antarctica. Soil samples were collected at the study site during the southern (pole) summer period from 0-5 cm and 5-10 cm depths, for chemical and biological analyses. Soil microbial biomass reached a maximal value of 144 µg g^-1 in soil samples taken from under the S. uncinata upper layer plant. qCO₂ ranged from 167 to 239 µg CO₂^.mgC_mic^.h^-1 at the 0-5 and 5-10 cm depths, respectively. CO₂ evolution showed values of 54.3 mg^.m^-2 h^-1 beneath plant cover and 55.9 mg^.m^-2 h^-1 in the open space. CO₂ evolved by substrate induced respiration in the soil samples taken under the plant cover in the summer period, oscillated between 0.25 and 4.78 µg CO₂ g^-1 h^-1. The data obtained from this short study may provide evidence that both activity and the composition and substrate utilization of the microbial community appear to change substantially across the moisture level and sample location.     

H+/OH- Excretion and Nutrient Uptake in Upper and Lower Parts of Lupin (Lupinus angustifolius L.) Root Systems

The cultivation of narrow-leafed lupins (Lupinus angustifoliusL.) increase rates of subsoil acidification, and this is thoughtto be partly related to their pattern of nutrient uptake andH⁺/OH^- excretion. The main hypothesis of this study was thatH⁺ and OH^- excretion is not distributed evenly over the entirelength of the root system but is limited to zones where excesscation or anion uptake occur. Seedlings of nodulated lupinswere grown in solution culture using vertically split pots thatallowed the upper and lower zones of the root system to be suppliedwith varying concentrations of K⁺ and NO^-₃. Net H⁺/OH^- excretionwas equated to the addition of NaOH/HCl required to maintaina constant pH in the nutrient solution during a 4-d treatmentperiod and nutrient uptake was measured by depletion from solutionin each zone of the split pots. The excess of cation over anion uptake was positively correlatedwith H⁺ excretion in each rooting zone. In zones where K⁺ wassupplied at 1200 µM, cation uptake was dominated by K⁺and up to twice as much H⁺ was excreted than in zones whereK⁺ was absent. In zones where NO^-₃ was supplied at 750 µM,the anion/cation uptake was balanced, however H⁺ excretion continuedto occur in the zone. When NO^-₃ was supplied at 5000 µM,anion uptake exceeded cation uptake but there was no OH^- excretion.Organic acid anions may be excreted by lupins to maintain theirinternal electroneutrality when anion uptake exceeds cationuptake. Rhizosphere pH would not increase unless the pK_a ofthe excreted organic anions was greater than the external pH.Copyright1993, 1999 Academic Press Lupinus angustifolius L., H⁺/OH^- excretion, nutrient uptake, cation-anion balance, vertical split root     

Net and Steady-state Cation Fluxes in Chlorella pyrenoidosa

The addition of K⁺ to Chlorella cells grown so as to be abnormallyrich in Na⁺ induces a net Na⁺ efflux and a concomitant uptakeof K⁺. The net Na⁺ extrusion shows first-order kinetics withtime constants of about 10 min for illuminated cells, and occursat rates in the region of 10 to 15 pmol cm1² s. The correspondingtime course for the net K⁺ influx also approximates to first-orderkinetics but is more complicated because it not only involvesa K⁺/Na⁺ component but also a K⁺/H⁺ exchange. The H⁺ extrusionusually represents less than 20 per cent of the net cation movementand may account both in magnitude and in rate for the differencebetween K⁺ and Na⁺ movements. The magnitudes of the net K⁺ andNa⁺ fluxes differed from steady-state flux rates in normal highK⁺-containing cells being as much as 20 times greater for K⁺and over 100 times greater for Na⁺. There is some indicationthat K⁺ competes for Na⁺ entry into Na⁺-rich cells, suggestingthat both the Na⁺/Na⁺ and K⁺/Na⁺ exchanges may share the sameentry site. The K⁺/Na⁺ exchange rates saturate at low externalK⁺ concentrations; the half-maximum rate was at about 0.2 mMK⁺. The Na⁺/K⁺ exchange is sensitive to temperature and between0 and 25 °C an activation energy of about 25 k cal/molewas calculated from the Arrhenius equation.     

The Transport and Metabolism of Urea in Chara australis: I. PASSIVE DIFFUSION, SPECIFIC TRANSPORT AND METABOLISM OF UREA, THIOUREA AND METHYLUREA

Most of the urea entering Chara australis cells is rapidly metabolizedto produce CO₂, which diffuses out of the cells into the surroundingmedium. A simple and convenient apparatus to measure both the¹⁴C-urea retained by cells and the ¹⁴CO₂ released into the mediumwas developed and used in a study of urea transport in Chara.The permeability coefficient for urea in the Chara plasmalemmawas estimated from the slope of an uptake versus concentrationfunction as 85 nm s^-1. Computer modelling of urea uptake andmetabolism suggests that this could be a 20% underestimate ofthe true value.The corresponding permeability coefficients forthiourea and N-methyl-urea were estimated in the same way as34 and 35 nm s^-1, respectively. These permeabilities are muchgreater than expected on the basis of either/water partitioncoefficients for the solutes and are consistent with the diffusionof urea and its similarly-sized analogues through aqueous poresin the plasmalemma.At external concentrations of urea less than20 mmol m^-3, the bulk of the uptake is effected by a specifictransport mechanism with an apparent K_m for urea of less than1.0 mmol m^-3. This transport system operates most rapidly withexternal pH in the range 6.5–7.5 and is influenced bythe nitrogen status of the cell.Evidence is produced here suggestingthat the specific transport of urea may be an active process. Key words: Chara, urea uptake, metabolism, diffusion, specific transport     

Effects of nonselective cation channels and PI3K on endothelin-1-induced PYK2 tyrosine phosphorylation in C6 glioma cells

We recently demonstrated that endothelin-1 (ET-1) activates two types of Ca²⁺-permeable nonselective cation channels (designated NSCC-1 and NSCC-2) in C6 glioma cells. In the present study, we investigated the effects of NSCCs on the ET-1-induced proline-rich tyrosine kinase 2 (PYK2) phosphorylation in C6 glioma cells. In addition, we examined the effects of phosphoinositide 3-kinase (PI3K) on the ET-1-induced NSCCs activation and PYK2 phosphorylation. The PI3K inhibitors wortmannin and LY-294002 inhibited ET-1-induced Ca²⁺ influx through NSCC-2 but not NSCC-1. On the other hand, addition of these inhibitors after stimulation with ET-1 failed to suppress Ca²⁺ influx through NSCC-2. PYK2 phosphorylation was abolished by blocking Ca²⁺ influx through NSCCs. The PI3K inhibitors blocked the NSCC-2-dependent part of ET-1-induced PYK2 phosphorylation. These results indicate that 1) NSCC-2 is stimulated by ET-1 via a PI3K-dependent cascade, whereas NSCC-1 is stimulated via a PI3K-independent cascade; 2) PI3K seems to be required for the activation of the Ca²⁺ entry, but not for its maintenance; 3) Ca²⁺ influx through NSCC-1 and NSCC-2 plays an essential role in ET-1-induced PYK2 phosphorylation; and 4) PI3K is involved in the ET-1-induced PYK2 phosphorylation that depends on the Ca²⁺ influx through NSCC-2. endothelin; phosphoinositide 3-kinase; nonselective cation channel; proline-rich tyrosine kinase 2; glioma cell     

New insights into the molecular and cellular workings of the cardiac Na+/Ca2+ exchanger

The cardiac Na⁺/Ca²⁺ exchanger (NCX1) is almost certainly the major Ca²⁺ extrusion mechanism in cardiac myocytes, although the driving force for Ca²⁺ extrusion is quite small. To explain multiple recent results, it is useful to think of the exchanger as a slow Ca²⁺ buffer that can reverse its function multiple times during the excitation-contraction cycle (ECC). An article by the group of John Reeves brings new insights to this function by analyzing the role of regulatory domains of NCX1 that mediate its activation by a rise of cytoplasmic Ca²⁺. It was demonstrated that the gating reactions are operative just in the physiological range of Ca²⁺ changes, a few fold above resting Ca²⁺ level, and that they prevent the exchanger from damping out the influence of mechanisms that transiently increase Ca²⁺ levels. Furthermore, exchangers with deleted regulatory domains are shown to reduce resting Ca²⁺ to lower levels than achieved by wild-type exchangers. A study by the group of Kenneth Philipson demonstrated that the NCX1 regulatory domain can bind and respond to Ca²⁺ changes on the time scale of the ECC in rat myocytes. At the same time, studies of transgenic mice and NCX1 knockout mice generated by the Philipson group revealed that large changes of NCX1 activity have rather modest effects on ECC. Simple simulations predict these results very well: murine cardiac ECC is very sensitive to small changes of the Na⁺ gradient, very sensitive to changes of the sarcoplasmic reticulum Ca²⁺ pump activity, and very insensitive to changes of NCX1 activity. It is speculated that the NCX1 gating reactions not only regulate coupled 3Na⁺:1Ca²⁺ exchange but also control the exchanger’s Na⁺ leak function that generates background Na⁺ influx and depolarizing current in cardiac myocytes. excitation-contraction cycle     

Carbon and nitrogen isotope ratios, nitrogen content and heterotrophy in New Zealand mistletoes

The carbon isotope ratio ('¹³C) of New Zealand mistletoes (-29.51ǂ.10‰) and their hosts (-28.89ǂ.12‰) is generally more negative, and shows less difference between mistletoes and their hosts, than found in previous studies. In 37% of the examined pairs, the '¹³C of mistletoes was less negative than that of their hosts. These reversals were not associated with the relative position (proximal or distal) of the host material with regard to the mistletoe. Differences between host and mistletoe tended to be greater on hosts with less negative '¹³C. Both nitrogen content and isotope ratio ('¹⁵N) of the mistletoe leaves were strongly correlated with those of their hosts. Nitrogen contents of mistletoe leaves were similar to those of their hosts at low nitrogen contents but proportionately less on hosts with a high nitrogen content, whereas '¹⁵N of mistletoes was consistently similar to that of their hosts. The '¹³C of mistletoes was related to both host nitrogen content and '¹⁵N, but '¹³C in host tissue was related to neither, suggesting that the mistletoes derived both nitrogen and carbon from their hosts. The '¹³C of both hosts and mistletoes were significantly related to leaf conductance and carbon dioxide concentration but relationships with transpiration and water use efficiency were not significant. In all cases there was no clear separation between the responses of hosts and mistletoes. This may be related to the similarity of stomatal conductance, transpiration and photosynthesis in the studied mistletoes and their hosts and is consistent with the small differences in '¹³C between mistletoes and hosts found in this study. Consequently, the estimation of mistletoe heterotrophy from carbon discrimination is confounded, as the small difference between host and mistletoe carbon discrimination could equally well result from either similarities in photosynthesis and water relations or heterotrophic assimilation of host-derived carbon. The differences between our study and previous studies (which are mostly from seasonally dry or semi-arid to arid environments) may be related to the temperate environment in which these mistletoes grow. Water is freely available so that the mistletoe is able to obtain sufficient water and dissolved nutrients without having to maintain the high transpiration rate and low water potentials that are needed to extract water from a water-stressed host. Similarly, mistletoe photosynthesis is less inhibited by water stress. The physiological similarities between mistletoe and hosts from a temperate environment are reflected in their similar '¹³C values.     

Time course of active and passive liquid and solute movement in the isolated perfused rat lung model

The isolated perfused rat lung model (IL)is used to study alveolar epithelial transport properties. Most of theprevious studies have been done over a short period of time and havenot used the same preparation as a control and intervention group. Weevaluated whether the IL preparation could be used for a prolonged period of time (5 h) and studied the rates of activeNa⁺ transport, lung liquidclearance, and passive movement of solutes. ActiveNa⁺ transport and lung liquidclearance were stable from 1 to 5 h. The passive movement of smallsolutes (Na⁺, mannitol) did notchange significantly, and albumin movement increased slightly at thefifth hour. Total RNA isolated from IL after 5 h was intact, and theNa⁺-K⁺-ATPaseactivity in alveolar type II cells isolated at the end of 5-hexperiments was equal toNa⁺-K⁺-ATPasefunction from freshly isolated alveolar type II cells. Finally, wemeasured the stimulatory effect of the -adrenergic-agonist terbutaline and the inhibitory effect of theNa⁺-K⁺-ATPase-antagonistouabain by using the same animal as a control. Accordingly, theisolated perfused lung model is functionally stable for at least 5 h,and it could be utilized to evaluate the effect of differentinterventions by using the same preparation.

     

Taste responses to divalent cations in the frog glossopharyngeal nerve: competitive inhibition of the Mg2+ response by Ca2+

In the frog glossopharyngeal nerve, single water fibers respondto low CaCl₂ (1–2 mM) and relatively high MgCl₂ (100 mM).In the present study, it was found that stimulation by a mixtureof low CaCl₂ and relatively high MgCl₂ led to a small response.This suggests that the Ca⁺ response is inhibited by the presenceof Mg²⁺ and the Mg²⁺ response is inhibited by the presence ofCa²⁺. Hence, it is suggested that there are different receptorsites for divalent cations in single water fibers of the frogglossopharyngeal nerve, a calcium receptor site (X_Ca) responsiblefor the Ca²⁺ response and a magnesium receptor site (X_Mg) responsiblefor the Mg²⁺ response. It has been reported that Mg²⁺ inhibitsthe Ca²⁺ response by competing with Ca²⁺ for X_Ca (Kitada andShimada, 1980). In the present study, the inhibition of theMg²⁺ response by Ca²⁺ was examined quantitatively under theassumption that the magnitude of the neural response is proportionalto the amount of MgX_Mg complex minus a constant (the thresholdconcentration of the MgX_Mg complex). The results obtained indicatethat Ca²⁺ competes with Mg²⁺ for X^Mg. The apparent dissociationconstants for MgX_Mg complex and CaX_Mg complex, which were obtainedfrom the present study, were 8.0 x 10^–2 M and 7.2 x 10^–4M, respectively. Thus, competition between Ca⁺ and Mg²⁺ forthe distinct receptor sites involved in taste reception wasdemonstrated by the results described in this paper. Since thedivalent cations do not always bring about activation of tastereceptors, the responses to salts in the frog glossopharyngealnerve cannot be explained in terms of changes in the surfacepotential outside the taste cells. The present results suggestthat there exist multiple specific receptor sites for cationsinvolved in salt taste responses, and only the binding of eachseparate cation to its appropriate receptor sites leads to activationof the receptor and the initiation of impulses in sensory nerveendings.     

Zip3 plays a major role in zinc uptake into mammary epithelial cells and is regulated by prolactin

During lactation, a substantial amount of Zn²⁺ is transferred by the mammary gland from the maternal circulation into milk; thus secretory mammary epithelial cells must tightly regulate Zn²⁺ transport to ensure optimal Zn²⁺ transfer to the suckling neonate. To date, six Zn²⁺ import proteins (Zip1–6) have been identified; however, Zip3 expression is restricted to tissues with unique requirements for Zn²⁺, such as the mammary gland, which suggests that it may play a specialized role in this tissue. In the present study, we have used a unique mammary epithelial cell model (HC11) to characterize the role of Zip3 in mammary epithelial cell Zn²⁺ transport. Confocal microscopy demonstrated that Zip3 is localized to the cell surface in mammary epithelial cells and transiently relocalized to an intracellular compartment in cells with a secretory phenotype. Total ⁶⁵Zn transport was higher in secreting cells, while gene silencing of Zip3 decreased ⁶⁵Zn uptake into mammary epithelial cells, particularly in those with a secretory phenotype. Finally, reduced expression of Zip3 ultimately resulted in cell death, indicating that mammary epithelial cells have a unique requirement for Zip3-mediated Zn²⁺ import, which may reflect the unique requirement for Zn²⁺ of this highly specialized cell type and thus provides a physiological explanation for the restricted tissue distribution of this Zn²⁺ importer. zinc transport; mammary gland; lactation     

On the compatibility of carbon uptake rates calculated from stable and radioactive isotope data: implications for the design of experimental protocols in aquatic primary productivity

Basic differences between the ¹³C and the ¹⁴C techniques ofmeasuring carbon uptake by phytoplankton exist at the levelof the isotopic analysis. With the first method, a ratio ofisotopic abundances is measured on the sample, whereas an absoluteamount of isotope is estimated with the second method. If acarbon source other than the labeled one is utilized by thephytoplankton during incubation, the stable isotope method maybe biased. Specific uptake values will be underestimated bythis effect. It is, however, possible to obtain unbiased estimatesof the ¹³C-labeled carbon uptake by using an equation containinga term describing the final particulate carbon concentration.Only under this condition will carbon uptake rates derived from¹³C isotope data be always compatible with the ¹³C method ofmeasuring primary production ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP), Second International Workshop heldat the National Oceanographic Institute, Haifa, Israel in April–May1984.     

The Correlation between Cd2+ Sensitivity and Cd2+ Uptake in the Strains of Saccharomyces cerevisiae

The sensitivity of twelve strains of Saccharomyces cerevisiaeto Cd²⁺ was examined in correlation with the uptake of Cd²⁺.Strains of S. cerevisiae were grouped into three categoriesdepending on the sensitivity of cells grown on agar-plates containingvarious concentrations of Cd²⁺. 1) The sensitive group did notgrow in 0.1 mM Cd²⁺. 2) The sub-tolerant group was capable ofgrowth at 0.3 min Cd²⁺, but not at 0.4 mM Cd²⁺. 3) The tolerantgroup was capable of growth at 0.4 mM Cd²⁺ or higher. In thesestrain groups the increase in sensitivity to Cd²⁺ was associatedwith an increase in the activity of Cd²⁺ absorption. ¹ This study is dedicated to the late president J. Ashida ofEhime University. (Received November 25, 1982; Accepted February 14, 1983)