Inhibition of short-circuit current by triaminopyrimidine in isolated toad urinary bladder

Potassium Regulation in Medaka (Oryzias latipes) Larvae Acclimated to Fresh Water: Passive Uptake and Active Secretion by the Skin Cells

Molecular mechanisms of Na⁺, Cl⁻, and Ca²⁺ regulation in ionocytes of fish have been well investigated. However, the regulatory mechanism of K⁺ in fishes has been largely unknown. In this study, we investigated the mechanism of K⁺ regulation in medaka larvae acclimated to fresh water. Using a scanning ion-selective electrode technique (SIET) to measure the K⁺ fluxes at skin cells, significant K⁺ effluxes were found at ionocytes; in contrast, significant K⁺ influxes were found at the boundaries between keratinocytes. High K⁺ water (HK) acclimation induced the K⁺ effluxes at ionocytes and suppressed the K⁺ influxes at keratinocytes. The K⁺ effluxes of ionocytes were suppressed by VU591, bumetanide and ouabain. The K⁺ influxes of keratinocytes were suppressed by TAP. In situ hybridization analysis showed that mRNA of ROMKa was expressed by ionocytes in the skin and gills of medaka larvae. Quantitative PCR showed that mRNA levels of ROMKa and NKCC1a in gills of adult medaka were upregulated after HK acclimation. This study suggests that medaka obtain K⁺ through a paracellular pathway between keratinocytes and extrude K⁺ through ionocytes; apical ROMKa and basolateral NKCC1a are involved in the K⁺ secretion by ionocytes.

Molecular and physicochemical characterization of hemoglobin from the high-altitude Taiwanese brown-toothed shrew (Episoriculus fumidus)

Red-toothed shrews (subfamily Soricinae) exhibit the highest mass-specific rates of O₂ consumption recorded among eutherian mammals, though surprisingly no data appears to be available on the functional characteristics of their hemoglobin (Hb). As a first step in addressing this shortcoming, we investigated the O₂ binding characteristics of Taiwanese brown-toothed shrew (Episoriculus fumidus) Hb and its temperature and pH dependence in the absence and presence of anionic red blood cell effectors. Although comparative data regarding the intrinsic O₂ affinity of other shrew species are currently unavailable, our data suggest that the sensitivity of this high-elevation endemic species' Hb to allosteric effector molecules is similar to that of the two lowland species of white-toothed (crocidurine) shrews examined to date. The efficient exploitation of blood O₂ reserves by E. fumidus appears to be achieved via synergistic modulation of O₂ affinity by Cl⁻ and organic phosphates that moreover dramatically lowers the overall enthalpy of oxygenation of their Hb. Oxygen unloading is presumably further enhanced by a relatively high Bohr effect (ΔLog P₅₀/ΔpH = -0.69) and marked reduction in the titratable histidine content (predicted low proton buffering value) of the component globin chains relative to human HbA. Notably, however, the limited data available suggest these latter attributes may be widespread among shrews and hence likely are not adaptations to chronic altitudinal hypoxia per se.

Historic perspectives and recent advances in major animal models of hypertension

Hypertension and related cardiovascular diseases are the leading causes of death in many countries. The etiology of human essential hypertension is largely unknown. It is highly likely that hypertension is a complex and multifactorial disease resulting from the interaction of multiple genetic and environmental factors. Animal models of hypertension have been proved to be useful to study the pathogenesis of, and to find a new therapy for, hypertension. The aim of this article is to briefly review the most widely used rodent models of experimental hypertension, including history and recent advances. These models are classified as genetically-induced, environmentally-induced, pharmacologically-induced, and renal-induced hypertension according to the way of induction; the typical representatives of each of these major types of experimental hypertension are spontaneous hypertension, cold-induced hypertension, DOCA-salt-induced hypertension, and renal-induced hypertension, respectively. The processes of induction of hypertension, possible pathogenesis, characteristics, advantages, and limitations of these animal models are reviewed. In addition, the clinical implications of the above experimental models of hypertension are addressed.

Decrease in litter size in the shrew Crocidura suaveolens (Mammalia, Insectivora) from Corsica (France): Evolutionary response to insularity?

Populations of the lesser white-toothed shrew, Crocidura suaveolens (Pallas, 1811), from Corsica show an increase in adult body size associated with a decrease in litter size. The average number of embryos in wild Corsican females is smaller (mean 2.6, n = 62) than in mainland females (mean 4.6, n = 173). A breeding experiment was run for 4 years, yielding three generations. Under standard breeding conditions, the differences between island and mainland populations were maintained and were significant (median litter size was 2 for Corsica and 5 for the mainland). These differences in life-history traits were therefore proved experimentally to be genetically determined. Hypotheses concerning the mechanisms responsible for these differences are discussed.

Mechanisms of corneal drug penetration. I: In vivo and in vitro kinetics

Corneal penetration studies were conducted in unanesthetized albino rabbits using various organic compounds representing both polar and nonpolar species. Very low molecular weight compounds demonstrate rapid uptake into the aqueous humor despite the lipid-like barrier imposed by the corneal epithelium. Evidence that these compounds may have access to a diffusional channel for corneal transport is presented. In vitro permeability studies were also conducted in an effort to quantitate the corneal diffusion of compounds covering a range of molecular weights and partition coefficients; the results corresponded well with the results of in vivo experiments. Calculations of energies of activation, taken from Arrhenius plots, indicate that the diffusion of drug across the cornea may be by two different mechanisms that depend on the physical-chemical characteristics of the perfusant. One mechanism appears similar to drug movement in an aqueous environment and is characterized by an activation energy similar to that for diffusion in water. The other relates to the expected partitioning of a compound across cellular membranes represented by a relatively high activation energy for diffusion. For hyrdophilic compounds, the epithelium appears to be rate limiting to drug movement, whereas for hydrophobic compounds, the stroma is rate limiting. In the presence of calcium-chelating agents, glycerol demonstrated an increase in corneal penetration in vivo. This effect appears to be reversible at specific concentrations of chelator. In contrast, divalent cations reduced corneal penetration of glycerol. The known calcium chelator EDTA was shown to penetrate the cornea, conjunctiva, and iris/ciliary body from a topically applied dose. The implications of this observation pertain to toxicity effects when EDTA is incorporated into ocular drug products for stability purposes, or novel stratagems for improving ocular bioavailability of topically applied drugs are employed. The addition of calcium-chelating agents to in vivo mounted corneas demonstrated increases in permeability of the cornea to glycerol which were directly related to the concentration of chelating agent used. These results paralleled the findings of similar in vivo studies. The results of these studies are consistent with a currently proposed 'pore' model for the penetration of drugs through the cornea which demonstrates both a partition coefficient and molecular weight dependency on the permeability of the cornea to transported compounds.

Structure-activity relationship of amiloride analogs as blockers of epithelial Na channels: II. Side-chain modifications

The overall on- and off-rate constants for blockage of epithelial Na channels by amiloride analogs were estimated by noise analysis of the stationary Na current traversing frog skin epithelium. The (2-position) side chain structure of amiloride was varied in order to obtain structure/rate constant relationships. Hydrophobic chain elongations (benzamil and related compounds of high blocking potency) increase the stability of the blocking complex (lowered off-rate), explained by attachment of the added phenyl moiety to a hydrophobic area near the site of side chain interaction with the channel protein. Some other chain modifications show that the on-rate, which is smaller than a diffusion-limited rate, varies with side chain structure. In several cases this effect is not attributable to steric hindrance on encounter, and implies that the side chain interacts briefly with the channel protein (encounter complex) before the main blocking position of the molecule is attained. The encounter complex must be labile since the overall rate constants of blockage are not concentration-dependent. In two cases, changes at the 2-position side chain and at other ring ligands, with known effects on the blocking rate constants, could be combined in one analog. The rate constants of blocking by the resulting compounds indicate that the structural changes have additive effects in terms of activation energies. Along with other observations (voltage dependence of the rate constants and competition with the transported Na ion), these results suggest a blocking process of at least two steps. It appears that initially the 2-position side chain invades the outward-facing channel entrance, establishing a labile complex. Then the molecule is either released completely (no block) or the 6-ligand of the pyrazine ring gains access to its receptor counterpart, thus establishing the blocking complex, the lifetime of which is strongly determined by the electronegativity of the 6-ligand.

Determination of the driving force for the sodium pump (ENa) and of active and passive conductances (GNa and Gsh) in isolated toad skin: influence of antidiuretic hormone

1. Values of the sodium potential (ENa), active conductance (GNa) and passive conductance (Gsh) were measured in the isolated skin of the toad Pleurodema thaul placed in an Ussing chamber, and Isaacson's test was performed with 2,4,6-trieminopyrimidine (TAP) and with amiloride. 2. The numerical estimates obtained in the presence of TAP were ENa 122.85 +/- 15.17 mV, GNa 0.493 +/- 0.09 mS/cm2 and Gsh 1.145 +/- 0.23 mS/cm2. 3. After exposure to ADH these values were as follows: ENa 85.76 +/- 12.17 mV, GNa 1.191 +/- 0.20 mS/cm2 and Gsh 0.935 +/- 0.14 mS/cm2. 4. Addition of 0.5 x 10(2-) TAP produced a 53.90 +/- 5.10% decrease in transepithelial potential and a 37.90 +/- 4.90% fall in short-circuit current. 5. Exposure to ADH increased the transepithelial potential difference 34.20 +/- 13.20% and the short-circuit current to 78.00 +/- 20.50% above the control values. 6. Comparison of the efficiency and mechanism of action of TAP and amiloride in the determination of electrical parameters shows that both agents induce a similar decrease in Gsh, a finding which could indicate that TAP blocks toad skin apical membrane Na+ channels without affecting tight junction conductance.

Excitatory effect of amino acids on identified neuron R14 of Aplysia. I. Glycine-induced depolarization and its ionic mechanism

The ionic mechanism of the membrane effect of glycine on identified neuron R14 of Aplysia was investigated with conventional intracellular recording and voltage-clamp techniques. Both localized and bath applications of glycine markedly depolarize R14. Bath-applied glycine induced an inward current that gradually reached a maximum and remained at that level until glycine was washed out. Displacement of the holding potential from -46 to -121 mV increased the inward current. The extrapolated reversal potential was +38.6 mV. Reduction of [Na+]o reversibly decreased the inward current. Alterations of [K+]O, [Cl-]O, and [Ca2+]O, as well as bath-applied ouabain and sodium cyanide, did not affect the inward current. These results suggest that glycine can induce an Na+ current and that the glycine-induced inward current does not reflect an active uptake by an Na+-coupled transport system of glycine into the neuron.