Structural and developmental differences between three types of Na channels in dorsal root ganglion cells of newborn rats

Summary The changes in Na current during development were studied in the dorsal root ganglion (DRG) cells using the whole-cell patch-clamp technique. Cells obtained from rats 1–3 and 5–8 days after birth were cultured and their Na currents were compared. On top of the two types of Na currents reported in these cells (fast-FA current and slow-S current) a new fast current was found (FN). The main characteristics of the three currents are: (i) The voltages of activation are –37, –36, and –23 mV for the FN, FA and S currents, respectively. (ii) The activation and inactivation kinetics of FN and FA currents are about five times faster than those of the S current. (iii) The voltages at which inactivation reaches 50% are –139, –75 and –23 mV for the FN, FA and S currents, respectively.The kinetics and voltage-dependent parameters of the three currents and their density do not change during the first eight days after birth. However, their relative frequency in the cells changes. In the 1–3 day-old rats the precent of cells with S, FA, and mixed S+FN currents is 22, 18, and 60% of the cells, respectively. In the 5–8 day-old, the percent of cells with S, FA, and FN+S is 10, 66 and 22%. The relative increase in the frequency of cells with FA current during development can contribute to the ease of action potential generation compared with cells with FN currents, which are almost completely inactivated under physiological conditions. The predominance of FA cells also results in a significant decrease in the relative frequency of cells with the high-threshold, slow current.Antibodies directed against a part of the S₄ region of internal repeat I of the sodium channel (C ₁ ⁺ , amino acids 210–223, eel channel numbering) were found to shift the voltage dependence of FA current inactivation (but not of FN or S currents) to more negative potentials. The effect was found only when the antibodies were applied externally. The results suggest that FN, FA and S types of Na currents are generated by channels, which are different in the topography of the C ₁ ⁺ region in the membrane.     

Effects of predatory risk and resource renewal on the timing of foraging activity in a gerbil community

The foraging decisions of animals are often influenced by risk of predation and by the renewal of resources. For example, seed-eating gerbils on sand dunes in the Negev Desert of Israel prefer to forage in the bush microhabitat and during darker hours due to risk of predation. Also, daily renewal of seed resource patches and timing of nightly foraging activity in a depleting environment play important roles in species coexistence. We examined how these factors influence the timing of gerbil foraging by quantifying foraging activity in seed resource patches that we experimentally renewed hourly during the night. As in previous work, gerbils showed strong preference for the safe bush microhabitat and foraged less in response to high levels of illumination from natural moon light and from artificial sources. We demonstrate here for the first time that gerbils also responded to temporal and spatial heterogeneity in predatory risk through their timing of activity over the course of each night. Typically, gerbils concentrated their activity early in the night, but this changed with moon phase and in response to added illumination. These results can be understood in terms of the nature of patch exploitation by gerbils and the role played by the marginal value of energy in determining the cost of predation. They further show the dynamic nature of gerbil foraging decisions, with animals altering foraging efforts in response to time, microhabital, moon phase, illumination, and resource availability.     

Mechanism of aluminium-induced porphyrin synthesis in bacteria

In previous studies, aluminium was found to retard bacterial growth and enhance porphyrin formation in Arthrobacter aurescens RS-2. The aim of this study was to establish the mechanism of action of aluminium which leads to increased porphyrin production. Cultures of Arthrobacter aurescens RS-2 were incubated in the absence and presence of 0.74 mm aluminium. After 6 and 24 h of incubation, various parameters of the haem biosynthetic pathway were determined. After 6 h of incubation with aluminium, the activities of the enzymes aminolevulinate synthase (ALAS), aminolevulinate dehydratase (ALAD), porphobilinogen deaminase (PBGD) and uroporphyrinogen decarboxylase (UROD) were increased by 120, 170, 190 and 203%, respectively, while that of ferrochelatase (FC) was found to be unchanged. However, after 24 h of incubation, no change in the activities of ALAS and ALAD was noted, while an about 2-fold increase in PBGD and UROD activities were observed. FC activity was decreased by 63%. It was concluded that aluminium exerts its effect by inducing the enzymes PBGD and UROD rather than by a direct or indirect effect on ALAS. Its effect on the final step in the haem biosynthetic pathway is discussed.     

Flowering seasonality and flower characteristics ofLoranthus acaciae Zucc. (Loranthaceae): Implications for advertisement and bird-pollination

The flowering biology and pollination ecology ofLoranthus acaciae was studied at Hazeva in the northern Arava Valley in Israel. Flowers at anthesis had red anthers, a red stigma and a green corolla which turned red as a postfloral phenomenon. Their flowering period was approximately 10 months long (from mid-June until mid-April) during which time two main flowering patterns were distinguished. Some plants flowered twice a year, with separate summer and winter flowering periods; other plants flowered continuously, with two peaks, one in the summer and one in the winter. Several significant differences between summer and winter flowering and fruiting were found: (1) the summer flowering period was shorter than that of winter, (2) flowering synchrony between individual plants was lower in summer than in winter, (3) in summer the plants produced a larger proportion of female flowers, whereas in winter most of the plants produced a larger proportion of hermaphrodites, (4) in summer a limited number of plants produced smaller flowers while the majority produced normal-sized flowers, whereas in winter the entire population produced only normal-sized flowers, and (5) fruit set percentage was lower in summer than in winter.L. acaciae was found to be self-compatible, but, since it was not spontaneously self-pollinated, it showed high dependence on pollinator activity. In summer the flowers were visited by a wide spectrum of pollinators, both birds and insects, while in winter flowers were visited almost exclusively by the orange-tufted sunbird (Nectarinia osea osea, Nectariniidae). These seasonal changes in flowering characteristics and pollinator activity could explain why reproductive success is higher in winter than in summer.     

ABO incompatibility and reproductive failure. I. Prenatal selection

An analysis of previous spontaneous abortions and the frequencies of blood-group combinations in mother-child pairs was carried out in 500 gravidae. The rate of previous spontaneous abortions in blood-group-O women whose latest child has blood group B is significantly higher than in all other women. On the other hand, the combination mother B/child AB is rarer than expected, but no increase in the rate of previous spontaneous abortions is obvious among these women. These discrepancies are interpreted as an indication that prenatal selection associated with ABO incompatibility may operate at various stages from fertilization through pregnancy, and that different incompatible combinations may be subject to selection at different stages.     

Increased placental concentrations of 25-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 in pregnant rats treated with human chorionic gonadotropin

Pregnant rats were injected intrajugularly with 2500 i.u. human chorionic gonadotropin (HCG) toward the end of gestation (days 18-19) and 7.0 pmoles of tritiated 25-hydroxyvitamin D3 [( 3H]25(OH)D3) the following day. They were sacrificed ten to 24 hours later. [3H]25(OH)D3 and the in vivo produced [3H]24,25-dihydroxyvitamin D3 [( 3H]24,25(OH)2D3) in lipid extracts from maternal serum, kidneys, placenta and fetal tissues were separated by Sephadex LH-20 chromatography, and high performance liquid chromatography (HPLC). HCG treatment of pregnant rats increased significantly 25(OH)D3 levels in the placenta and kidneys and 24,25(OH)2D3 level in the placenta. Fetal metabolites levels were unaffected by HCG treatment. Serum and kidney levels of 25(OH)D3 and 24,25(OH)2D3 in pregnant rats were significantly lower than in non-pregnant rats. Serum and kidney levels of both metabolites in non-pregnant female rats treated with HCG did not differ from the untreated controls. HCG may, therefore, be involved in regulation of fetoplacental vitamin D metabolism.     

Orientation of the hemes of high potential cytochromes relative to photosynthetic membranes, as shown by the linear dichroism of oriented preparations

The orientations of high potential cytochromes with respect to photosynthetic membranes was investigated in spinach chloroplasts and in Rhodopseudomonas viridis. The general approach consists in detection with polarized light of photoinduced absorbance changes related to the oxidation of the cytochromes. The orientation of cytochrome c-558 was measured at room temperature in chromatophores and whole cells of Rps. viridis, oriented on glass slides and in a magnetic field, respectively. The orientation of cytochrome b-559 of green plants was detected at 77 K in magnetically oriented chloroplasts. In both cases the dichroic ratio for the band shows that the heme plane makes an angle greater than 35°C with the membrane plane. Moreover, the dichroic ratio is not constant throughout the and β bands, for both cytochrome c-558 and b-559. Linear dichroism spectra of oriented pure horse heart cytochrome c and cytochrome c₂ of Rhodopseudomonas sphaeroides in stretched polyvinyl alcohol films show that the variations of the dichroic ratio in the and β bands can be explained by the occurrence of x- and y-polarized transitions absorbing at slightly different wavelengths.     

Potassium ion accumulation at the external surface of the nodal membrane in frog myelinated fibers.

Potassium accumulation associated with outward membrane potassium current was investigated experimentally in myelinated fibers and analyzed in terms of two models-three-compartment and diffusion in an unstirred layer. In the myelinated fibers, as in squid giant axons, the three-compartment model satisfactorily describes potassium accumulation. Within this framework the average space thickness, theta, in frog was 5,900 +/- 700 A, while the permeability coefficient of the external barrier, PK, was (1.5 +/- 0.1) X 10(-2) cm/s. The model of ionic diffusion in an unstirred aqueous layer adjacent to the axolemma, as an alternative explanation for ion accumulation, was also consistent with the experimental data, provided that D, the diffusion constant, was (1.8 +/- 0.2) X 10(-6) cm/s and l, the unstirred layer thickness, was 1.4 +/- 0.1 micron, i.e., similar to the depth of the nodal gap. An empirical equation relating the extent of potassium accumulation to the amplitude and duration of depolarization is given.