Effects of disassembly of actin microfllaments on the AVP-induced regulation of sodium channel densities in frog skin epithelium

Vasopressin (AVP) stimulates the absorption of Na⁺ across frog skin epithelium by increasing the number of open apical channels (N_o) mainly through a large expansion of the total number of channels (N_T) at the membrane. This study investigates with blocker-induced noise analysis the potential role of actin in the regulation of AVP-induced changes in channel densities. Particularly we examined the idea that actin regulates the insertion of channels into apical cell membranes, consistent with the model for its hydrosmotic action. Treating cells with cytochalasin B (CB) for 2 h to disassemble the prominent subapical actin network did not significantly alter the stimulation of the I_sc by AVP. Importantly, the treatment had no significant effect on the AVP-induced activation of Na⁺ channels, nor on the increases in N_T indicating that an intact actin network is not required for the natriferic actions on the channels. This result is disparate from well known effects of CB on the AVP-induced hydrosmotic response and we assume that these distinct responses are produced by different mechanisms. Other mechanisms need to be considered to explain the mechanism whereby new Na⁺ channels are recruited to the apical membrane. In particular, mechanisms of intracellular trafficking involved in the redistribution of epithelial Na⁺ channels remain unresolved and need to be refined.     

Relationship between saturation deficit and the survival and locomotory activity of surface dwelling Collembola

The springtails Tomocerus minor, Orchesella cincta and Isotoma viridis survive only in air with a saturation deficit near zero. Their survival time is inversely related to saturation deficit and not simply to the relative humidity. The different species show different reactions to the same saturation deficit values, which agree with the humidity preferences of the species in their natural habitats.Drought stimulates Collembola to start locomotory activity which may lead them to aggregate in optimal humidity conditions. Thus this reaction may play a part in increasing the survival rate of these animals.

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Zusammenfassung Die Beziehung zwischen Sättigungsdefizit (S.D.) und Überlebenszeit und zwischen S.D. und lokomotorischer Aktivität wurde an drei epedaphischen Collembolen-Arten untersucht. Die Überlebensdauer verhält sich umgekehrt proportional zum S.D., und steht in keinem direkten Verhältnis zur relativen Luftfeuchte. Die verschiedenen Arten weisen unterschiedliche Reaktionen zu den gleichen S.D.-Werten auf. Diese Reaktionen stimmen mit den Feuchte-Präferenzen der drei Arten in ihren natürlichen Habitats überein. Hohe S.D.-Werte führen zu lokomotorischer Aktivität, mittels welcher die Tiere optimale Feuchtigkeitsverhältnisse erreichen können.Diese Ergebnisse weisen auf einen ökologischen Überlebensmechanismus hin.

     

Activity-stimulating phenomena caused by field-disturbance in the use of pitfall-traps

Summary The influence of several mechanical field disturbances, when using pitfall-traps for activity determinations, is analysed. The disturbances result in higher catches of surface-dwelling Collembola; the animals being stimulated to higher locomotory activity.The disturbance by walking in the field results in a multiple of the control catch. This effect lasts for about 1 hour.The digging-in effect lasts for about two days but more or less different for the various species. The cause is sought in the CO₂ production in the soil, which is considerable after soil disturbances. The reaction of Collembola to rather high CO₂ concentrations in the air results in an increase of the locomotory activity.Another interpretation, namely attraction to CO₂ sources, could not be demonstrated.

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Zusammenfassung Der Einfluß verschiedener mechanischer Störungen im Feld bei der Anwendung der Fallenfangmethode wurde analysiert. Die Störungen stimulieren die Tiere zu größerer lokomotorischer Aktivität, wodurch sich höhere Fangtotale von epedaphischen Collembolen ergeben.Die Störung, welche durch vielfältiges Herumlaufen im Fangterrain entsteht, ergibt Fangtotale, die um ein Vielfaches höher sind als die Kontrollfänge. Dieser Effekt hält eine Stunde an.Eine andere Störung entsteht durch das Eingraben der Fallen. Es resultiert ebenfalls in eine höhere Anzahl gefangener Tiere. Dieser Effekt aber dauert etwa zwei Tage. Die Ursache liegt vermutlich in der Zunahme der CO₂-Produktion im Boden als Folge des Eingrabens. Erhöhte CO₂-Konzentration in der Luft resultiert tatsächlich in einer Zunahme der lokomotorischen Aktivität.Attraktion durch die CO₂-Quelle konnte nicht nachgewiesen werden.

     

Mitochondrial Potentiation Ameliorates Age-Related Heterogeneity in Hematopoietic Stem Cell Function

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Resistance of indigenous African cattle to the tickAmblyomma hebraeum

Field collections ofAmblyomma hebraeum Koch adults from six breeds of cattle were conducted on two farms in the Northern Transvaal. The density ofA. hebraeum adults was highest on Simmentaler cows, followed by Santa gertrudis, Africaner, Bonsmara, Brahman and Nguni. Indigenous breeds, namely Nguni, Bonsmara and Africaner, have the highest level of resistance toA. hebraeum. The relationship between tick abundance and the high level of resistance found in the indigenous breeds is discussed.     

Differential effects of phorbol ester (PMA) on blocker-sensitive ENaCs of frog skin and A6 epithelia

Activation ofprotein kinase C with phorbol 12-myristate 13-acetate (PMA) causedcomplex transient perturbations of amiloride-sensitive short-circuitNa⁺ currents(I_Na) in A6epithelia and frog skins that were tissue and concentration dependent.A noninvasive channel blocker pulse method of noise analysis (18) wasused to investigate how PMA caused time-dependent changes of apicalmembrane epithelial Na⁺ channel(ENaC) single-channel currents, channel open probabilities (P_o), andchannel densities(N_T). In A6epithelia, 5 and 50 nM PMA caused within 7 min concentration-dependentsustained decreases ofP_o (~55% belowcontrol, 50 nM) and rapid compensatory transient increases ofN_T within 7 min(~220% above control, 50 nM), resulting in either small transientincreases of I_Naat 5 nM PMA or small biphasic decreases ofI_Na at 50 nM PMA.In contrast to A6 epithelia, 50 and 500 nM PMA in frog skin causedafter a delay of at least 10 min transient increases ofN_T to~60-70% above control at 30-60 min. Unlike A6 epithelia,P_o was increased~15% above control within 7 min and remained within±10-15% of control for the duration of the 2-h experiments.Despite differences in the time courses of secondary inhibition oftransport in A6 epithelia and frog skin, the delayed downregulation oftransport was due to time-dependent decreases ofN_T from theirpreelevated levels in both tissues. WhereasP_o is decreasedwithin minutes in A6 epithelia as measured by noise analysis or bypatch clamp (8), the discrepancy in regulation ofN_T in A6epithelia as measured by noise analysis and patch clamp is most likelyexplained by the inability of on-cell patches formed before treatmentof tissues with PMA to respond to regulation of their channeldensities.