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At least 307 records · Page 17Linked to original sources
PubMed1990

Electrogenic 2 Na+/1 H+ exchange in crustaceans.

Hepatopancreatic brush border membrane vesicles of the freshwater prawn, Macrobrachium rosenbergii and the marine lobster, Homarus americanus exhibited 22Na uptake which was Cl-independent, amiloride sensitive, and stimulated by a transmembrane H gradient (Hi greater than Ho). Sodium influx by vesicles of both species were sigmoidal functions of [Na]o, yielding Hill coefficients that were not significantly different (P greater than 0.5) than 2.0. Estimations of half-saturation constants (KNa) were 82.2 mM (prawn) and 280.1 mM (lobster), suggesting a possible adaptation of this transporter to environmental salinity. Trans-stimulation and cis-inhibition experiments involving variable [H] suggested that the exchangers in both species possessed single internal cation binding sites (pK 6.5-6.7) and two external cation binding sites (prawn, pK 4.0 and 5.7; lobster pK 3.5 and 6.1). Similar cis inhibition studies using amiloride as a competitive inhibitor of Na uptake supported the occurrence of dual external sites (prawn, Ki50 and 1520 microM; lobster Ki9 and 340 microM). Electrogenic Na/H exchange by vesicles from both crustaceans was demonstrated using equilibrium shift experiments where a transmembrane potential was used as the only driving force for the transport event. Transport stoichiometries of the antiporters were determined using Static Head analysis where driving forces for cation transfer were balanced using a 10:1 Na gradient, a 100:1 H gradient, and a stoichiometry of 2.0. These electrogenic 2 Na/1 H exchangers appear thermodynamically capable of generating sufficient gastric acidification for organismic digestive activities.

Amiloride↗
PubMed1975

Evidence that alpha-dihydrograyanotoxin II does not bind to the sodium gate.

The basis for the ability of alpha-dihydrograyanotoxin II (alpha-2HG-II) to promote Na+ conductance in axons was sought. The apparent binding of tritiated alpha-2HG-II to neural and other preparations was studied, using equilibrium dialysis, with lobster axon membranes, Torpedo electroplax, housefly head, and rat brain, liver and kidney. In every case the "binding" was nonsaturating and was suggested to involve nonspecific partitioning into the tissue. Supporting evidence was the similarity of extent of "binding" in all tissues and its relative insensitivity to neuropharmacological agents. Alpha-2HG-II did not affect the Na+ conductance of phospholipid bilayers, nor did it permit transport of 22Na into a bulk organic phase. It was concluded that alpha-2HG-II did not bind to the sodium gate, but possibly to a sodium permease present at a frequency of less than one per mu2 of cell membrane.

Animals↗
PubMed1975

Ultraviolet photoalteration of ion channels in voltage-clamped lobster giant axons.

An analysis of the ultraviolet light-induced changes in ionic conductances of lobster giant axon membranes has been carried out using the double sucrose gap voltage-clamp technique. The predominant effect of monochromatic light from a xenon arc source in the 255 to 305 nm region is an irreversible reduction in the magnitude of sodium conductance, without change in sodium channel activation or inactivation kinetics. A considerably smaller reduction in the magnitude of potassium conductance occurs, with some slowing of potassium channel activation kinetics. Leakage conductance is essentially not altered. The fall in sodium conductance follows an exponential time course toward a zero asymptote. The rate constant for conductance decrease was used as an assay for the wavelength dependence. The sodium conductance was maximally sensitive at 290 nm. It is suggested that individual sodium channels are closed upon absorption of single photons by aromatic amino acid residues in membrane proteins.

Action Potentials↗
PubMed1988

The interaction of homologous series of alkanols with sodium channels in nerve membrane vesicles.

The potency of members of the homologous series of alkanols to block 22Na uptake through sodium channels stimulated by veratridine was studied in membrane vesicles obtained from lobster walking leg nerves. A cut-off was revealed at the level of 1-undecanol. However, secondary isomers of inactive primary homologues, such as 5-dodecanol and 5-tridecanol, were able to block ion flux. From the concentration required for an equipotent effect, it was calculated that the standard free energy for adsorption of primary alkanols is -725 cal/mol CH2. Furthermore, since the concentration required for an equipotent effect for primary isomer was found to be lower than that obtained for secondary isomers, it is concluded that the latter are less potent than the former. The similarity between this set of results and those obtained in intact frog sciatic nerve (J. Requena et al., J. Membrane Biol., 84:229-238, 1985) offers further support to the notion that the procedure employed to isolate the membrane vesicles does preserve the Na channels. However, the mechanism of alcohol inhibition of the Na channel in isolated membrane vesicles would seem to be somewhat different from that preferred in axons. While in vesicles the block needs to be thought in terms of a reduction in the number of conducting Na channel, in axons this is considered to be the less likely mode of action, mainly because under veratridine it is not possible to invoke a shift in the steady-state activation or inactivation.

Alcohols↗
PubMed1990

Nonselective cation channel activated by patch excision from lobster olfactory receptor neurons.

A nonselective cation channel activated by patch excision was characterized in inside-out patches from spiny lobster olfactory receptor neurons. The channel, which was permeable to Na+, K+ and Cs+, had a conductance of 320 pS and was weakly voltage dependent in the presence of micromolar divalent cations. Millimolar internal divalent cations caused a voltage- and concentration-dependent block of Na+ permeation. Analysis of the voltage dependence indicated that the proportion of the membrane's electric field sensed by Mg2+ was greater than 1, suggesting that the channel contains a multi-ion pore. Internal divalent cations also reduced the frequency of channel opening in a concentration-dependent, but not voltage-dependent, manner, indicating that different cation binding sites affect gating and conductance. While block of gating prevented determining if internal divalent cations permeate the channel, a channel highly permeable to external divalent cations was observed upon patch excision to the inside-out configuration. The monovalent and divalent cation conductances shared activation by patch excision, weak voltage dependence, and steady-state activity, suggesting that they are the same channel. These data extend our understanding of this type of channel by demonstrating permeation by monovalent cations, detailing Mg2+ block of Na+ permeation, and demonstrating the channel's presence in arthropods.

Animals↗
PubMed1996

The modulus of elasticity of lobster aorta microfibrils.

The presence of elastic fibres in the extracellular matrix (ECM) provides physiologically important elastic properties for many tissues. Until recently, microfibrils, one component of the ECM, were thought primarily to serve as a scaffolding on which elastin is deposited during development to form elaunin fibres [1]. The most prominent protein that forms mammalian microfibrils is fibrillin. It is known that mutations in the fibrillin gene cause a heterogenous connective tissue disease called Marfan syndrome [2], so information on mechanical properties of microfibrils or their role in tissue function would be useful. Microfibrils are also found in the ECM of some invertebrate tissues, and there is growing evidence that the protein forming the structure is homologous to mammalian fibrillin [3,4]. It has been shown that the microfibril-based arterial wall of the lobster has viscoelastic properties [5], and we have now utilized this primitive artery to measure the modulus of elasticity of microfibrils. It is similar to that of the rubber-like protein elastin.

Animals↗
PubMed1977

Molt induction in lobsters (Homarus americanus) by intramuscular injection of ecdysterone triacetate.

The principal component of the successful, molt-inducing ecdysterone acetate mixture was established as ecdysterone triacetate and shown to have the same activity as the acetate mixture. The triacetate induced ecdysis in male lobsters collected in winter and summer and in female lobsters collected in winter. Intramuscular injections of triacetate in ethanol were as effective as those with oil emulsions.

Animals↗