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J P Reuben

Publications and source records attributed to J P Reuben.

47 records · Page 3Linked to original sources
PubMed1972

Membrane calcium activation in excitation-contraction coupling.

Depolarization thresholds for eliciting tension and Ca electrogenesis have been compared in isolated crayfish muscle fibers. Just-detectable tensions and Ca spikes induced after treatment with procaine were elicited with intracellularly applied depolarizing currents of fixed duration. Both thresholds were found to increase in a similar manner in fibers exposed to increased concentrations of Ca in the bathing solution or addition of other divalent cations (Mg, Mn, Ni). However, antagonistic effects between divalent cations were also demonstrated. Substitution of increasing amounts of NaSCN for NaCl in the standard saline produced a progressive decrease in both thresholds. The correlation in the change in thresholds for the two processes supports the hypothesis that a change in membrane Ca conductance is an integral step in excitation-contraction coupling.

Animals↗
PubMed1971

Regulation of tension in the skinned crayfish muscle fiber. I. Contraction and relaxation in the absence of Ca (pCa is greater than 9).

In isolated skinned crayfish muscle fibers bathed in solutions that were buffered to be virtually free of Ca(2+) (pCa 8-10) the substrate for both contraction and relaxation is the MgNTP complex. Tension increased up to 50% of the maximum capability of the fiber as the substrate MgATP increased to an optimum (pMgATP = 5.5). Relaxation was induced by further increases in MgATP. Similar bell-shaped curves of tension vs. pMgNTP were obtained with UTP and ITP, but optimum pMgUTP was about 4.5 and optimum pMgITP was about 2.6. The relation between equilibrium tension and pMgNTP is described by an equation analogous to that for the kinetics of enzymes regulated by substrate inhibition.

Adenosine Triphosphate↗
PubMed1970

Effects of caffeine on crayfish muscle fibers. I. Activation of contraction and induction of Ca spike electrogenesis.

Contractions are evoked in single muscle fibers of crayfish by intracellular as well as extracellular applications of caffeine. Responses to external applications in concentrations above 2 mM could be induced indefinitely. With concentrations above 5 mM the caffeine-induced responses were highly repeatable. Tensions were transient even when the caffeine remained in the bath. There was no change in resting potential, but during the contraction the effective resistance decreased about 10%. A number of factors (change in pH, Ca, K, and Cl) modified the responses. The time course of the tension was greatly prolonged when the transverse tubular system (TTS) was s swollen and was again shortened when the TTS was caused to shrink. An increased permeability to Ca induced by caffeine was evidenced by the transformation of the normally graded electrical responses to Ca spikes, which are insensitive to tetrodotoxin. The overshoot is a function of both external Ca and caffeine. A 10-fold change in Ca changed the overshoot by 19 mv in the presence of 10 mM caffeine and by 29 mv in 80 mM caffeine. The role of the increased permeability to Ca for caffeine-induced contractions will be analyzed in the accompanying paper.

Animals↗
PubMed1970

Effects of caffeine on crayfish muscle fibers. II. Refractoriness and factors influencing recovery (repriming) of contractile responses.

When caffeine evokes a contraction, and only then, crayfish muscle fibers become refractory to a second challenge with caffeine for up to 20 min in the standard saline (5 mM K(o)). However, the fibers still respond with contraction to an increase in K(o), though with diminished tension. Addition of Mn slows recovery, but the latter is greatly accelerated during exposure of the fiber to high K(o), or after a brief challenge with high K(o). Neither the depolarization induced by the K, nor the repolarization after its removal accounts for the acceleration, which occurs only if the challenge with K had itself activated the contractile system; acceleration is blocked when contractile responses to K are blocked by reducing the Ca in the bath or by adding Mn. Recovery is accelerated by redistribution of intracellular Cl and by trains of intracellularly applied depolarizing pulses, but not by hyperpolarization. The findings indicate that two sources of Ca can be mobilized to activate the contractile system. Caffeine mobilizes principally the Ca store of the SR. Depolarizations that are induced by high K(o), by transient efflux of Cl, or by intracellularly applied currents mobilize another source of Ca which is strongly dependent upon the entry of Ca from the bathing medium. The sequestering mechanism of the SR apparently can utilize this second source of Ca to replenish its own store so as to accelerate recovery of responsiveness to a new challenge with caffeine.

Animals↗
PubMed1969

Ionic permeability of the inhibitory postsynaptic membrane of lobster muscle fibers.

Reversal potentials (E(IPSP)) of the inhibitory postsynaptic potential and the membrane resting potentials (E(M)) of lobster muscle fibers were determined with intracellular recording under a variety of ionic conditions. E(IPSP) is solely dependent on the electromotive force of anionic batteries; i.e., on the electrochemical gradient for a "mobile" fraction of intracellular Cl (Cl(i)) which is considerably smaller than the total intracellular Cl. The active inhibitory membrane is more permeable to certain "foreign" anions in the order NO(3) > SCN > Br > Cl. The membrane is impermeable to BrO(s), isethionate, and methylsulfate, but is slightly permeable to acetate and propionate. The level of Cl(i) appears to be determined in part by some active (pump?) process and most of the anions studied appear to interfere with the steady-state level of Cl(i).

Animals↗
PubMed1968

Correlated morphological and physiological studies on isolated single muscle fibers. II. The properties of the crayfish transverse tubular system: localization of the sites of reversible swelling.

Living muscle fibers of crayfish become dark during efflux of Cl(-). This change in appearance is correlated with occurrence of vacuolation in the fixed fibers. The vacuoles begin at and are mainly confined to the terminals of the transverse tubular system (TTS) which are in diadic contact with the sarcoplasmic reticulum (SR). In electron micrographs swellings more than 1 micro in diameter may be seen connected to the sarcolemma or sarcolemmal invaginations by relatively unswollen tubules about 300-500 A wide. Darkening of the living fibers can be reversed by causing an influx of Cl(-). Vacuoles are then absent in the fixed preparations. These findings accord with the conclusion that the membrane of the TTS is anion permselective. Localization of the selectivity to the membrane of the terminals of the TTS strengthens the hypothesis that a channeling of current flow is responsible for initiation of excitation-contraction coupling. During the swelling, and upon its reversal, the area of the membrane of the terminals must change reversibly by about two to four orders of magnitude. The absence of changes in the dimensions of the unit membrane indicates that the expansion of the membrane and its subsequent shrinkage involve reversible incorporation of cytoplasmic material into the membrane phase.

Chlorides↗
PubMed1967

Crayfish muscle: permeability to sodium induced by calcium depletion.

Membrane of crayfish muscle fibers becomes selectively permeable to sodium when the calcium concentration of the bathing medium is reduced. Removal of calcium or its reduction below 1 or 2 millimole per liter causes large transient depolarizations up to 70 millivolts in amplitude. They resemble pro longed action potentials and occur only in the presence of sodium. The responses are abolished when tris(hydroxymethyl)aminomethane or lithium is substituted for sodium, and are blocked by tetrodotoxin even in the presence of sodium.

Animals↗
PubMed1966

Osmometrically determined characteristics of the cell membrane of squid and lobster giant axons.

Lobster and squid giant nerve fibers respond differently when subjected to osmotic challenges. The axons proper, as distinct from the total (fiber) complex formed by the axon and connective sheath, both behave as "fast" osmometers for changes in the concentration of NaCl, but the maximum degree of swelling in hyposmotic media is by about 60% in lobster and only by 20% in squid. The relative volume intercepts of the van't Hoff relation are about 0.2 for lobster and 0.4 for squid. The sheaths of both axons undergo only small, apparently passive changes in volume. Lobster axons are permeable to Cl, but squid axons are impermeable to this anion. Lobster axons are also permeable to glycerol. The implications of the data as to the nature of volume regulation of cells are discussed.

Animals↗
PubMed1965

Correlated morphological and physiological studies on isolated single muscle fibers. I. Fine structure of the crayfish muscle fiber.

Single fibers isolated from walking leg muscles of crayfish have 8- to 10-micro sarcomeres which are divided into A, I, and Z bands. The H zone is poorly defined and no M band is distinguishable. Changes in the width of the I band, accompanied by change in the overlap between thick and thin myofilaments, occur when the length of the sarcomere is changed by stretching or by shortening the fiber. The thick myofilaments (ca. 200 A in diameter) are confined to the A band. The thin myofilaments (ca. 50 A in diameter) are difficult to resolve except in swollen fibers, when they clearly lie between the thick filaments and run to the Z disc. The sarcolemma invaginates at 50 to 200 sites in each sarcomere. The sarcolemmal invaginations (SI) form tubes about 0.2 micro in diameter which run radially into the fiber and have longitudinal side branches. Tubules about 150 A in diameter arise from the SI and from the sarcolemma. The invaginations and tubules are all derived from and are continuous with the plasma membrane, forming the transverse tubular system (TTS), which is analogous with the T system of vertebrate muscle. In the A band region each myofibril is enveloped by a fenestrated membranous covering of sarcoplasmic reticulum (SR). Sacculations of the SR extend over the A-I junctions of the myofibrils, where they make specialized contacts (diads) with the TTS. At the diads the opposing membranes of the TTS and SR are spaced 150 A apart, with a 35-A plate centrally located in the gap. It appears likely that the anion-permselective membrane of the TTS which was described previously is located at the diads, and that this property of the diadic structures therefore may function in excitation-contraction coupling.

Animals↗