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R J Podolsky

Publications and source records attributed to R J Podolsky.

At least 37 records · Page 2Linked to original sources
PubMed1981

Isotonic contraction of skinned muscle fibers on a slow time base: effects of ionic strength and calcium.

The force development by calcium-activated skinned frog skeletal muscle fibers and the motion on a slow time base after a quick decrease in load were studied at 0-1 degrees C as a function of the ionic strength and the degree of activation. The ionic strength was varied between 50 and 190 mM by adding appropriate concentrations of KCl to the bathing solution. Under these conditions, the fibers could be maximally activated for several cycles at low ionic strength without developing residual tension. We found that the steady isometric force in fully activated fibers linearly decreased when the KCl concentration was increased from 0 to 140 mM. The steady isotonic motion at a given relative load in fully activated fibers was almost the same at KCl concentration greater than or equal to 50 mM. In 0 and 20 mM KCl, the isotonic velocity decreased continuously for more than 300 ms. At a given relative load, the initial velocity of the motion in 0 and 20 mM KCl was about 0.6 and 0.9 times, respectively, that in 140 mM KCl. The initial velocity decreased further when residual tension developed; this observation provides additional evidence that residual tension may reflect the presence of an internal load. The effect of calcium on the motion was examined at 70 mM KCl. In this solution, the motion during the velocity transient at a given relative load appeared to be the same at different levels of activation. The speed of the subsequent motion was almost steady at high calcium levels but decreased continuously in low calcium levels. These results support the idea that at low ionic strength the response of the fiber to calcium is switch-like, but that other factors also affect the contraction mechanism under these conditions.

Animals↗
PubMed1978

Contraction transients of skinned muscle fibers: effects of calcium and ionic strength.

Calcium and ionic strength are both known to modify the force developed by skinned frog muscle fibers. To determine how these parameters affect the cross-bridge contraction mechanism, the isotonic velocity transients following step changes in load were studied in solutions in which calcium concentration and ionic strength were varied. Analysis of the motion showed that calcium has no effect on either the null time or the amplitude of the transients. In contrast, the transient amplitude was increased in high ionic strength and was suppressed in low ionic strength. These results are consistent with the idea that calcium affects force in skeletal muscle by modulating the number of force generators in a simple switchlike "on-off" manner and that the steady force at a given calcium level is proportional to cross-bridge number. On the other hand, the effect of ionic strength on force is associated with changes in the kinetic properties of the cross-bridge mechanism.

Animals↗
PubMed1977

Regulation by magnesium of intracellular calcium movement in skinned muscle fibers.

The effect of Mg on Ca movement between the sarcoplasmic reticulum (SR) and myofilament space (MFS) was studied in skinned muscle fibers by using isometric force as an indicator of MFS Ca. In Ca-loaded fibers at 20 degrees C, the large force spike induced by Ca in 1 mM Mg (5 mM ATP) was strongly inhibited in 3 mM Mg, and force development was extremely slow. After a brief Ca stimulus in 1 mM Mg, relaxation in Ca-free solution was significantly faster in 3 mM Mg. These changes were due to altered Ca movements, since the effect of 3 mM Mg on steady force in CaEGTA solutions was small. Changes in Mg alone induced force transients apparently due to altered Ca movement. In relaxed fibers, decreasing the Mg to 0.25 mM caused phasic force development. In contracting fibers in Ca solutions, increasing the Mg caused a large transient relaxation. The effects of increased Mg were antagonized by 0.5 mM Cd, an inhibitor of the SR Ca transport system. The results indicate that active Ca uptake by the SR in situ is stimulated by Mg, and that it can affect local MFS [Ca++] in the presence of a substantial Ca source. These results provide evidence that an increased rate of Ca uptake in 3 mM Mg could account for inhibition of the large force spike associated with Ca-induced Ca release in skinned fibers.

Animals↗
PubMed1977

Influence of magnesium on chloride-induced calcium release in skinned muscle fibers.

Chloride-induced Ca release in skinned muscle fibers was studied by measuring isometric force transients and 45Ca loss from fiber to washout solutions. Skinned fibers prepared from muscles soaked in normal Ringer solution made large force transients in 120 mM Cl solution with 5 mM ATP and 1 mM Mg, but 3 mM Mg was inhibitory. Mg inhibition was antagonized by low temperature and by Cd, agents which slow active Ca uptake by the sarcoplasmic reticulum (SR). In low Mg++, Cl stimulated rapid 45Ca release from the SR in sufficient amounts to account for the force response. The increased 45Ca release was inhibited by EGTA, suggesting that release requires free Ca under these conditions. The 45Ca initially released was partially reaccumulated later. Reaccumulation was increased in higher Mg++. These results provide additional evidence that the Ca uptake rate is an important determinant of net release, and suggest that Mg++ acts primarily on this mechanism. Skinned fibers prepared from muscles soaked in low Cl solutions could give force responses to Cl solutions with 3 mM and 6 mM Mg. This observation suggests that the Cl stimulus varies with the [Cl] gradient across the internal membranes, and supports the hypothesis that applied Cl causes membrane depolarization.

Animals↗
PubMed1976

X-ray diffraction of actively shortening muscle.

Low angle x-ray diffraction patterns were obtained from resting and activated frog sartorius muscles by means of a position-sensitive detector. Although the intensity ratio I10/I11 decreased many-fold upon activation, it was nearly the same during isometric and isotonic contraction. Thus, motion has a much smaller effect on the low order equatorial pattern than the transition from rest to activity. Analysis of the 10 and 11 reflections separately showed that I10 and I11 change reciprocally upon activation, and that they both increase by a small amount in the transition from isometric to isotonic contraction. If the intensity ratio can be taken as a measure of cross-bridge number, the results provide evidence that the drop in force in an actively shortening muscle is due primarily to the influence of motion on the configuration, rather than the number, of cross-bridges.

Animals↗
PubMed1975

Some self-consistent two-state sliding filament models of muscle contraction.

The general formalism required to treat two-state sliding filament models of muscle contraction, including free energy considerations, is first reviewed and amplified. This formalism is then used to examine, and modify as needed, three models studied previously by Podolsky and Nolan, in which cross-bridge attachment-detachment and ATP turnover are not tightly coupled. No attempt is made here to establish an optimal, self-consistent model of this type because our interest is primarily in methadology rather than in fitting experimental results. But it appears from this preliminary study that such a model, with satisfactory mechanical and thermodynamic properties, could be found. An extremely simple but unrealistic two-state model is also studied which is of interest because it demonstrates the fact that it is possible, in principle at least, for sliding filament models to work with very high thermodynamic efficiencies (50-100 percent). An appendix is included that is concerned with the form of the dependence of certain first-order rate constants on the concentrations of ATP, ADP, and P.

Adenosine Diphosphate↗
PubMed1974

Contraction transients of skinned muscle fibers.

The contraction kinetics of calcium-activated skinned muscle fibers were studied after step decreases in load by means of a quickly responding recording system. The steady velocity at a given relative load was close to that found in electrically stimulated, intact muscle fibers. The presteady motion had the same shape as that of intact fibers, but the time scale of the transient was nearly two times slower. The duration of the initial phase of the motion, where the velocity was greater than the steady value, and the time at which the subsequent low velocity phase ended, were both stretched out to the same extent. Changing the temperature had the same effect on the length of these two phases of the transient. The results indicate that both phases of the transient are produced by the same underlying factors and can be taken as evidence that the entire transient originates in the crossbridge mechanism. In this case the experimental technique described here provides a basis for distinguishing between chemical parameters that affect contractility by (a) controlling the number of sites at which crossbridges can be formed, as opposed to (b) changing the kinetic properties of a given number of sites.

Actins↗
PubMed1974

Ionic strength and the contraction kinetics of skinned muscle fibers.

The influence of KCl concentration on the contraction kinetics of skinned frog muscle fibers at 5-7 degrees C was studied at various calcium levels. The magnitude of the calcium-activated force decreased continuously as the KCl concentration of the bathing solution was increased from 0 to 280 mM. The shortening velocity at a given relative load was unaffected by the level of calcium activation at 140 mM KCl, as has been previously reported by Podolsky and Teichholz (1970. J. Physiol. [Lond.]. 211: 19), and was independent of ionic strength when the KCl concentration was increased from 140 to 280 mM. In contrast, the shortening velocity decreased as the KCl concentration was reduced below 140 mM; the decrease in velocity was enhanced when the fibers were only partially activated. In the low KCl range, the resting tension of the fibers increased after the first contraction cycle. The results suggest that in fibers activated at low ionic strength some of the cross bridges that are formed are abnormal in the sense that they retard shortening and persist in relaxing solution.

Adenosine Triphosphate↗
PubMed1974

Muscle compliance and the longitudinal transmission of mechanical impulses.

The time required for a mechanical impulse to propagate from one end to the other was measured directly in frog sartorius muscles and in fiber bundles from the semitendinosus muscle. When the fibers were fully activated, the transmission velocity was 170 mm/ms. In resting fibers the transmission time was three to four times greater than in activated fibers. Control experiments indicated that the transmission time across the tendons was negligible. A muscle compliance of 55-80 A per half sarcomere was estimated from these data. The "measurement time" of the method was calculated to be about 15 micros. This relatively short measurement time makes the method potentially useful for detecting changes in cross-bridge compliance.

Animals↗
PubMed1972

Length-force relation of calcium activated muscle fibers.

Calcium activated skinned frog muscle fibers develop a large relative force at a sarcomere length of 1.0 micrometer. Since the normal myofilament lattice is perturbed at this length, regularity of the lattice does not appear to be an important factor in the contraction mechanism.

Animals↗
PubMed1972

Calcium uptake and force development by skinned muscle fibres in EGTA buffered solutions.

1. The uptake of (45)Ca and the development of force by segments of skinned muscle fibres were measured in Ca solutions buffered with EGTA. When the Ca ion concentration in these solutions was above the contraction threshold, Ca was accumulated by the sarcoplasmic reticulum during the delay phase before force developed. The uptake rate increased, and the length of the delay decreased, when the concentration of the calcium buffer was increased.2. The maximum accumulation of Ca was 2-3 mM. Force developed, ending the delay phase, when the calcium uptake approached this maximum level.3. The pattern of force development suggested that this process was often accompanied by a sudden release of accumulated Ca.4. The relation between the kinetics of (45)Ca uptake and the interaction of calcium with EGTA and the sarcoplasmic reticulum is discussed. The data indicate that at the low concentrations necessary for relaxation the sarcoplasmic reticulum takes up Ca fast enough to account for the rate at which force falls when intact muscle fibres relax.

Acetates↗
PubMed1972

Intracellular calcium movements in skinned muscle fibres.

1. Force development and (45)Ca movement between the internal membranes and the myofilament space were measured in segments of skinned muscle fibres.2. At 20 degrees C calcium ions in the myofilament space of calcium loaded fibres caused the internal membranes to release a substantial amount of calcium which produced a large force spike. Calcium produced a similar force spike in fibres containing only the physiological level of calcium when the temperature was lowered to about 6 degrees C.3. The force spike at 6 degrees C was inhibited when the magnesium concentration in the bathing solution was increased from 1 to 6 mM. The pattern of force development in preparations transferred directly from oil to a free calcium solution indicated that the normal concentration of Mg(2+) in the myofilament space of intact fibres is of the order of 10(-3)M.4. A possible role for the regenerative calcium release mechanism in the physiological activation of muscle fibres is suggested.

Action Potentials↗
PubMed1970

Regenerative calcium release within muscle cells.

Free calcium appears to trigger the release of stored calcium from the sarcoplasmic reticulum of skinned skeletal muscle fibers immersed in solutions with a low concentration of magnesium ion.

Animals↗