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Biomedical subjects

J Gulati

Publications and source records attributed to J Gulati.

At least 55 records · Page 3Linked to original sources
PubMed1985

Contraction kinetics of intact and skinned frog muscle fibers and degree of activation. Effects of intracellular Ca2+ on unloaded shortening.

This study addresses a long-standing controversy on the effects of the degree of activation on cross-bridge kinetics in vivo, by utilizing isolated intact and skinned fiber preparations. Steady force levels ranging from 0.1 to 0.76 P0 were achieved at 0 degrees C with temperature-step stimulation of intact fibers by varying the amount of caffeine in the bathing medium. The speed of unloaded shortening (by slack test) was found to be practically constant, which suggests that intracellular Ca2+ in the intact preparation has relatively little effect on isotonic shortening. Along with the results on tetanically stimulated fibers (force, P0), we observed a minor but significant trend for the speed to decline with lowered force levels. This trend is explained by the presence of a constant internal load equaling approximately 1% P0. The effect of Ca2+ on the shortening behavior of skinned fibers was examined at 0 and 10 degrees C. At 0 degrees C, there was practically no effect of Ca2+ on the shortening response in slack tests. At 10 degrees C, there was also no Ca2+ effect during the first activation cycle, but in subsequent cycles the speed of shortening was reduced during partial activation, which indicates that there were permanent changes in the fiber properties under these experimental conditions. The latter result could be explained if the internal load had increased to approximately 5% P0 in the modified skinned fiber (compared with 1% P0 in intact fiber). These findings show that isotonic contraction of frog fibers is intrinsically unaffected by the variations in intracellular Ca2+ that modulated the force over a nearly complete range. The results provide support for the idea that Ca2+ influences the force development in vivo by on-off switching mechanisms.

Animals↗
PubMed1984

Isotonic contraction of temp-step activated muscle fibers with varied tonicity: effects of cell volume and the degree of activation.

These studies on intact fibers describe the effects of calcium, ionic strength and volume on the contraction properties. The results provide firm evidence that cell volume affects the speed but not the force. On the other hand, sarcoplasmic ionic strength affects the force development, with no effect on unloaded speed of shortening. These results suggest that there are essential differences in the rate limiting steps for isometric and isotonic properties of the cross-bridge mechanism. The studies at various degrees of activation indicate that Ca acts as a simple "on-off" switch for cross-bridge activation, in intact fibers.

Animals↗
PubMed1984

Intrinsic shortening speed of temperature-jump-activated intact muscle fibers. Effects of varying osmotic pressure with sucrose and KCl.

Effects of intracellular ionic strength on the isotonic contraction properties of both intact fibers and skinned fibers give insights into the cross-bridge mechanism, but presently there is fundamental disagreement in the results on the two fiber preparations. This paper, which studies the effects on contraction of varying the osmotic pressure of the bathing medium with impermeant and permeant solutes, explains the above controversy and establishes the physiological significance of the previous results on skinned fibers. Fast-twitch fibers, isolated singly from tibialis and semitendinosus muscles of frogs, were activated by a temperature-jump technique in hyperosmotic solutions with either 100 or 150 mM sucrose (impermeant), or 50 or 75 mM KCl (permeant). Intracellular ionic strength was expected to rise in these solutions from the standard value of approximately 190 to 265 mM. Cell volume and the speed of unloaded shortening both decreased with sucrose and were constant with KCl. On the other hand, isometric force decreased equally with equiosmolar addition of either solute; this is additional evidence that contractile force decreases with ionic strength and is independent of fiber volume. Therefore, for the main cross-bridges, force per bridge is constant with changes in the lateral separation between the myofilaments. The next finding, that at a fixed cell volume the contraction speed is constant with KCl, provides clear evidence in intact fibers that the intrinsic speed of shortening is insensitive to increased ionic strength. The data with KCl are in agreement with the results on skinned fibers. The results suggest that in the cross-bridge kinetics in vivo the rate-limiting step is different for force than that for shortening. On the other hand, the decrease in speed with sucrose is associated with the shrinkage in cell volume, and is explained by the possibility of an increased internal load. A major fraction of the internal load may arise from unusual interactions between the sliding filaments; these interactions are enhanced in the fibers compressed with sucrose, but this does not affect the intrinsic kinetics of the main cross-bridges.

Animals↗
PubMed1983

Magnesium ion-dependent contraction of skinned frog muscle fibers in calcium-free solution.

Skinned frog fibers were reversibly activated in Ca-free solutions containing 0 mM KCl, 23 microM free Mg, and having an ionic strength of approximately 50 mM. Contractile force was nearly maximal at 22 degrees - 25 degrees C and decreased at lower temperatures. Maximal force in Ca-free solution at 50 mM ionic strength was close to twice the calcium-activated force with pCa 5 and 190 mM ionic strength. The force in Ca-free solution could be reduced to zero by raising the concentration of free Mg from 23 microM to 1.0 mM at the same ionic strength (50 mM). On stretching the fiber from 2.0 to 3.2 micron the force decreased; this effect was similar to that seen with Ca-activated fiber and the data support the idea that Ca-free tension is made at the cross-bridge level. Isotonic contraction during Ca-free activation showed a velocity transient as in Ca-activated fiber at 190 mM ionic strength, but the transient in the present case was very much prolonged. This finding suggests that contraction mechanisms for force generation and for shortening are essentially the same in the two conditions, but that certain rate constants of cross-bridge turnover are slower for the Ca-free contraction. Also, the results indicate that, in low ionic strength, Ca binding to thin filaments is not essential for unmasking the cross-bridge attachment sites, which suggests that the steric blocking mechanism is modified under these conditions.

Animals↗
PubMed1982

Tonicity effects on intact single muscle fibers: relation between force and cell volume.

Contraction of isolated, intact frog muscle fibers under increasing tonicity of the external solution was studied by adding (i) effectively impermeant sodium chloride and sucrose and (ii) permeant potassium chloride. Force of isometric contraction decreased as a function of tonicity, independent of the permeability of the solute. In contrast, cell volume changed with tonicity in impermeant solutes and was constant with potassium chloride. The results are evidence that ionic strength in the sarcoplasm directly influences the contraction mechanism. Also, the findings show that force development is unaffected by changes in fiber volume, suggesting that the force per cross-bridge is constant at different distances between the thin and myofilaments. Finally, in light of the length-force relation, the results support the idea that cross-bridges are independent force generators.

Animals↗
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↗
PubMed1976

Potassium accumulation in muscle: a test of the binding hypothesis.

Living frog skeletal muscle can accumulate potassium in vitro to concentrations up to 580 millimolar. Both the amount of potassium accumulated and the relationship between intracellular and extracellular potassium concentrations indicate that potassium is "free" under all conditions, rather than bound to cellular macromolecules. The data also indicate that at most 20 percent of the cell water is "bound" in the sense that it excludes electrolytes.

Animals↗
PubMed1976

Force-velocity characteristics for calcium-activated mammalian slow-twitch and fast-twitch skeletal fibers from the guinea pig.

Twitch fiber bundles from a slow (soleus) and a fast (extensor digitorum longus) mammalian muscle after mild chemical treatment were activated with calcium and relaxed in calcium-free solution. Like the electrically activated whole muscles, the force-velocity relationship was such that, at each relative load, the steady speed of shortening for the fast fibers was about two times greater than that for the slow twitch fibers. The duration of pre-steady motion in the two types of fibers was the same. The data provide direct evidence that the difference in the shortening characteristics of the two types of fibers is due to differences in their cross-bridge properties.

Animals↗
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↗
PubMed1972

Cooperative critical thermal transition of potassium accumulation in smooth muscle.

The steady-state levels of potassium and sodium of taenia coli of guinea are critically affected by varying temperature in the narrow range 12 degrees to degrees C. For the accumulation of both cations the critical temperature, T(c), is 13.8 degrees C the presence of millimolar external potassium. The value of T(c), decreases 10.0 degrees C when the external potassium is raised to 10 millimolar. Since, at a fixed Temperature, the potassium accumulation follows a cooperative mechanism, the results are compared with the quantitative predictions of this approach. The itical thermal transition behavior can be described in terms of the cooperative cumulation process.

Animals↗
PubMed1972

Cooperative thermal effects on the accumulation of potassium and sodium in frog muscle.

Sodium-rich frog muscles are found to extrude sodium and reaccumulate potassium at 0 degrees C. The uptake of potassium by these muscles is studied at three different temperatures as a function of external potassium concentration, K(ex). The steady-state potassium content of the tissue is related to K(ex). by a sigmoidal cooperative curve at all temperatures. These results are compared with findings on a mammalian smooth muscle.

Animals↗
PubMed1971

Cooperative control of potassium accumulation by ouabain in vascular smooth muscle.

The effects of ouabain on potassium accumulation were studied in the dog carotid artery. It was confirmed that vascular smooth muscle lost potassium in the presence of ouabain greater than 10(-9) molar. This effect could be reversed by systematically increasing potassium in the external medium. The action of ouabain on ion accumulation was represented quantitatively with the application of a recent biophysical approach.

Animals↗
PubMed1971

Metabolic cooperative control of electrolyte levels by adenosine triphosphate in the frog muscle.

This study examines the effects of metabolic inhibitors on the content of cellular K, Na, and adenosine triphosphate (ATP). ATP and K are seen to fall in the inhibited tissues. The ATP content is correlated with the K content. The role of ATP is examined according to a recent biophysical approach. It is suggested that ATP may control the electrolyte levels by inducing conformational changes in the cytoplasmic proteins.

Adenosine Triphosphate↗