Effects of calcium and ionic strength on shortening velocity and tension development in frog skinned muscle fibres.

Abstract

SUMMARY1. The influence of Ca2+ concentration and ionic strength on the maximum velocity of shortening (Vmax) and the tension generating capability of frog skinned muscle fibres has been studied at temperatures between 1 and 101C.2. Fibre segments were mounted between a force transducer and servo motor, where they could be viewed and photographed through a microscope. Segments in which the striations became non-uniform during activation were discarded.3. Velocity was obtained as a function of load by stepping the tension to values less than the steady isometric tension. Vmax was then determined by an extrapolation technique. Vmax was also obtained using a second, independent method by measuring the times required to take up various amounts of slack imposed on the segments. 4. Vmax was significantly influenced by the Ca2+ concentration, decreasing by about one half when the Ca2+ concentration was reduced to give steady tensions less than half-maximal. 5. Vma. was not influenced by changes in ionic strength, in the range 0-09-018 M.Steady tension was found to increase as ionic strength was decreased in the same range.6. These results indicate that the effect of changes in ionic strength is to alter the numbers or stiffness of attached cross-bridges, while there is no apparent influence of ionic strength on the steady-state kinetics of the actin-myosin interaction during unloaded shortening. The mechanism responsible for the influence of Ca2+ on Vmax is unknown, though possible sites of action for Ca2+ are discussed.

Effect of cross-bridge kinetics on apparent Ca2+ sensitivity.

“…An increase in maximum tension does accompany the left shifts of the curve induced by ionic strength reduction and reduction in phosphate concentration. Similar increases in tension with reduction in ionic strength have been reported by others for skinned fibers (Ashley and Moisescu, 1977 ;Julian and Moss, 1981 ;Gordon et al, 1973 ;Thames et al, 1974) and for intact fibers (April et al, 1968 ;Edman and Hwang, 1977 ;Gordon and Godt, 1970) . We observe a large increase in tension with reduction of phosphate from 7.5 to 0 mM in 5 mM substrate but no such increase in 0.25 mM substrate.…”

“…V. of shortening (Edman and Hwang, 1977) and maximum tension (April and Brandt, 1973) have been reported to increase in intact fibers bathed in hypotonic media. In skinned fibers bathed in media of reduced ionic strength, no increase in Vmaz has been observed, although the maximum tension does increase (Julian and Moss, 1981) . Gulati and Podolsky (1978) argue from their skinned fiber data that some kinetic parameters of the cross-bridge cycle must change with reduction in ionic strength .…”

Mechanisms underlying rhythmic locomotion: dynamics of muscle activation

“…Camilleri and Hull determined the parameter values of a Hill-type model to fit the data of rapid front-arm release movements at low muscle activation levels, and found that the model parameter v max (shortening velocity at zero load) in function q(v) deviated considerably from the value determined from the tetanic isovelocity experiments (Camilleri and Hull, 2005). The dependence of v max on the calcium ion concentration was also discussed in the earlier literature (Julian and Moss, 1981;Gordon et al, 2000). To compensate for this model error, v max was parameterized by the muscle activation level in some applications (Otten, 1987;Winters and Stark, 1985), and the resulting models no longer had the decoupled multiplicative structure.…”

“…The structure and parameters of each component function are then determined based on the observed muscle properties and the tension data from isometric and isovelocity experiments. Because of the complexity of muscle dynamics (Gordon et al, 2000;Julian and Moss, 1981;Balnave and Allen, 1996) such models can fail to predict the tension under functionally relevant conditions that are remote from the condition under which the model is developed. Perreault and colleagues modeled the cat soleus muscle and found that the error between the model prediction and data was consistently large for a variety of length change and stimulation patterns; sometimes greater than 50% during large length changes (Perreault et al, 2003).…”

“…Muscle activation dynamics can be too complex to be captured by the simple multiplicative structure; for example, rapid shortening can deactivate the thin filaments, and strongly bound cross bridges can activate skinned fibers in the absence of calcium ions (Gordon et al, 2000). In fact, a number of studies have demonstrated that multiplicative models can fail to predict tensions accurately when the MN stimulation rate is low and/or there are continuous changes in muscle velocity (Perreault et al, 2003;Camilleri and Hull, 2005;Julian and Moss, 1981;Gordon et al, 2000). A common cause of the failure is the assumption that the activation, length and velocity factors in the multiplicative structure are independent of each other.…”