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Modulation by substrate concentration of maximal shortening velocity and isometric force in single myofibrils from frog and rabbit fast skeletal muscle
Author(s) -
Tesi C.,
Colomo F.,
Nencini S.,
Piroddi N.,
Poggesi C.
Publication year - 1999
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1999.0847u.x
Subject(s) - myofibril , isometric exercise , chemistry , biophysics , anatomy , muscle contraction , sarcomere , skeletal muscle , tibialis anterior muscle , substrate (aquarium) , grenouille , contraction (grammar) , myofilament , myosin , myocyte , biochemistry , medicine , biology , salientia , endocrinology , ecology , xenopus , gene
1 The effects of magnesium adenosine triphosphate (MgATP; also referred to as ‘substrate’) concentration on maximal force and shortening velocity have been studied at 5 °C in single and thin bundles of striated muscle myofibrils. The minute diameters of the preparations promote rapid diffusional equilibrium between the bathing medium and lattice space so that during contraction fine control of substrate and product concentrations is achieved. 2 Myofibrils from frog tibialis anterior and rabbit psoas fast skeletal muscles were activated maximally by rapidly (10 ms) exchanging a continuous flux of pCa 8.0 for one at pCa 4.75 at a range of substrate concentrations from 10 μM to 5 mM. At high substrate concentrations maximal isometric tension and shortening velocity of both frog and rabbit myofibrils were very close to those determined in whole fibre preparations from the same muscle types. 3 As in frog and rabbit skinned whole fibres, the maximal isometric force of the myofibril preparations decreases as MgATP concentration is increased. The maximal velocity of unloaded shortening ( V 0 ) depends hyperbolically on substrate concentration. V 0 extrapolated to infinite MgATP (3.6 ± 0.2 and 0.8 ± 0.03 l 0 s −1 in frog and rabbit myofibrils, respectively) is very close to that determined directly at high substrate concentration. The K m is 210 ± 20 μM for frog tibialis anterior and 120 ± 10 μM for rabbit psoas myofibrils, values about half those found in larger whole fibre preparations of the same muscle types. This implies that measurements in whole skinned fibres are perturbed by diffusional delays, even in the presence of MgATP regenerating systems. 4 In both frog and rabbit myofibrils, the K m for V 0 is about one order of magnitude higher than the K m for myofibrillar MgATPase determined biochemically in the same experimental conditions. This confirms that the difference between the K m values for MgATPase and shortening velocity is a basic feature of the mechanism of chemomechanical transduction in muscle contraction.