Contraction of glycerinated rabbit slow-twitch muscle fibers as a function of MgATP concentration.
We have measured the isometric tension and force-velocity relationships of glycerinated rabbit slow-twitch semimembranosus muscle as a function of MgATP concentration ([MgATP]) and have compared the results with those obtained previously from fast-twitch psoas muscle. We find that isometric tension decreases as [MgATP] increases. The magnitude of the decrease is not as great as observed in psoas. Maximum shortening velocity (Vmax) exhibits classical Michaelian saturation behavior with respect to [MgATP] with a Michaelis constant (Km) for half-maximal velocity of 18 microM and a value at saturating [MgATP] of 0.6 muscle lengths/s. Similar values were observed in fibers from soleus, another slow-twitch muscle. The corresponding values in rabbit psoas muscle are 150 microM and 1.6 lengths/s. Compared with psoas, in semimembranosus muscle Km decreases by a factor of approximately 10, whereas Vmax decreases by about a factor of 3. Thus, although in a nonphysiological regime, at low [MgATP], a "fast" muscle actually has a lower shortening velocity than a "slow" muscle.
Pate
E
E
Department of Pure and Applied Mathematics, Washington State University, Pullman 99164.
Lin
M
M
Franks-Skiba
K
K
Cooke
R
R
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D005990:Glycerol / Q000494:pharmacology*; D007537:Isometric Contraction; D007700:Kinetics; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism*; D009994:Osmolar Concentration; D011817:Rabbits; D012044:Regression Analysis
D000255:Adenosine Triphosphate; D005990:Glycerol