Stable maintenance heat rate and contractile properties of different single muscle fibres from Xenopus laevis at 20 degrees C.

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Abstract
1. Different types of fibres were isolated from the iliofibularis muscle of Xenopus laevis. Resting length was adjusted to obtain a sarcomere length of 2.3 .mu.m as judged by laser diffraction. Heat production was measured during tetanic contractions lasting 0.3-2 s at 20.degree. C. From twenty fibres the force-velocity relationship was determined as well. 2. After correcting the records for heat loss, and taking the relevant heat capacities into account, stable maintenance heat rate was determined by fitting the corrected heat records from 0.35 s after the onset of contraction onwards by a straight line. The value obtained was then normalized on the dry weight of the fibre. The force-velocity relationships were fitted according to Hill''s equation, yielding values for a and Vmax (Hill, 1938). 3. Stable maintenance heat rate (hb) depended on fibre type and ranged from 0.05 to 0.86 W g-1 dry wt. Isometric tetanic force per cross-sectional area (P0) varied between 190 and 427 kN m-2. Therefore the variations in hb were not proportional to the variations in P0. 4. The maximum velocity of shortening (Vmax) differed considerably from fibre to fibre (4.6-10.3 lengths s-1). Between Vmax and hb a non-linear relationship was found. The curvature of this relationship was such that its slope (.DELTA.hb/.DELTA.Vmax) increased with Vmax. 5. A proportional relationship was found between the rate of force redevelopment, following a period of rapid shortening and hb. 6. Maximum power output during loaded shortening as derived from the force-velocity (P-V) relationships was on average 2.2 times higher than hb. 7. The curvature of the force-velocity relationship, as reflected by the value of a/P0, varied between fibres from 0.18 to 0.53. A larger value of a/Po, i.e. a less curved relationship, corresponded with a larger value of hb. 8. Since hb reflects the rate at which ATP is hydrolysed during contraction, the results suggest that the maximum velocity of shortening and the in vivo actomyosin ATPase under isometric conditions are non-linearly related. This would imply that these two variables are not dominated by the same rate-limiting step of the cross-bridge cycle.