title: Force relaxation, labile heat and parvalbumin content of skeletal muscle fibres of Xenopus laevis.
abstract: 1. Measurements were made of stable (hb) and labile (ha) maintenance heat rate, slowing of relaxation as a function of tetanus duration, and parvalbumin (PA) content in intact single muscle fibres of types 1 and 2 from Xenopus laevis. The majority of experiments were performed at 20 degrees C. In addition, total and myofibrillar ATPase activity was measured in skinned Xenopus fibres, also of types 1 and 2; these studies were performed at 4 degrees C. 2. In agreement with a previous study hb was significantly higher in type 1 (175 +/- 13 mW (g wet wt)-1; n = 8) than in type 2 fibres (88 +/- 9 mW (g wet wt)-1; n = 7). The value of ha was 236 +/- 22 and 117 +/- 16 mW (g wet wt)-1, respectively (mean +/- S.E.M.). ha decayed with a time constant of 0.27 +/- 0.02 (n = 8) and 0.33 +/- 0.02 s (n = 7). 3. The early relaxation rate of tetanic force, extrapolated to the onset of stimulation (yo + yb; where yo is 'extra' rate of relaxation and yb steady rate) was 85.6 +/- 4.2 s-1 for type 1 fibres (n = 8) and 62.7 +/- 7.3 s-1 for type 2 fibres (n = 7). Relaxation rate at the end of a 1.8 s tetanus (yb) was 29.4 +/- 1.6 and 33.3 +/- 1.5 s-1, respectively; thus, there was more slowing with tetanus duration in type 1 fibres. The time constant for slowing of relaxation with tetanus duration was similar to that for decay of ha. 4. Parvalbumin concentration, [PA], was 0.45 +/- 0.04 mM in type 1 (n = 7) and 0.22 +/- 0.04 mM (n = 7) in type 2 fibres. 5. For individual fibres positive correlations were found between the 'extra' rate of relaxation (yo), labile heat (ha) and [PA]. Significantly more labile heat was liberated than can be accounted for by the enthalpy change of Ca2+ binding to PA. 6. For five fibres (type 1) studied both at 20 and 10 degrees C, the magnitude of slowing of relaxation, expressed as yo/(yo + yb), was 0.58 +/- 0.03 at 20 degrees C and 0.65 +/- 0.03 at 10 degrees C. 7. Both slowing of relaxation and labile heat were depressed in the second of two closely spaced tetani in type 1 fibres. Repriming of both effects followed similar, biphasic time courses and required more than 10 min for completion at 20 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Physiology II, Karolinska Institutet, Stockholm, Sweden.
authors.forename: J
authors.initials: J
authors.lastname: Lännergren
authors.forename: G
authors.initials: G
authors.lastname: Elzinga
authors.forename: G J
authors.initials: GJ
authors.lastname: Stienen
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004558:Electric Stimulation; D004591:Electrophoresis, Polyacrylamide Gel; D004734:Energy Metabolism / Q000502:physiology; D005260:Female; D006358:Hot Temperature; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000502:physiology; D009126:Muscle Relaxation / Q000502:physiology; D009132:Muscles / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D009210:Myofibrils / Q000201:enzymology; D010320:Parvalbumins / Q000378:metabolism*; D011485:Protein Binding; D014982:Xenopus laevis
chemical_list: D010320:Parvalbumins; D000251:Adenosine Triphosphatases; D002118:Calcium