title: Relation between force and calcium ion concentration in different fibre types of the iliofibularis muscle of Xenopus laevis.
abstract: Calcium activated isometric force was measured in segments of single muscle fibres of the iliofibularis muscle of Xenopus laevis skinned by freeze-drying. A subdivision in five different fibre types was made, based on the location of the fibres inside the muscle, fibre diameter and a quantitative histochemical assay for succinate dehydrogenase activity. The Ca2+ sensitivity was characterized by fitting a Hill curve to the force levels reached at different Ca2+ concentrations. The parameter n of this equation indicates the steepness and pK the midpoint of this force-pCa relation. A considerable variability in the Ca2+ sensitivity characteristics was found between different fibres. The parameter n varied between 1.1 and 4.2 while pK varied between 5.5 and 6.6. The distribution of the data indicates the presence of three groups with different Ca2+ sensitivity; a group of fibres with low Ca2+ sensitivity but with considerable variation of the steepness of the Ca2+ sensitivity curves (type 1 fibres), an intermediate group (type 2, 3 and 4 fibres) with also considerable variation in steepness of the Ca2+ sensitivity curves, in which the lowest values for n are found in type 3 and 4 fibres and a group with high Ca2+ sensitivity and low n containing at least one tonic (type 5) fibre. At sub-saturating Ca2+ concentrations occasionally a transient decrease of the rate of force development was found which resembled the force oscillation reported for some mammalian muscle fibres.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D005260:Female; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects*; D009132:Muscles / Q000201:enzymology; D013385:Succinate Dehydrogenase / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D013385:Succinate Dehydrogenase; D002118:Calcium