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The length of cooperative units on the thin filament in rabbit psoas muscle fibres
Journal article   Open access

The length of cooperative units on the thin filament in rabbit psoas muscle fibres

Wei Ding, Hideaki Fujita and Masataka Kawai
Experimental physiology, Vol.87(6), pp.691-697
11/2002
DOI: 10.1113/eph8702448
PMID: 12447449
url
https://doi.org/10.1113/eph8702448View
Published (Version of record) Open Access

Abstract

The length of cooperative units along the thin filament of rabbit psoas single muscle fibres was determined by reducing filament length by treatment with the thin filament severing protein, gelsolin, in the presence of Ca(2+) and 2,3-butanedione 2-monoxime (BDM). The average time for 50 % reduction in isometric tension was 6.7 min at 22 degrees C. The pCa-tension relationship was measured at 22 degrees C, pH 7.00 and ionic strength 200 mM, and the data were fitted to the Hill equation to determine the half-saturation point (K) and the cooperativity (n). Our results demonstrate that the cooperativity does not change much when the remaining isometric tension was in the range 20-100 %. The cooperativity quickly diminished when the remaining tension was reduced to less than 20 %. Our results further demonstrate that the change in the pK value was minimal and averaged 0.075 (less Ca(2+) sensitive) as the thin filament length was reduced. We infer from the first observation that the thin filament cooperativity extends up to 0.2 microm, which includes the maximum of about four basic cooperative units consisting of seven actin molecules, one tropomyosin dimer and one troponin complex. We infer from the second observation that the Ca(2+) sensitivity is slightly reduced by removal of the cooperative interaction between neighbouring cooperative units.
 Rabbits Gelsolin - pharmacology Animals Muscle Contraction - drug effects Psoas Muscles - drug effects Muscle Contraction - physiology Male Muscle Fibers, Skeletal - drug effects Muscle Fibers, Skeletal - physiology Psoas Muscles - physiology

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