The pCa-tension characteristics of single skinned fibres isolated from the anterior and posterior latissimus dorsi muscles of the chicken
Journal of Experimental Biology
The anterior latissimus dorsi (ALD) muscles of the chicken are mostly composed of multiply innervated slow fibres, and the posterior latissimus dorsi (PLD) muscles consist mostly of focally innervated fast fibres (Ginsborg, 1960) . The rate of development of isometric tension is 8-10 times higher in the PLD than in the ALD, and the unloaded speed of shortening is 4-5 times higher (Canfield, 1971). On tetanic stimulation ALD fibres are able to maintain a steady isometric tension for more than 2
... on for more than 2 min, whereas the PLD begins to fatigue after only 1 s (Canfield, 1971). The fatigue resistance and economy (Goldspink, Larson & Davies, 1970) of the ALD when contracting isometrically are probably related to its postural role in holding the wings against the body. The rate of relaxation of PLD fibres following short tetani is eight times faster than for the ALD (Canfield, 1971). This difference is attributable in part to the less highly developed sarcoplasmic reticulum in the ALD (Page, 1969) . The present study investigates the pCa-tension relationship of these fibres. Male chickens (breed Thornber 404: Light Sussex cross Rhode Island Red), aged 90-100 days were used in all experiments. Single fibre segments (~3mm length, 50-100/im diameter) were dissected from the anterior and posterior latissimus dorsi muscles. Fibres were isolated and chemically skinned for 30 min with 1 % Brij (polyoxy-ethylene 20 cetyl ether) in relaxing solution as described previously (Altringham & Johnston, 1982) . The basic relaxing solution contained 10 mM imidazole pH6-95 (at 35 °C), H0mM-KCl, 3mM-MgCl 2 , 5rnM-EGTA, 10 HIM phosphocreatine, 2-5mM-ATP and 20/igml" 1 creatine phosphokinase (Sigma, Poole, England) . Activating solutions were made by addition of 0-5 mM-CaCh to the basic relaxing solution.