Specific tension measurements in single soleus and medial gastrocnemius muscle fibers of the cat

Abstract

Direct measurements of the sizes of and forces produced by single fibers of the cat soleus and medial gastrocnemius muscles were made to determine whether or not different fiber types have characteristically distinct specific tensions. Single fibers (5-mm lengths), whose sarcolemmas had been chemically removed using a 5-mM EGTA “skinning” solution, were attached to a photodiode force transducer. Each single fiber was first placed in “relaxing” solution (22 ± 1°C, pH 7.0, pCa 8), its sarcomere length set at 2.7 μm using its laser diffraction pattern, and its diameter measured using the calibrated graticule of a microscope eyepiece (±2 μm). Subsequently, each fiber was transferred to an activating bathing solution ( pCa 3.6) in which the fiber produced its maximum tension. The specific tension values for single soleus muscle fibers displayed a threefold range (1.19 to 3.53 kg/cm 2) with a mean value of 2.30 ± 0.61 (SD) kg/cm 2 ( N = 42). The medial gastrocnemius fibers studied had a fourfold range in specific tensions (1.05 to 4.47 kg/cm 2) and a mean value of 2.42 ± 0.61 kg/cm 2 ( N = 104). Many medial gastrocnemius fibers ( N = 64) were type-identified using a standard actomyosin ATPase histochemical assay. Type I medial gastrocnemius fibers had mean specific tension values of 2.45 ± 0.47 kg/cm 2 ( N = 18), whereas, type II single fibers had mean specific tension values of 2.43 ± 0.67 kg/cm 2 ( N = 46). Our results suggest that there is no significant difference between the specific tensions of the different muscle fiber types within the cat medial gastrocnemius muscle.