Fiber type populations and Ca2+-activation properties of single fibers in soleus muscles from SHR and WKY rats

Electrophoretic analyses of muscle proteins in whole musclehomogenates and single muscle fiber segments were used to examine myosin heavy chain (MHC) and myosin light chain 2 (MLC2) isoform composition and fiber type populations in soleus muscles from spontaneously hypertensive rats (SHRs) and their age-matchednormotensive controls [Wistar-Kyoto (WKY) rats], at threestages in the development of high blood pressure (4 wk, 16 wk, and 24 wk of age). Demembranated (chemically skinned with 2% Triton X-100),single fiber preparations were used to determine the maximumCa²⁺-activated force percross-sectional area, calcium sensitivity, and degree of cooperativityof the contractile apparatus andCa²⁺-regulatory system withrespect to Ca²⁺. The results showthat, at all ages examined, 1) SHRsoleus contained a lower proportion of MHCI and MLC2 slow (MLC2s) and ahigher proportion of MHCIIa, MHCIId/x, and MLC2 fast (MLC2f )isoforms than the age-matched controls;2) random dissection of single fibers from SHR and WKY soleus produced four populations of fibers: type I (expressing MHCI), type IIA (expressing MHCIIa), hybrid typeI+IIA (coexpressing MHCI and MHCIIa), and hybrid type IIA+IID (coexpressing MHCIIa and MHCIId/x); and3) single fiber dissection from SHRsoleus yielded a lower proportion of type I fibers, a higher proportionof fast-twitch fibers (types IIA and IIA+IID), and a higher proportionof hybrid fibers (types I+IIA and IIA+IID) than the homologous musclesfrom the age-matched WKY rats. Because the presence of hybrid fibers isviewed as a marker of muscle transformation, these data suggest thatSHR soleus undergoes transformation well into adulthood. Our data showalso that, for a given fiber type, there are no significant differencesbetween SHR and WKY soleus muscles with respect to any of theCa²⁺-activation propertiesexamined. This finding indicates that the lower specific tensionsreported in the literature for SHR soleus muscles are not due tostrain- or hypertension-related differences in the function of thecontractile apparatus or regulatory system.