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 muscle homogenates and single muscle fiber segments were used to examine myosin heavy chain (MHC) and myosin light chain 2 (MLC2) isoform composition and fiber type populat ions in soleus muscles from spontaneously hypertensive rats (SHRs) and thei r age-matched normotensive controls [Wistar-Kyoto (WKY) rats], at three sta ges in the development of high blood pressure (4 wk, 16 wk, and 24 wk of ag e). Demembranated (chemically skinned with 2% Triton X-100), single fiber p reparations were used to determine the maximum Ca2+-activated force per cro ss-sectional area, calcium sensitivity, and degree of cooperativity of the contractile apparatus and Ca2+-regulatory system with respect to Ca2+. The results show that, at all ages examined, 1) SHR soleus contained a lower pr oportion of MHCI and MLC2 slow (MLC2s) and a higher proportion of MHCIIa, M HCIId/x, and MLC2 fast (MLC2f) isoforms than the age-matched controls; 2) r andom dissection of single fibers from SHR and WKY soleus produced four pop ulations of fibers: type I (expressing MHCI), type IIA (expressing MHCIIa), hybrid type I+IIA (coexpressing MHCI and MHCIIa), and hybrid type IIA+IID (coexpressing MHCIIa and MHCIId/x); and 3) single fiber dissection from SHR soleus yielded a lower proportion of type I fibers, a higher proportion of fast-twitch fibers (types IIA and IIA+IID), and a higher proportion of hyb rid fibers (types I+IIA and IIA+IID) than the homologous muscles from the a ge-matched WRY rats. Because the presence of hybrid fibers is viewed as a m arker of muscle transformation, these data suggest that SHR soleus undergoe s transformation well into adulthood. Our data show also that, for a given fiber type, there are no significant differences between SHR and WKY soleus muscles with respect to any of the Ca2+-activation properties examined. Th is finding indicates that the lower specific tensions reported in the liter ature for SHR soleus muscles are not due to strain- or hypertension-related differences in the function of the contractile apparatus or regulatory sys tem.