Basic morphology and function of the TA muscles of different β-KO mouse strains.

<p>(A) Body masses for the different β-KO mouse strains (white bars) and their respective controls (black bars). No significant differences were observed across the strains. (B) Comparison of TA muscle mass in β-KO mouse strains (white bars) and their respective controls (black bars). There was no significant difference in TA mass for either β₁-KO or β₂-KO mice when compared with littermate controls, but β₁/β₂-KO mice had a significantly lower TA mass than control (*<i>P</i><0.05 compared with control, <i>t</i>-test, <i>n</i> = 5-6 per group). (C) and (D) maximal tetanic force production by TA muscles measured as either absolute force (P_o) or force normalized to muscle size (specific force, or sP_o). β₁-KO mice had no significant differences in maximal force whether expressed as either P_o or sP_o. In contrast, β₂-KO mice had significant deficits in maximal force when expressed as either P_o or sP_o. β₁/β₂-KO mice had significantly lower maximal forces than control when expressed as P_o, but there was no significant deficit in force production when force was normalized to muscle size (sP_o) (*<i>P</i><0.05 compared with control, <i>t</i>-test, <i>n</i> = 5–6 per group). (E) Comparison of cross-sectional area (CSA) of TA muscle fibers from β-KO mouse strains (white bars) and their respective controls (grey bars). Neither β₁-KO nor β₂-KO mice had significant changes in muscle fiber CSA compared with their wild-type littermates, but fiber CSA was significantly lower in TA muscles from β₁/β₂-KO mice compared with control. Data for each strain were pooled from >1500 fibers from <i>n</i>≥9 mice (*<i>P</i><0.05, Mann-Whitney test by ranks).