In the present work we test the hypothesis that sustained transgenic overexpression of insulin‐like growth factor‐1 (IGF‐1) in skeletal muscle prevents age‐related decreases in myoplasmic Ca2+ concentration and consequently in specific force in single intact fibres from the flexor digitorum brevis (FDB) muscle from the mouse. Measurements of IGF‐1 concentration in FDB muscle showed higher levels in transgenic than in wild‐type mice at all ages. The specific tetanic force decreased significantly in single muscle fibres from old (286 ± 22 kPa) compared to young wild‐type (455 ± 28 kPa), young transgenic (423 ± 43 kPa), and old transgenic mice (386 ± 15 kPa) (P < 0.05). These results are consistent with measurements in whole FDB muscles. The peak Ca2+ concentration values in response to prolonged stimulation were: 1.47 ± 0.15, 1.70 ± 0.29, 0.97 ± 0.13 and 1.7 ± 0.22 μm, in fibres from young wild‐type, young transgenic, old wild‐type and old transgenic mice, respectively. The effects of caffeine on FDB fibres support the conclusion that the age‐related decline in peak myoplasmic Ca2+ and specific force is not explained by sarcoplasmic reticulum Ca2+ depletion. Immunohistochemistry in muscle cross‐sections was performed to determine whether age and/or IGF‐1 overexpression induce changes in fibre type composition. The relative percentages of type IIa, IIx and I myosin heavy chain (MHC) isoforms did not change significantly with age or genotype. Therefore, IGF‐1 prevents age‐related decline in peak intracellular Ca2+ and specific force in a muscle that does not exhibit changes in fibre type composition with senescence.

In the present study we measured, for the first time, the isometric specific force (SF, force normalized to cross sectional area) generated by single intact fibers from fast- (extensor digitorum longus, EDL) and slow-twitch (soleus) muscles from young adult (2-6), middle-aged (12-14) and old (20-24 month-old) mice. SF has also been measured in single intact flexor digitorum brevis fibers from young mice. Muscle fibers have been classified into fast- or slow-twitch based on the contraction kinetics. Maximum SF recorded in EDL and soleus fibers from young and middle-aged mice did not differ significantly. A significant age-dependent decline in maximum SF was recorded in EDL and soleus fibers from young or middle-aged to old mice. The SF was 377 +/- 18, 417 +/- 20 and 279 +/- 18 kPa for EDL fibers from young, middle-aged and old mice, respectively and 397 +/- 17, 405 +/- 24 and 320 +/- 33 kPa for soleus fibers from age-matched mice, respectively. The frequency needed to elicit maximum force in EDL and soleus fibers from middle-aged to old mice did not differ significantly. In conclusion, the specific force developed by both fast and slow-twitch single intact muscle fibers declines with aging and more significantly in the former.

IGF-1 is a potent growth factor for both motor neurones and skeletal muscle. Muscle IGF-1 is known to provide target-derived trophic effects on motor neurones. Therefore, IGF-1 overexpression in muscle is effective in delaying or preventing deleterious effects of ageing in both tissues. Since age-related decline in muscle function stems partly from motor neurone loss, a tetanus toxin fragment-C (TTC) fusion protein was created to target IGF-1 to motor neurones. IGF-1-TTC retains IGF-1 activity as indicated by [(3)H]thymidine incorporation into L6 myoblasts. Spinal cord motor neurones effectively bound and internalized the IGF-1-TTC in vitro. Similarly, IGF-1-TTC injected into skeletal muscles was taken up and retrogradely transported to the spinal cord in vivo, a process prevented by denervation of injected muscles. Three monthly IGF-1-TTC injections into muscles of ageing mice did not increase muscle weight or muscle fibre size, but significantly increased single fibre specific force over aged controls injected with saline, IGF-1, or TTC. None of the injections changed muscle fibre type composition, but neuromuscular junction post-terminals were larger and more complex in muscle fibres injected with IGF-1-TTC, compared to the other groups, suggesting preservation of muscle fibre innervation. This work demonstrates that induced overexpression of IGF-1 in spinal cord motor neurones of ageing mice prevents muscle fibre specific force decline, a hallmark of ageing skeletal muscle.

In the present work, we investigate whether changes in excitation-contraction (EC) coupling mode occur in skeletal muscles from ageing mammals by examining the dependence of EC coupling on extracellular Ca 2+ . Single intact muscle fibres from flexor digitorum brevis muscles from young (2-6 months) and old (23-30 months) mice were subjected to tetanic contractile protocols in the presence and absence of

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