Insulin-like growth factor-1 prevents age-related decrease in specific force and intracellular Ca2+ in single intact muscle fibres from transgenic mice

Abstract

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 microM, 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.

Figure 1. Maximum specific force in FDB muscles from young adult and old, wild-type and IGF-1 transgenic mice

Tetanic contraction declined significantly in old wild-type compared to young, young transgenic and old IGF-1 transgenic mice (P < 0.05). No statistically significant differences were detected among young, young transgenic and old transgenic(P > 0.05). Twitch specific force was not significantly different among groups. #, statistically significant. Data values are represented as the mean +s.e.m.

Figure 2. Force-frequency and [Ca²⁺]-frequency relationships in single intact FDB muscle fibres from young and old wild-type and IGF-1 transgenic mice

Tension (A) and myoplasmic Ca²⁺ concentration (B) have been normalized to the maximum value for each fibre. Data points are expressed as means ±s.e.m. Results have been fitted to a single exponential function. No statistically significant difference was detected among groups (P > 0.05).

Figure 4. Maximum specific twitch force, tetanic force, and peak intracellular Ca²⁺ concentration in single intact FDB muscle fibres from young adult and old wild-type and IGF-1 transgenic mice

A, tetanic contraction declined significantly in old wild-type compared to young wild-type and transgenic and old transgenic mice (P < 0.05). No significant differences in twitch specific force were detected among the four groups. B, maximum intracellular Ca²⁺ concentration recorded in response to twitch or tetanic pulses in intact FDB fibres. #, statistically significant difference between the old wild-type and the other three groups (P < 0.05). No statistically significant difference was detected among young, young transgenic, and old transgenic mice. Data values are represented as means +s.e.m.

Figure 3. Contraction and intracellular Ca²⁺ recordings in single intact FDB fibres

Representative records of twitch (A) and tetanic (B) contractions and intracellular Ca²⁺. Traces in a and b correspond to contraction and intracellular Ca²⁺ transients, respectively, recorded in single intact FDB fibres from young and old wild-type and transgenic mice. Both maximum specific force and peak intracellular Ca²⁺ concentration are significantly lower in old wild-type than in young or old transgenic mice. Fibre tension is expressed as normalized values to cross-sectional area, in kilopascals. Ca²⁺ concentration is expressed in absolute values, as explained in Methods. Contraction and intracellular Ca²⁺ transients have the same time scale. Dashed lines indicate the force and myoplasmic Ca²⁺ baselines.

Figure 5. Effects of caffeine on maximum specific force and intracellular Ca²⁺ concentration in single intact fibres from young and old wild-type and transgenic mice

Specific force (A) and peak myoplasmic Ca²⁺ concentration (B) before and after 5 mm caffeine (within 30 min incubation). * and #, statistical significance within groups (before and after caffeine) and between old wild-type mice and the remaining groups before caffeine (P < 0.05), respectively. **, statistically significant difference between muscle fibres from young and old wild-type mice treated with caffeine.

Figure 6. Fibre-type composition in ageing and IGF-1 transgenic FDB muscle

Representative serial sections of FDB muscle immunostained with labelled antibodies for type I, IIa and IIb fibres from young and old wild-type and IGF-1 transgenic mice. Quantification of the relative percentages of muscle fibre subtypes is shown in Table 2.