Increased extraocular muscle strength with direct injection of insulin-like growth factor-I

Abstract

Purpose: Previous work has demonstrated the effectiveness of insulin-like growth factor (IGF)-II in increasing force generation in extraocular muscle (EOM). Studies in the literature have suggested that IGF-I would be even more effective than IGF-II. This study was performed to assess the effects on muscle mass and force generation of IGF-I injection in adult rabbit superior rectus muscle.

Methods: Adult rabbits received a single injection of IGF-I at one of several doses into one superior rectus muscle. One week after treatment, the rabbits were euthanatized, and the superior rectus muscle from each orbit was removed. Force generation was measured using an in vitro apparatus, and injected muscles were compared with the contralateral control. A second group of animals were injected similarly, and the muscles were examined at 1 week for changes in cross-sectional area of individual myofibers.

Results: EOMs demonstrate significant numbers of cells expressing the IGF receptor. After the EOMs were injected with IGF-I, there were significant increases both in muscle force generation and cross-sectional area at all doses tested in this study. Doses of 10 and 25 microg IGF-I were most effective.

Conclusions: Direct muscular injection of IGF-I effectively increases EOM force generation without the potential biomechanical hazards of surgery such as permanently altered muscle length or insertional position on the globe.

Figure 1

(a) Orbital layer of normal superior rectus muscle and (b) normal tibialis anterior muscle, both immunostained for expression of the insulin-like growth factor receptor, and (c) global layer of normal superior rectus muscle double stained for IGF-R and laminin. Arrows: indicate cells positive for the IGF-receptor. Bar: (a, b) 50 μm; (c) 20 μm.

Figure 2

Effect of a single injection of 10, 25, or 50 μg IGF-I on myofiber cross-sectional area 1 week after treatment. *Significant difference from control.

Figure 3

Representative records of twitch (a) and tetanic (b) contractions of control and IGF-I-treated superior rectus muscles 1 week after injection.

Figure 4

Force generation, expressed as stress, of superior rectus muscles 1 week after a single injection with 1 (a), 5 (b), 10 (c), 25 (d), or 50 (e) μg IGF-I at twitch and 10-, 20-, 40-, 100-, 150-, and 200-Hz stimulation frequencies. (f) Control rabbits injected with saline only. n equals six for control and treated muscles. *Significant difference from control.

Figure 5

Effect of dose of IGF-I on force generation, expressed as stress, at both twitch and tetanic stimulation (200 Hz). All doses resulted in significantly increased force generation compared with control muscles (*). Both 10 and 25 μg resulted in significantly greater force generation than 1, 5, or 50 μg (**).

Figure 6

Fatigability was determined by stimulation of control and IGF-I-treated superior rectus muscles at 150 Hz every 2 seconds for 600 seconds or until there was a 50% reduction in generated muscle force. This is a representative trace from one experiment, in which 10 μg IGF-I was injected in the left superior rectus and saline only in the right superior rectus muscle.

Figure 7

Effect of 25- and 50-μg IGF-I injections on the percentage of myofibers positive for neonatal MyHC expression (a) and developmental MyHC expression (b) compared with control superior rectus muscles. *Significant difference from controls.