Local Changes of IGF-I mRNA, GH Receptor mRNA, and Fiber Size in Rat Plantaris Muscle Following Compensatory Overload.

Abstract

Skeletal muscle is hypertrophied by compensatory overload [1][2][3]. The magnitude of the hypertrophy after compensatory overload is different between the regions in a rodent muscle [4]. In humans, region-specific hypertrophy exists in consecutive cross-sectional areas of the elbow flexors [5]. Region-specific hypertrophy in a muscle is related to the different muscle fiber recruitment pattern [6,7]. However, what causes region-specific hypertrophy is unclear.Many studies have shown that growth hormone Japanese Journal of Physiology, 53, 53-60, 2003 Key words: IGF-I, GH receptor, hypertrophy, hypophysectomy, muscle fiber. Abstract:We investigated whether local insulin-like growth factor-I (IGF-I) mRNA and growth hormone (GH) receptor mRNA expressions in plantaris muscle were related to the region-specific hypertrophy following compensatory overload. Adult male normal or hypophysectomized rats were subjected to unilateral distal-half removals of the gastrocnemius and soleus muscles. The contralateral hindlimb was used as the control. Two weeks later, fiber areas in the distal and proximal parts of the plantaris muscle were measured. All the fiber areas measured in the distal part of the plantaris muscle in normal and hypophysectomized rats were significantly increased following the compensatory overload. In the proximal part, fiber areas of type I, IIA, and IIC were significantly increased, but fiber area of type IIB did not change. IGF-I mRNA expressions in the distal and proximal parts were increased 3 d after the compensatory overload in normal and hypophysectomized rats. The increase of IGF-I mRNA expression in the distal part 3 d after compensatory overload was greater than those in the proximal part. IGF-I mRNA expressions in the distal and proximal parts were increased 14 d after the compensatory overload in hypophysectomized rats, but not in normal rats. GH receptor mRNA expressions were decreased following compensatory overload, and almost disappeared 14 d after the compensatory overload in hypophysectomized rats. Thus muscle fiber hypertrophy following compensatory overload was different among the parts in a muscle and IGF-I mRNA was expressed in concert with the region-specific hypertrophy, but not GH receptor mRNA.

Systematic review of the synergist muscle ablation model for compensatory hypertrophy

“…Tenotomy has the disadvantage of the rapid reconnection of the cut tendon, which limits functionality. 65 Most commonly studied muscles Compensatory hypertrophy by the ablation of synergists is an efficient model for studies on muscle hypertrophy, 2,3,7,[10][11][12][13][14][15][16][17][18][19][20] as the fast increase in muscle mass reduces the duration of the experiment. In recent years, changes have occurred in the standard of surgery (bilateral or unilateral) and the relationship between the number of days and increase in muscle mass, which justifies this systematic review.…”

“…9 Unilateral vs. bilateral surgery Based on the data analyzed, both unilateral and bilateral synergist ablation lead to an increase in muscle mass, with no statistically significant difference between the two types of surgery. Researchers working with unilateral surgery report a mean increase in muscle mass of 46.8 ± 2.6% 14 days following synergist ablation, 4,12,61,63 whereas those working with bilateral surgery report an increase of 52.3 ± 3.1% in the same period. 4,12,61,63 Although the number of studies involving unilateral ablation (n=4) was smaller than the number involving bilateral ablation (n=14), the similar increase in muscle mass demonstrates the benefits of unilateral surgery, which reduces the number of animals used in experiments and is in line with the goals of the International Council for Laboratory Animal Sciences.…”

“…The muscle adaptation process can be induced by stretching/ immobilization, 44,46 compensatory mechanisms (chronic). 1,[2][3][4][5][6]8,9,12,14,[18][19][20][61][62][63][64][65] and exercise/training. 33,46 Evidence of this is derived from a large number of studies demonstrating that overload leads to an increase in muscle mass and cross-sectional area of the muscle fibers and induces chronic changes in the balance between the synthesis and degradation of proteins.…”

“…2,[7][8][9] Compensatory hypertrophy through the ablation of synergists of plantar flexion is one of the ways to produce chronic overload experimentally. 3,7,12,13,20,59 The ablation of synergists for compensatory hypertrophy consists of the surgical removal of all or part of synergistic muscles, which can be either unilateral or bilateral, to generate chronic functional overload that causes hypertrophy. 3,7,12,13,20,59 According to Parvaresh et al, 1 complete muscle removal can compromise the neurovascular supply, which increases edema and the recovery of the animal in the postoperative period.…”

“…2,3,7,[10][11][12][13][14][15][16][17][18][19][20] The most studied muscles are plantar flexors in the rear paw of rats. As skeletal muscle has different types of fibers (type I [slow--twitch] and type II [fast-twitch -IIa, IIb, IIx/IId]), 21 a number of authors justify the choice of the plantaris muscle due to its diversity of fiber types (type I: 8 ± 2%; type IIA: 19 ± 3%; type IIB/D: 74 ± 4%) and its different adaptation possibilities.…”