EFFECT OF THYROID STATUS ON SOLEUS AND TFL (WHOLE) MUSCLES

MYOFIBRILLAR ATPase ACTIVITY

The weight of the hyperthyroid anim als (Table 8.1) w as slightly (9%) b u t significantly (P < 0.05) higher th an the hypothyroid anim als. The

basal metabolic rate w as lower (2%) in hypo- th an hyperthyroid anim als as reflected by the decreased core tem perature of hypo- anim als with respect to hyperthyroid anim als, although it was non-significant.

Table 8.1: Animal and muscle weights of hyper- and hypothyroid rats.

Hyperthyroid Hypothyroid Rat weight (g) Core temperature (®C) 4 0 7 . 0 0 ± 1 1 . 5 1 (N = 6) * 3 7 3 . 1 7 ± 1 5 . 0 9 (N = 6) 3 7 . 0 0 ± 0 . 2 6 3 6 . 3 3 ± 0 . 1 7 Soleus weight (mg) Soleus/body ratio (mg/g) 1 6 6 . 2 0 ± 6 . 3 0 1 5 7 . 8 0 ± 4 . 9 0 0 . 4 1 ± 0 . 0 2 0 . 4 2 ± 0 . 0 2 TFL weight (mg) TFL/body ratio (mg/g) 3 1 3 . 4 0 ± 1 4 . 3 0 3 0 3 . 9 0 ± 1 1 . 7 0 0 . 7 7 ± 0 . 0 4 0 . 8 1 ± 0 . 0 5 * P < 0.05, hyper- vs. hypothyroid

The m ean soleus or TFL m uscle weights were not significantly affected and therefore there w as no real effect on the m uscle w eight/body weight ratio betw een hyper- and hypothyroid anim als. This resu lt is co n sisten t w ith resu lts obtained by previous investigators (Nicol & Bruce, 1981; Nicol & Johnston, 1981; Nicol & Maybee, 1982) who used sim ilar regimes to produce hypo- or hyperthyroidism as in the present study. Thus, this result suggests th a t the growth and development of the anim als was effectively norm al, w ithout any net anabolic or catabolic effects on either the rats as a whole or the individual m uscles, despite

the differing thyroid hormone levels.

For the soleu s m u scles, the M g2+, EGTA myofibrillar ATPase

activity (Table 8.2) w as 100% higher (p < 0.005) in hyper- than hypothyroid

anim als, whereas for the TFL m uscles it w as 41% higher (significant at

the one tailed t-test level, P < 0.05). Similarly, the Mg2+ activated Ca2+

regulated myofibrillar ATPase activity (Table 8 .2 , Fig. 8.1 ) of the soleus

m uscles w as 81% higher (P < 0.05) in hyper- than hypothyroid animals,

w hereas for the TFL m u scles it w as 16% higher (non-significant) in

hyper- than hypothyroid animals.

Table 8.2: Mg2+, EGTA and Mg2+ activated Ca2+ regulated myofibrillar ATPase activity of soleus and TFL muscles from hyper- and hypothyroid animals. Activity units are pmoles of Pi/mg of protein/min and yield is expressed as mg of protein/g muscle. Values are mean ± SEM from between 5 and 6 animals.

Hyperthyroid Hypothyroid Soleus EGTA activity 0 . 0 7 6 ± 0 . 0 0 8 + 0 . 0 3 8 ± 0 . 0 1 0 Ca2+ activity 0 . 1 7 6 ± 0 . 0 1 6 * 0 . 0 9 7 ± 0 . 0 0 9 Ca2+ sensitivity (%) 58.20 ± 1 . 9 1 6 4 . 5 3 + 5. 80 yield 8 7 , 7 4 ± 3 . 2 7 7 8 . 8 3 ± 6 . 2 5 TFL EGTA activity 0. 097 ± 0 . 0 1 2 0.069 ± 0 . 0 0 6 Ca2+ activity 0 . 4 1 8 ± 0 . 0 3 7 0 . 3 5 9 ± 0.039 Ca2+ sensitivity (%) 7 6 . 0 9 ± 2 . 7 6 8 2 . 6 2 ± 4 . 5 8 yield 8 7 . 2 3 ± 3 . 3 1 85. 82 ± 3. 81 * P < 0.05, P < 0.005, hyper- vs. hypothyroid

Fig. 8.1: Mg2+ activated Ca2+ regulated myofibrillar ATPase activity of hyper- {--- *---) and hypothyroid soleus ( m uscles and of hyper- (--- *---) and hypothyroid TFL (...z s r - - - - ) muscles. The Ca2+ activated myofibrillar ATPase activity of the myofibrils was calculated from the slope of the Initial straight line portion of the curves. Values are mean ± SEM (which are indicated by the vertical bars or the size of the symbols used) from between 5 and 6 animals.

TFL “8

I

I

The 2 .3 - 3 .7 fold higher myofibrillar ATPase activity in fast hyper- and hypothyroid m uscles com pared to slow hyper- and hypothyroid m uscles observed in the present study agrees favourably with the study of Baldwin et al. (1977) who found a 2.3 - 3 .5 fold increase in fast euthyroid m uscles compared to slow euthyroid muscles.

The 81% Increase in th e (Mg2+ activated C a 2 + regulated)

m yofibrillar ATPase activity in hyper- com pared to hypothyroid soleus m uscles is in quantitative agreem ent with the result obtained by Nwoye et al. (1982) who found myofibrillar ATPase activity to be 82% higher. Furtherm ore, the resu lt is also in quantitative agreem ent w ith the resu lts of investigators (lanuzzo et al., 1977, 1980 Jo h n sto n et al.,

Although, the present result contradicts the results of Fitts et al.

(1980) who found no change in Mg2+ activated actomyosin ATPase activity

in hyper- compared to euthyroid soleus m uscles. The results obtained by Fitts et al. (1980) could be a consequence of com paring hyper- w ith euthyroid m uscles as opposed to hyper- w ith hypothyroid, w here the results could become significant. On the other hand, a recent study, by Fitzsim ons et al. (1990) found no difference in the myofibrillar ATPase activity betw een hyper- and hypothyroid soleus m uscles. The reason for this discrepancy does not seem apparent.

In fast m uscle, no previous results are known to exist for the TFL m uscle, b u t resu lts obtained on other fast m uscles, are in general agreem ent with the results obtained for the TFL m uscles in the present study. For example, a fast m uscle such as the (white) gastrocnem ius has show n either a non-significant increase (8%) in the myofibrillar ATPase activity in hyper- com pared to hypothyroid m uscles (Fitzsimons et al.,

1990) or a non-significant decrease in both the m yofibrillar ATPase

activity (14%) and the Ca2+ activated myosin ATPase activity (9%) of

hypothyroid m uscles com pared to euthyroid m uscles (Leijendekker &

Hardveld, 1987). Furtherm ore, lanuzzo et al. (1980) found the Ca2+

activated myosin ATPase activity of the fast plantaris m uscle, relatively unchanged in hyper- compared to hypothyroid muscles.

However, m easurem ents of myofibrillar ATPase activity in the (red) gastrocnem ius m uscle have show n a 53% (significant) increase in the

myofibrillar ATPase activity of hyper- compared to hypothyroid m uscles

(Fitzsimons et al., 1990). Moreover, m easurem ents of Ca2+ activated

m yosin ATPase activity in the fast EDL m uscle have show n a 34%

(significant) increase in the Ca2+ activated m yosin ATPase activity of hyper- com pared to hypothyroid m uscles (Nwoye & M om m aerts, 1981).

Although, these results contradict the ones obtained for the TFL m uscles in the p resen t stu d y an d previous stu d ies (lanuzzo et ah, 1980; Leijendekker & Hardveld, 1987; Fitzsim ons et al., 1990) they can be explained as follows:

In the red gastrocnem ius and the EDL m uscles the m ajority fibre type is FOG w hereas in the TFL, p lantaris and w hite gastrocnem ius m uscles the m ajority fibre type is FG (Ariano et al., 1973; A rm strong & Phelps, 1984). Therefore, the difference in sensitivity of thyroid levels to the ATPase reaction seem s to lie in the fibre types, and to this extent m uscles composed prim arily of FG fibres are less likely to be affected th an m uscles composed of FOG fibres because investigators (e.g., Nwoye & M om m aerts, 1981) have found fast m uscle to be less responsive to thyroid horm one levels th a n slow m uscle, and sim ilarly it h as been reported th a t w ithin fast m uscles, the fast m uscles composed prim arily of FG fibres are even less responsive th a n fast m uscles com posed prim arily of FOG fibres (Sickles et al., 1987; Fitzsimons et al., 1990) and hence the apparent contradiction in the results.

The Ca2+ sensitivity betw een hyper- and hypothyroid m uscles of the soleus or the TFL w as unchanged. This concurs w ith the resu lts obtained by Leijendekker & Hardveld (1987) who found no significant effect on the Ca2+ sensitivity of m yofibrillar ATPase activity betw een hypo- and euthyroid m uscles. The unchanged Ca2+ sensitivity between hyper- and hypothyroid m uscles of the soleus or the TFL m uscles tends to suggest th at changes in the ATPase activity were due to the contractile proteins alone, as there w as no significant loss of calcium sensitivity. Furtherm ore, since actin is highly conserved (Vanderckhove & Weber, 1979) th en the ATPase activity changes are specifically due to changes in the m yosin molecule and consequently due to the isoform s of myosin

heavy and light chains.

The yield of myofibrillar pro tein /g of m uscle w as not significantly different betw een hyper- and hypothyroid soleus or TFL m uscles, and there w as also no significant difference betw een the soleus and TFL m uscles, irrespective of thyroid sta tu s (i.e. no significant difference betw een the four groups studied). This agrees w ith resu lts obtained by previous investigators (lanuzzo et al., 1977, 1980; Fitts et al., 1980) who found no change in the actom yosin or myosin yield either from slow or fast hypo- and hyperthyroid muscles.

This finding reinforces the above conclusion of no significant change in the growth and developm ent of the anim als. Secondly it suggests th a t mild dysthyreosis does not change the contractile m aterial in either the soleus or TFL m uscles to any significant effect. Thirdly it concurs w ith resu lts of previous investigators (e.g.. Close, 1972) who have show n no significant difference in the contractile m aterial in slow and fast euthyroid muscles.