CONCLUSIONS

The m ain finding of th is project h as b een th a t significant differences in m axim al isom etric ten sio n , m axim um velocity of shortening and T /pC a relationships of single sldnned fibres from mildly hypo- and hyperthyroid rats have been observed. Moreover, these changes have been observed in the sam e fibre type as identified histochemically. Furtherm ore, these changes occurred to a sim ilar extent in both the soleus and TFL m uscle fibres.

The increased maximal isometric tension and decreased m axim um velocity of sh o rte n in g of hypothyroid fibres in co m p ariso n to hyperthyroid fibres resem bles the p attern observed w hen com paring euthyroid soleus fibres with euthyroid TFL fibres. Moreover, the observed right hand shift in the T /pC a relationship of hyperthyroid soleus m uscle fibres w ith respect to hypothyroid soleus m uscle fibres resem bles the reported (e.g. Danieli-Betto et al., 1990) right hand shift of fast m uscle fibres with respect to slow m uscle fibres, along with other characteristics such as the steepness of the T /pC a relationship.

Therefore, a tentative hypothesis based on these resu lts is th a t mild hypo- and hyperthyroidism changes the dynamic equilibrium of the regulatory and contractile proteins from slow to fast in the direction hypo- to hyperthyroid and from fast to slow in the direction hyper- to hypothyroid. More specifically it is suggested th a t different isoforms of m yosin light chains are m ainly responsible for the changes in maximal isom etric ten sio n and m axim um velocity of shortening, since these changes occurred in the sam e type of fibre as identified histochemically.

F u rth er evidence for th is hypothesis comes from experim ents undertaken in conditions of acidic pH and increased inorganic phosphate concentration.

In conditions of acidic pH (pH 6 .6), the results in com parison to n eu tral pH for hypo- and hyperthyroid fibres were generally consistent w ith those reported (e.g. Fablato & Fabiato, 1978; Cooke et al., 1988) in euthyroid fibres, i.e. a decrease in m axim al isom etric ten sio n and m axim um velocity of shortening, and a shift to the right for the T /pC a relationship.

Similarly, a t increased inorganic phosphate concentration (7.5mM

Pi) the results in com parison to norm al conditions (i.e. OmM) for hypo- and hyperthyroid fibres were generally consistent w ith those reported (e.g. B randt et al., 1982; Cooke et al., 1988) in euthyroid fibres, i.e. a decrease in maximal isometric tension, a shift to the right for the T /pC a relationship and an unchanged m axim um velocity of shortening.

Thus, these results in conditions mimicking fatigue are consistent w ith a change in the dynam ic equilibrium of the regulatory and

contractile proteins.

An interesting resu lt w as the effect of su p ra m axim al calcium concentrations to reduce m axim al isometric tension of both soleus and TFL m uscle fibres in the p resence of either inorganic p h o sp h ate concentration (7.5mM) or reduced pH (pH 6 .6). This effect of calcium was

probably unm asked due to the effect of dysthyreosis and it m ay use a m echanism (most probably by decreasing the num ber of active cross bridges) in the fibre to show th a t fu rth er activity will lead to fibre dam age. A decrease due to su p ra m axim al calcium concentrations at acidic pH - to a lesser extent - h as been reported in cardiac cells (Fabiato & Fabiato, 1976; Godt & Nosek, 1989) b u t not in m uscle fibres.

On the whole m uscle level, myofibrillar ATPase activity w as found to be higher in hyper- as opposed to hypothyroid m uscles irrespective of m uscle type. It w as also found th a t a t the group level a higher m yofibrillar ATPase activity in hyperthyroid soleus m uscles correlated

w ith a greater percentage of FOG fibres, w hilst a higher myofibrillar ATPase activity in hyperthyroid TFL m uscles correlated with a greater p ercen tag e of FG fibres in co m p ariso n w ith th e hypothyroid counterparts.

Moreover, the results of myofibrillar ATPase activity correlate well w ith the m axim um velocity of shortening values obtained at the single fibre level. This suggests th at the relationship between m axim um velocity

of shortening and myofibrillar ATPase activity holds in anim als which are

hypo- or hyperthyroid.

As an hypothesis, it is proposed th a t w ithin the three broad categories of fibre types as identified histochemically a continuum exists (due to the m any possible com binations of the different isoforms of the p ro tein s of the co n tractile and th e reg u lato ry a p p a ra tu s) and dysthyreosis changes the dynamic equilibrium of this continuum which is reversible since it is known th a t the euthyroid state can be restored easily from both states (Ramsay, 1974) and due to the plasticity of m uscle (Fig 9.1.).

Fig. 9.1: A possible mechanism of change induced by hyper- or hypothyroid treatment.

^

8 0

^— ►FOG^*— ►FG^

No m atter w hat the fibre type e.g., w hether it is between the SO or FOG type, it will shift to the left in the hypothyroid state and to the right in the hyperthyroid state, th u s changing the contractile function of the fibre accordingly. Obviously, if the fibre is at one extrem e then fu rth er change in th a t sam e direction is limited, by the sam e token change in the other direction is m uch greater. As to where a fibre lies along the co ntinuum depends on its com position w ith regards to contractile and regulatory proteins.

One possible m echanism to explain th e change seen w ith dysthyreosis is th a t the genetic basis for the control of the different proteins involved in deciding where along this continuum a fibre will lie is shared. It is logical th a t a selective advantage m ight be gained from a co-ordinated control of regulatory and contractile proteins. Evidence for co-ordinated expression of myosin light chains and troponin subunits in avian skeletal m uscle has been reported (Mikawa et al., 1981).