The influence of experimental
hyperthyroidism on the
catecholamine induced stimulation of
rubidium ion transport in the soleus (
SOL), a slow-twitch muscle and the extensor digitorum longus (EDL), a fast-twitch skeletal muscle of rats was studied.
Thyroxine administration (800 micrograms/kg/day), for ten days induced a rise of
ouabain-sensitive 86Rb uptake in
SOL muscle, without affecting the
ouabain-insensitive uptake, whereas both fractions of 86Rb uptake were increased in EDL muscle from
hyperthyroid rats.
Isoproterenol (5 mumol/l) caused a two-fold rise in
ouabain-sensitive Rb uptake of euthyroid
SOL muscle, while in
hyperthyroid SOL it could stimulate only the
ouabain-insensitive fraction of 86Rb influx. On the other hand, the stimulating action of
isoproterenol on euthyroid EDL muscle was due to an enhancement of
ouabain-insensitive Rb uptake, but in
hyperthyroid EDL it failed to stimulate the
ouabain-insensitive transport and caused a marked rise in
ouabain-sensitive Rb uptake. The changes in
catecholamine mediated transport properties in
SOL muscle may be related to fibre type transformation induced by
thyroid hormone, although in EDL the changes of
catecholamine stimulation are unlikely due to fibre type conversion. Basal and
isoproterenol stimulated cAMP levels were significantly reduced in both EDL and
SOL muscles from
hyperthyroid rats, in contrast with an insignificant decrease in net
rubidium uptake caused by
isoproterenol at the same concentration.