Effects of isoproterenol on rubidium transport in slow- and fast-twitch muscles from euthyroid and hyperthyroid rats

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 g/kg/day), for ten days induced a rise of ouabainsensitive⁸⁶Rb uptake in SOL muscle, without affecting the ouabain-insensitive uptake, whereas both fractions of⁸⁶Rb uptake were increased in EDL muscle from hyperthyroid rats. Isoproterenol (5 mol/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 of⁸⁶Rb 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 trans-formation 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.The work was supported by a grant from the Hungarian Ministry of Health (No 17/2-06/072)