Skeletal muscle β-adrenoceptor distribution and responses to isoproterenol in hyperthyroidism

To determine whether hyperthyroidism selectively increases β-adrenergic receptor density in vessels or fibers of human skeletal muscle, we characterized β-receptor distribution autoradiographically in muscle biopsies of 18 subjects aged 26 ± 1 yr before and after daily administration of 100 μg 3,5,3'-triiodothyronine (T₃) for 2 wk. To establish whether vascular and metabolic responses to β-adrenergic stimulation are concomitantly altered, we quantified calf blood flow and plasma concentrations of glucose, lactate, glycerol, free fatty acids (FFA), insulin, and C-peptide during graded-dose isoproterenol infusion in eight of these individuals. Differences in β-adrenergic receptor density among muscle fiber types and vascular components were highly significant (type I > type IIa > type IIb muscle fibers, P < 0.001; and type I muscle fibers > resistance arterioles, P < 0.05). Hyperthyroidism increased β-adrenergic receptor density in all types of muscle fibers (+31-50%; P < 0.01) but not in resistance arterioles. There was no change in calf blood flow or plasma glucose, glycerol, FFA, insulin, or C-peptide responses to isoproterenol. A rise in lactate during stages 3 and 4 of isoproterenol infusion (P < 0.01) was observed before but not after T₃ administration. Thus hyperthyroidism increases β-adrenergic receptor density in fibers but not vessels of human skeletal muscle without increasing either metabolic or vascular responses to selective β-adrenergic stimulation.