THE INFLUENCE OF THYROID STATES UPON RESPONSES OF THE RAT AORTA TO CATECHOLAMINES

Mechanical responses to various stimulants and the distribution of adrenoceptor subtypes were investigated in smooth muscle cells of the rat aorta in eu- (control), hyper- and hypo-thyroid states. Concentration-response relationships for K showed that the KCl EC50 value was slightly higher (28.4 mM K) in hypothyroid than in euthyroid and hyperthyroid states (22.5 mM and 22.8 mM K, respectively). The order of maximum amplitudes of KCl contraction was control > hypothyroid > hyperthyroid. The EC50 values for the noradrenaline (NA)-induced contraction for eu-, hypo- and hyper-thyroid states were 7.4, 12.5 and 5.5 nM, respectively. The density of α1-adrenoceptors was increased to 1.4 times the control in the hypothyroid state and reduced to 0.26 times the control in the hyperthyroid state. The EC50 values for the phenylephrine-induced contraction in eu-, hypo- and hyper-thyroid states were 43.4, 247 and 41.5 nM, respectively, and those for clonidine 104, 1360 and 56.5 nM, respectively. The IC50 values for prazosin against 0.1 μM NA-induced contractions in eu-, hypo- and hyper-thyroid states were 5.8, 3.5 and 0.7 nM, respectively, and those for yohimbine 8.9, 5.4 and 4.1 μM, respectively. The IC50 values for isoprenaline against phenylephrine-induced contractions were 63 nM and 2.4 μM in hyper- and eu-thyroid states, respectively, and the corresponding IC25 values for eu- and hypo-thyroid states were 72 and 423 nM, respectively. The density of β-adrenoceptors was increased to 3.91 times the control in the hyperthyroid and reduced to 0.68 times the control in the hypothyroid states. The amount of adenosine 3':5'-cyclic monophosphate (cyclic AMP) synthesized by isoprenaline increased in parallel with the density of β-adrenoceptors in the three thyroid states. NA consistently increased the amount of inositol 1,4,5-trisphosphate (IP3) within 30 s in the three thyroid states. In the hypothyroid state, the amount of IP3 was slightly increased and in the hyperthyroid condition, it occurred to a lesser extent than in the euthyroid state. The results indicate that in hypothyroidism, the density of α1-adrenoceptors is increased and that of β-adrenoceptors reduced in vascular beds. The latter but not the former is accompanied by corresponding mechanical responses. In hyperthyroidism, the density of α1-adrenoceptors is decreased and β-adrenoceptors increased with parallel changes in mechanical responses.