Effect of Induced Thyroid Dysfunction upon Uterine Responsiveness in Strips from Pregnant and Nonpregnant Rats

Abstract

The influence of thyroid dysfunction on rat myometrial responsiveness to oxytocin, acetylcholine, CaCl₂ and BaCl₂ was assessed in uterine strips from pregnant and nonpregnant rats. In preparations obtained from hypothyroid nonpregnant rats, oxytocin and acetylcholine concentration-response curves were significantly displaced to the left, whereas myometrial strips from hyperthyroid nonpregnant rats were only supersensitive to acetylcholine. In relation to the pregnant state, T₃ treatment caused an increased myometrial sensitivity to both oxytocin and acetylcholine. In this condition, concentration-response curves for oxytocin did not differ from nonpregnant control animals, whereas acetylcholine was about 300-fold more potent at the ED₅₀ levels compared with those in nonpregnant animals. Either in pregnant or nonpregnant state, maximal responses and ED₅₀ to BaCl₂ did not differ. In addition, thyroid dysfunction did not modify the pattern of CaCl₂-induced contractions in isolated myometrial strips from pregnant rats. This data extended to the myometrium, previous evidence in the literature indicating that thyroid dysfunction may affect uterine responsiveness to agonists.

Maternal thyroid function during gestation is related to breech presentation at term

“…The TSH receptor (TSHR) is expressed not only in the thyroid, but also in adipose tissue, brain, orbital tissue, lymphocytes and bone. Only two studies were found, one showing reduced responsiveness to uterine agonists in hypothyroid rats, 22 and another concluding that this might be caused by a reduction in uterine myometrial Ca 2 + channel function. 21 However, until now, no TSHR has been described in the uterus.…”

“…Empirical evidence regarding such a direct effect is, however, scarce. Only two studies were found, one showing reduced responsiveness to uterine agonists in hypothyroid rats, 22 and another concluding that this might be caused by a reduction in uterine myometrial Ca 2 + channel function. 23 Uterine contractions are important for final cephalic presentation at term as has been suggested by a large Norwegian study including over 45,000 breech cases.…”

Chronic levothyroxine and acute T3 treatments enhance the amplitude and time course of uterine contractions in human

“…The existence of a large-conductance calcium-activated potassium channel (BK Ca ) is demonstrated in pregnant human myometrium and is even shown to be involved in uterine quiescence (2,17,22). A modification in BK Ca conductance is related to a change in the functionality of uterine contractile response (16,25).Several investigative groups have analyzed the effect of thyroid dysfunction on pregnant uterine tissue and in particular have observed an effect of low levels of T 4 on uterine contractile activities in gravid rats (24,30). Hypothyroidism has also been found to reduce calcium channel function in pregnant rat uterine tissue (30).…”

“…Several investigative groups have analyzed the effect of thyroid dysfunction on pregnant uterine tissue and in particular have observed an effect of low levels of T 4 on uterine contractile activities in gravid rats (24,30). Hypothyroidism has also been found to reduce calcium channel function in pregnant rat uterine tissue (30).…”

Levothyroxine treatment generates an abnormal uterine contractility patterns in an in vitro animal model

“…Moreover, thyroid hormones have been shown to influence calcium homeostasis and flux responsible for excitation and contractility, with T4 and T3 modulating its pharmacological control and secretion [8] . Therefore, uterine responsiveness to various uterotonic agents is significantly modified under thyroid disorders [9] , [10] .…”