Electrical field stimulation of myometrial strips from non‐pregnant and pregnant guinea‐pigs

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Abstract
Myometrial tissue strips from term pregnant and non‐pregnant guinea‐pigs were studied in vitro by electrical field stimulation, and the adrenoceptor sensitivity to exogenously applied noradrenaline was determined. The induced response was characterized by various sympatholytic and nerve blocking drugs. In previously contracted strips from non‐pregnant animals field stimulation provoked a relaxation, probably mediated via postganglionic adrenergic nerve terminals and myometrial β‐receptors. In strips from myometrial tissue surrounding fetuses, no response was seen after field stimulation. Relaxation induced by exogenous noradrenaline was more pronounced and provoked at lower concentrations in pregnant than in non‐pregnant tissue. However, in non‐fetus‐carrying uterine horns in unilateral pregnancies field stimulation still induced a relaxation, and the sensitivity to exogenous noradrenaline was less than in the contra‐lateral fetus‐carrying horn. The physiological findings were compared with the endogenous level of uterine noradrenaline transmitter and the presence of adrenergic nerves, demonstrated by the Falck‐Hillarp fluorescence technique. It is suggested that the altered effects at term pregnancy are due to a pregnancy induced postganglionic denervation of the adrenergic terminals in the fetus‐carrying uterine horn. This is accompanied by an increased postsynaptic adrenoceptor sensitivity to the transmitter in analogue with a denervation supersensitivity after adrenergic denervation. Functionally, the present results verify the recent structural and biochemical findings of an adrenergic nerve degeneration in fetus‐carrying myometrial tissue during pregnancy, probably representing a very special neuroendocrine neuron‐target relationship.