The effect of copper on the electrical activity of the rabbit myometrium.

Zipper, Medel & Prager (1969) reported a depressive effect on the implantation rate in the rabbit with intrauterine copper wires. They postulated that the effect of the copper IUD was local rather than systemic because only minute quantities of copper were necessary to produce the antifertility effect (Zipper, Tatum, Pastene, Medel & Rivera, 1969). The exact mechanism of the action of copper has not been clarified but the retention of the metal IUD might influence myometrial activity either by decreased or antiperistaltic activity. In women, the presence of an IUD could provide pre-labour like uterine activity (Bengtsson & Moawad, 1967), possibly induced by elevated blood levels of oxytocin or oxytocin-like substances (Gomez-Rogers, 1967). The pre¬ sent paper reports on the electrical activity of the myometrium in the rabbit after insertion of a copper IUD. Thirteen mature female rabbits (3 to 5 kg) were used in our experiments to study the spike burst activity recorded by three pairs of platinum electrodes, diameter 200 μ , implanted at 2-cm intervals on the upper, middle and lower portion of each uterine horn. The electrode leads were run under the skin and were anchored at the back of the neck. Electric potentials were fed into an a.c. amplifier of 40 to 150 Hz frequency limit and recorded by a pen-writing oscillograph. Nine of the thirteen rabbits received a daily intramuscular injection of 10 ^g oestradiol in arachidonic oil, (Leo Pharmaceutical) for 3 to 8 days before the experiment, and the remainder were in natural oestrus. A copper wire 150 μ in diameter and 6, 12 or 60 cm long was introduced at laparotomy through a vaginal incision into the lower portion of one of the uterine horns immediately adjacent to the cervical os. The myometrial activity of each rabbit was followed for 1 to 36 days after the insertion of a copper IUD and was compared with that of the control horn (Table 1 ). In seven rabbits, the small copper IUD (150 μ 6 cm) did not, for the most part, produce any significant increase in electrical activity during an observation period of 4 to 10 days. In two rabbits receiving oestrogen, a 12-cm copper wire produced some equivocal effects, but the highest incidence of increased fre¬ quency of the spike bursts was recorded in the presence of a 60-cm copper wire. In one case, a prolonged stimulatory action was observed for a period of 8 days after the IUD was removed (Table 1).