Biphasic effect of a prostacyclin methyl ester analog on uterine contractiliy in the chronically catheterized pregnant rhesus monkey in late gestation

We determined the in vivo effects of a prostacyclin methyl ester analog (PGI₂) administration on myometrial activity in twelve chronically- catheterized pregnant rheusus monkeys during the last third of pregnancy under three different states of myometrial contractility: postsurgical contractions, spontaneous contractures and oxytocin induced contractions. Prostacyclin (200 μg) was administered to four monkeys 2-3 h after surgery, to eight monkeys having only contractures, and to four monkeys having oxytocin induced contractions. Vehicle administration was performed in six animals having contractures and to four having oxytocin induced contractions. In all three experimental paradigms PGI₂ administration elicited a biphasic response in uterine contractility. An initial increase in the 5 min average intrauterine pressure (IUP) value from 2.3 ± 1.29 to 5.8 ± 2.29; 3.6 ± 2.38 to 7.6 ± 3.59; and 2.3 ± 0.65 to 8.6 ± 0.72 (SD); was observed in the post-surgical, spontaneous contracture and oxytocin induced contraction groups respectively (mmHg, P < 0.05). This increase was followed by a fall in the 5 min average IUP from 2.3 ± 1.29 to 0.6 ± 0.49; 3.6 ± 2.38 to 1.3 ± 0.86; and 2.3 ± 0.65 to 0.1 ± 0.11 in the post-surgical, spontaneous contracture and oxytocin induced contraction groups respectively (mmHg, P < 0.05). A fall in mean arterial blood pressure from 89 ± 9.0 to 55 ± 10.9 (mmHg, P < 0.05) and a compensatory tachycardia from 108 ± 18.2 to 164 ± 56.2 (beats.min^-1, P < 0.05) was observed by 10 min after PGI₂ administration. A late drop in uterine surface temperature from 37.7 ± 0.19 to 37.0 ± 0.22°C (P < 0.05) was also observed. We conclude that intraarterially PGI₂ administered to the pregnant monkey induces hypotension and tachycardia, a biphasic effect on myometrial contractility and a drop in uterine surface temperature.