Increased myometrial blood flow evoked by vasoactive intestinal polypeptide in the non-pregnant goat

1. The effect of vasoactive intestinal polypeptide (VIP) on myometrial blood flow was evaluated in anaesthetized goats. A solution of VIP, or vehicle alone, was infused into the right internal iliac artery for a period of 10 min. The myometrial blood flow in both uterine horns was measured from the third to the seventh min of the infusion by the gas clearance technique after local injection of (133)xenon in 10 mul. saline solution. Blood samples were collected from both utero-ovarian veins 5 min from the onset of the infusion and the plasma concentration of VIP determined by radio-immunoassay.2. During infusion of vehicle before VIP, myometrial blood flow was of the same magnitude in both uterine horns, i.e. 0.06-0.12 ml./min per g. The blood flow of the right horn increased to 0.20-0.39 ml./min per g during infusion of VIP (300 p-mole/min) in the ipsilateral artery, whilst that of the left horn rose to 0.13-0.26 ml./min per g. The effect was sometimes observed to last for more than 40 min.3. Increased myometrial blood flow was observed with infusion rates down to 3 p-mole/min. Once a response to VIP had been provoked, however, the vasculature sometimes became refractory to further stimulation.4. The plasma concentration of VIP increased in both utero-ovarian veins during unilateral infusion of the peptide.5. Methylene blue given through the infusion catheter stained tissue in both uterine horns, further evidencing that their blood supply is not entirely separate.6. Uterine motility was observed to diminish during the VIP infusions.7. During infusion of VIP (300 p-mole/min) heart rate rose from 146 +/- 6 to 158 +/- 7 beats/min. No significant change occurred in arterial blood pressure.8. It is concluded that the increase in blood flow is due to a local response and that, since VIP has been demonstrated in uterine nerve endings, it may act as a neuro-transmitter mediating vasodilatation in the uterus.