The effect of pregnancy on the changes in hind-limb vascular resistance following haemorrhage in the rabbit.

The change in vascular resistance of the skinned hind limb perfused at constant flow has been measured following rapid removal of 10% of the blood volume in anaesthetized pregnant rabbits (27-29 days gestation) and compared with that of non-pregnant rabbits. 5 s after haemorrhage was completed vascular resistance had risen in all rabbits, but in the pregnant ones this increase was only one-fifth of that in the non-pregnant ones. After section of either the carotid sinus and aortic nerves or the sympathetic innervation to the hind limb the increases of vascular resistance at this time were abolished. It is concluded that the previously described diminution during pregnancy of the protective function of the baroreflexes during haemorrhage results, to some extent, from their diminished role in a vascular bed not associated with the reproductive process. The blood withdrawn was not re-infused for 10 min, throughout which, in all rabbits with sinus, aortic and sympathetic nerves intact, the increase in vascular resistance was sustained. After section of the carotid sinus and aortic nerves and the sympathetic innervation there was, during this 10 min, a slow increase in vascular resistance comparable in final size to that which occurred immediately in the respective innervated preparations, i.e. it was smaller in the pregnant than in the non-pregnant rabbits. A possible difference in action during pregnancy of a vasoconstrictor substance released during haemorrhage is discussed briefly.