Autonomic control of the pelvic circulation: in vivo and in vitro studies in pregnant and nonpregnant sheep.

Pregnancy alters the autonomic control of the peripheral circulation. Present in vivo and in vitro studies were designed to investigate the mechanisms of these alterations. In the in vivo studies, pelvic and systemic circulatory responses to intra-arterial and intravenous injections of agonists were monitored in unanesthetized pregnant and nonpregnant ewes. In in vitro studies, a comparison was made of responses to transmural nerve stimulation (TNS) and to norepinephrine (NE) of different blood vessels obtained from pregnant and nonpregnant ewes. The results were the following: (1) Pelvic vascular response to all vasoactive stimuli was considerably more depressed during pregnancy than was systemic circulatory response. (2) In vitro reactivity to alpha-adrenergic stimulation with NE of different blood vessels obtained from pregnant animals was similar to that of vessels from nonpregnant animals. (3) The response to TNS of blood vessels obtained from pregnant ewes was less than that of vessels from nonpregnant ewes. The conclusions were that (a) probably largely related to dilution of a given dose of the stimulus in the greater blood volume and flow; and (b) the increased neurogenic tone of the peripheral circulation during pregnancy is most likely related to increased nerve firing rather than neural density in the vessel walls.