CHARACTERISTICS OF VASCULAR SMOOTH-MUSCLE IN THE MATERNAL RESISTANCE CIRCULATION DURING PREGNANCY IN THE RAT

OBJECTIVE: Our purpose was to determine if pregnancy results in a decrease in arterial sensitivity to receptor-independent stimuli and a change in vascular smooth muscle membrane potential. STUDY DESIGN: Mesenteric resistance arteries from late pregnant (n = 19) and age-matched virgin control (n = 20) Sprague-Dawley rats were studied in a pressurized arteriograph system or isometric myograph. RESULTS: Arteries from pregnant rats were less sensitive to membrane depolarization by K+ than were those from nonpregnant rats (mean effective concentration that produced a 50% response 49 vs 39 mmol/L, pregnant vs nonpregnant, p < 0.05). Arterial basal tone and the myogenic response to increasing pressure steps were also reduced in arteries from pregnant rats compared with nonpregnant controls. The vascular smooth muscle membrane of the arteries from the pregnant rats was hyperpolarized compared with that from the control rats (-64 mV from pregnant rats vs -57 mV from nonpregnant rats, p < 0.01). This was associated with a reduction in vasomotion in the arteries from the pregnant rats (I 0% for pregnant rats vs 45% from nonpregnant rats, p < 0.01). CONCLUSION: Pregnancy results in alterations of the vascular smooth muscle, including changes in the regulation of membrane potential and a reduced sensitivity to receptor-independent stimuli.