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 decr ease in arterial sensitivity to receptor-independent stimuli and a cha nge in vascular smooth muscle membrane potential. STUDY DESIGN: Mesent eric resistance arteries from late pregnant (n = 19) and age-matched v irgin control (n = 20) Sprague-Dawley rats were studied in a pressuriz ed arteriograph system or isometric myograph. RESULTS: Arteries from p regnant rats were less sensitive to membrane depolarization by K+ than were those from nonpregnant rats (mean effective concentration that p roduced a 50% response 49 vs 39 mmol/L, pregnant vs nonpregnant, p < 0 .05). Arterial basal tone and the myogenic response to increasing pres sure steps were also reduced in arteries from pregnant rats compared w ith nonpregnant controls. The vascular smooth muscle membrane of the a rteries from the pregnant rats was hyperpolarized compared with that f rom the control rats (-64 mV from pregnant rats vs -57 mV from nonpreg nant rats, p < 0.01). This was associated with a reduction in vasomoti on in the arteries from the pregnant rats (I 0% for pregnant rats vs 4 5% from nonpregnant rats, p < 0.01). CONCLUSION: Pregnancy results in alterations of the vascular smooth muscle, including changes in the re gulation of membrane potential and a reduced sensitivity to receptor-i ndependent stimuli.