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Departments of 1 Physiology and Pharmacology and 2 Medicine (Cardiology), Oregon Health and Science University, Portland, Oregon 97201
Pregnancy produces marked
systemic vasodilation, but the mechanism is unknown. Experiments
were performed in conscious rabbits to test the hypotheses that
increased nitric oxide (NO) production contributes to the increased
vascular conductance, but that the contribution varies among vascular
beds. Rabbits were instrumented with aortic and vena caval catheters
and ultrasonic flow probes implanted around the ascending aorta,
superior mesenteric artery, terminal aorta, and/or a femoral artery.
Hemodynamic responses to intravenous injection of
N
-nitro-L-arginine
(L-NA; 20 mg/kg or increasing doses of 2, 5, 10, 15, and 20 mg/kg) were determined in rabbits first before pregnancy (NP) and then
at the end of gestation (P). L-NA produced similar
increases in arterial pressure between groups, but the following
responses were larger (P < 0.05) when the rabbits were pregnant: 1) decreases in total peripheral conductance
[
3.7 ± 0.3 (NP), 5.0 ± 0.5 (P)
ml · min
1 · mmHg1],
2) decreases in mesenteric conductance [0.47 ± 0.05 (NP), 0.63 ± 0.07 (P)
ml · min1 · mmHg1],
3) decreases in terminal aortic conductance [0.43 ± 0.05 (NP), 0.95 ± 0.19 ml · min1 · mmHg1 (P)],
and 4) decreases in heart rate [41 ± 4 (NP),
62 ± 5 beats/min (P)]. Nevertheless, total peripheral and
terminal aortic conductances remained elevated in the pregnant rabbits
(P < 0.05) after L-NA. Furthermore,
decreases in cardiac output and femoral conductance were not different
between the reproductive states. We conclude that the contribution of
NO to vascular tone increases during pregnancy, but only in some
vascular beds. Moreover, the data support a role for NO in the
pregnancy-induced increase in basal heart rate. Finally, unknown
factors in addition to NO must also underlie the basal vasodilation
observed during pregnancy.
heart rate; cardiac output; arterial pressure; mesenteric flow; femoral flow; terminal aortic flow; total peripheral resistance; conductance
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