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DEVELOPMENTAL PHYSIOLOGY AND PREGNANCY

Repeated pregnancies (multiparity) increases venous tone and reduces compliance

Department of Physiology, University of Alberta, Edmonton, Alberta, Canada

Submitted 18 January 2005 ; accepted in final form 1 March 2005

In humans, multiparity (repeated pregnancy) is associated with increased risk of cardiovascular disease. In rats, multiparity increases the pressor response to phenylephrine and to acute stress, due in part to changes in tone of the splanchnic arterial vasculature. Given that the venous system also changes during pregnancy, we studied the effects of multiparity on venous tone and compliance. Cardiovascular responses to volume loading (2 ml/100 g body wt), and mean circulatory filling pressure (MCFP, an index of venomotor tone) were measured in conscious, repeatedly bred (RB), and age-matched virgin rats. In addition, passive compliance and venous reactivity of isolated mesenteric veins were measured by pressure myography. There was a greater increase in mean arterial pressure after volume loading in RB rats (+7.2 ± 2.5 mmHg, n = 8) than virgin rats (–1.4 ± 1.7 mmHg, n = 7) (P < 0.05). The increase in MCFP in response to norepinephrine (NE) was also greater in RB rats [half maximal effective dose (ED₅₀) 3.1 ± 0.5 nmol·kg^–1·min^–1, n = 6] than virgins (ED₅₀: 12.1 ± 2.7 nmol·kg^–1·min^–1, n = 6) (P < 0.05). Pressure-induced changes in passive diameter were lower in isolated mesenteric veins from RB rats (29.3 ± 1.8 µm/mmHg, n = 6) than from virgins (36.9 ± 1.3 µm/mmHg, n = 6) (P < 0.05). Venous reactivity to NE in isolated veins was also greater in RB rats (EC₅₀: 2.68 ± 0.37x10^–8 M, n = 5) than virgins (EC₅₀: 4.67 ± 0.93 x 10^–8 M, n = 8). We conclude that repeated pregnancy induces a long-term reduction in splanchnic venous compliance and augments splanchnic venous reactivity and sympathetic tonic control of total body venous tone. This compromises the ability of the capacitance (venous) system to accommodate volume overloads and to buffer changes in cardiac preload.

cardiac preload; mean circulatory filling pressure

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