Pregnancy-associated adaptations in [Ca2+]i-dependent and Ca2+ sensitization mechanisms of venous contraction: implications in pregnancy-related venous disorders

Abstract

Pregnancy is associated with significant adaptations in the maternal hemodynamics and arterial circulation, but the changes in the venous mechanisms during pregnancy are less clear. We hypothesized that pregnancy is associated with alterations in venous function, intracellular free Ca2+ concentration ([Ca2+]i), and Ca2+-dependent mechanisms of venous contraction. Circular segments of inferior vena cava (IVC) from virgin and late pregnant (Preg, day 19) Sprague-Dawley rats were suspended between two hooks, labeled with fura-2, and placed in a cuvet inside a spectrofluorometer for simultaneous measurement of contraction and [Ca2+]i (fura-2 340/380 ratio). KCl (96 mM), which stimulates Ca2+ influx, caused less contraction (35.6 ± 6.3 vs. 92.6 ± 19.9 mg/mg tissue) and smaller increases in [Ca2+]i (1.67 ± 0.12 vs. 2.19 ± 0.11) in Preg vs. virgin rat IVC. The α-adrenergic receptor agonist phenylephrine (Phe; 10−5 M) caused less contraction (23.8 ± 3.4 vs. 70.9 ± 12.9 mg/mg tissue) and comparable increases in [Ca2+]i (1.76 ± 0.10 vs. 1.89 ± 0.08) in Preg vs. virgin rat IVC. At increasing extracellular Ca2+ concentrations ([Ca2+]e) (0.1, 0.3, 0.6, 1, and 2.5 mM), KCl and Phe induced [Ca2+]e-contraction and [Ca2+]e-[Ca2+]i curves that were reduced in Preg vs. virgin IVC, supporting reduced Ca2+ entry mechanisms. The [Ca2+]e-contraction and [Ca2+]e-[Ca2+]i curves were used to construct the [Ca2+]i-contraction relationship. Despite reduced contraction and [Ca2+]i in Preg IVC, the Phe-induced [Ca2+]i-contraction relationship was greater than that of KCl and was enhanced in Preg vs. virgin IVC, suggesting parallel activation of Ca2+-sensitization pathways. The Ca2+ channel blocker diltiazem, protein kinase C (PKC) inhibitor GF-109203X, and Rho-kinase (ROCK) inhibitor Y27632 inhibited KCl- and Phe-induced contraction and abolished the shift in the Phe [Ca2+]i-contraction relationship in Preg IVC, suggesting an interplay between the decrease in Ca2+ influx and possible compensatory activation of PKC- and ROCK-mediated Ca2+-sensitization pathways. The reduced [Ca2+]i and [Ca2+]i-dependent contraction in Preg rat IVC, despite the parallel rescue activation of Ca2+-sensitization pathways, suggests that the observed reduction in [Ca2+]i-dependent contraction mechanisms is likely underestimated, and that the veins without the rescue Ca2+-sensitization pathways could be even more prone to dilation during pregnancy. These pregnancy-associated reductions in Ca2+ entry-dependent mechanisms of venous contraction, if occurring in human lower extremity veins and if not adequately compensated by Ca2+-sensitization pathways, may play a role in pregnancy-related venous disorders.