Local uteroplacental influences are responsible for the induction of uterine artery myogenic tone during rat pregnancy

Abstract

Uterine artery constrictor responses to elevation of intraluminal pressure (myogenic tone) are considerably enhanced in late pregnant rats, although the underlying causes remain unknown. A single uterine horn ligation model was used to differentiate local from systemic influences, and to test the hypothesis that factors associated with the site of placentation, rather than systemic hormonal changes, are primarily involved in the induction of this adaptive process. Radial uterine arteries were dissected from the gravid and nongravid uterine horns of late pregnant rats, cannulated, and pressurized. Changes in arterial diameter and smooth muscle [Ca(2+)](i) in response to the elevation of intraluminal pressure were studied using intact and endothelium-denuded arteries loaded with the ratiometric Ca(2+)-sensitive dye fura-2. Elevations of pressure from 10 to 60 and 100 mm Hg resulted in passive arterial distention of arteries from nongravid horns with a minor change in [Ca(2+)](i). In contrast, arteries from gravid horns developed myogenic tone associated with a significant elevation in [Ca(2+)](i). Synchronous oscillations in [Ca(2+)](i) and lumen diameter were frequently observed in vessels from gravid horns. Endothelial denudation augmented tone in the gravid horn but did not uncover myogenic tone in vessels from the nongravid horn. In summary, pregnancy-associated uterine artery myogenic behavior is due to an upregulation of calcium-handling mechanisms, occurs independently of the endothelium, and is induced by local uteroplacental influences.

Figure 1

A, Photograph showing the uterus of a late pregnant (20 day) rat that underwent surgical ligation of one uterine horn several weeks prior to breeding. Most of the picture is taken up by the pregnant horn that contained 9 pups. The nonpregnant horn can be seen in the lower left corner, along with the mesometrial arcade containing arteries and veins that perfuse the uterine corpus. Note difference in the size of the vasculature due to the predominance of local influences on remodeling, as previously described. B, Summary graph demonstrating a significant difference in the passive diameters of uterine radial arteries from nonpregnant versus pregnant horns of late pregnant rats. All vessels were pressurized to 60 mm Hg and maximally dilated with 10 μmol/L diltiazem and 100 μmol/L papaverine. * Significant difference at P < .05 (unpaired Student’s t-test); numbers in parentheses indicate the number of arteries tested.

Figure 2

A, Representative tracings showing the effects of stepwise elevation in intraluminal pressure from 10 to 60 and then to 100 mm Hg on smooth muscle cell (SMC) [Ca²⁺]_i and the diameter of a uterine radial artery from the non-pregnant horn of a late pregnant rat. The dotted lines indicate the maximal diameter of the same artery in the presence of 100 μmol/L papaverine and 10 μmol/L diltiazem. B, Smooth muscle and endothelial integrity of the same artery is evidenced by the presence of constrictor and dilator responses to phenylephrine and ACh, respectively, along with changes in SMC [Ca²⁺]_i.

Figure 3

A, Representative tracings demonstrating the significant elevation in smooth muscle [Ca²⁺]_i and constriction of a uteroplacental radial artery from the pregnant horn of a late pregnant rat in response to stepwise elevation of intraluminal pressure from 10 to 60 and then to 100 mm Hg. The dotted lines indicate the maximal diameter of the same artery in the presence of 100 μmol/L papaverine and 10 μmol/L diltiazem. B, The endothelial integrity of the artery pressurized to 60 mmHg is confirmed by a marked decrease in smooth muscle [Ca²⁺]_i and dilation in response to the application of 1 μmol/L ACh.

Figure 4

Summary graphs showing significant differences in pressure-induced SMC [Ca²⁺]_i responses and associated myogenic tone of uterine arteries from nonpregnant versus pregnant horns of late pregnant rats. No difference was found in the basal levels of SMC [Ca²⁺]_i in arteries from nonpregnant versus pregnant horns at 10 mm Hg. Myogenic tone is expressed as a percentage of the diameter measured in the presence of 100 μmol/L papaverine and 10 μmol/L diltiazem (D_max). *Significantly different at P < .05 (two-way repeated measures ANOVA); numbers in parentheses indicate the number of arteries tested.

Figure 5

Summary graphs demonstrating significant differences in pressure-induced smooth muscle [Ca²⁺]_i responses and myogenic tone in endothelium-denuded uterine arteries from nonpregnant versus pregnant horns of late pregnant rats. * Significantly different at P < .05 (two-way repeated measures ANOVA); numbers in parentheses indicate the number of arteries tested.

Figure 6

A, Representative tracings showing inhibition of pressure-induced SMC [Ca²⁺]_i elevation and myogenic tone by 10 μmol/L diltiazem. B, Summary graph demonstrating the effects of diltiazem on the level of SMC [Ca²⁺]_i in response to pressure elevation from 10 to 60 mm Hg. *Significantly different from SMC [Ca²⁺] levels at 10 mm Hg (paired Student’s t-test). C, Bar graph summarizing the effects of diltiazem on myogenic tone induced by pressure elevation from 10 to 60 mm Hg. Myogenic tone is expressed as a percentage of the fully dilated diameters (D_max) of each artery (in the presence of a combination of papaverine and diltiazem). *Significantly different from myogenic tone before application of diltiazem at P < 0.05 (paired Student’s t-test). On B and C, Numbers in parentheses indicate the number of arteries tested.