Coordinated and sufficient uterine vascular endothelial adaptations facilitate the dramatic increases in uterine blood flow necessary for fetal growth during pregnancy. These endothelial adaptations may be modulated by coordinated intercellular interactions via gap junction proteins which regulate eNOS activation. We hypothesized that in uterine artery endothelium (UA endo), expression of the gap junction proteins connexin (Cx) 43 and 37 is elevated during pregnancy compared to nonpregnant luteal and follicular phases. We further hypothesized that restricting pregnancy to a single uterine horn will induce unilateral increases of connexin expression only in UA endo ipsilateral to the gravid horn. Restricting pregnancy to a single uterine horn (gravid unilateral) was established by surgically severing the intercorneal vascular connections and then ligating one horn (i.e. contralateral nongravid unilateral) 2-3 months before breeding. UA endo were isolated from nonpregnant sheep (luteal, n=5 vessels; follicular, n=5) as well as late pregnant (120-130d, term = 147d) sheep (nongravid unilateral side, n=8; gravid unilateral side, n=8, control pregnant n=8). Cx43 and Cx37 protein expression as well as eNOS expression was determined by Western analysis. No difference was observed between nonpregnant luteal, follicular, and nongravid unilateral Cx43 and Cx37 expression in UA endo. Cx43 and Cx37 were significantly elevated by 2086% and 786%, respectively in control pregnant (P<0.001) whereas Cx43 was significantly elevated by 1392% in gravid unilateral (P<0.001) compared to luteal. Additionally, Cx43 expression was significantly elevated by 295% (P=0.003) in the UA endo of the gravid unilateral horn compared to the nongravid horn whereas Cx37 was barely detectable in both the nongravid and gravid horns. Further analysis of eNOS expression indicated that unlike Cx37, the patterns of Cx43 expression mimicked UA endo eNOS expression as well as the dramatic physiologic rises in uterine blood flows, and uterine artery shear stresses. These data demonstrate that there is a distinct difference in uterine vascular programming of gap junction proteins Cx43 and Cx37 during pregnancy. Both Cx43 and Cx37 are lower in gravid unilateral compared to pregnant, suggesting that the local responses of fetal restriction to a single horn has a direct negative effect on connexin expression during pregnancy. It is also noteworthy that in the unilateral gravid and pregnant horns, blood flow and shear stress are substantially increased which appears to be associated with the observed increase in Cx43 expression. These data suggest that the regulatory mechanisms pertaining to gap junction proteins may delineate a greater understanding of local rather then the systemic vascular adaptations during pregnancy. NIH HL49210, HD38843, HL87144, HL79020.(poster)