Effect of isolation versus pairing of female prairie voles on nitroprusside (NP)-induced changes in their aortic contractile responses to phenylephrine (PE). NP was not administered until PE-induced contractile tensions stabilized. *Isolation slightly but significantly attenuated the ability of submaximally-effective (though not maximally-effective) concentrations of NP to relax PE-induced contractions in aortic tissues with endothelium intact (left), as determined by both a significant group effect and a significant interaction effect from mixed-design 2-factor ANOVA in which factor 1 (independent factor) = isolated or paired animal group and factor 2 (repeated measures factor) = NP concentrations [F(1,32)=4.34, p=0.04; interaction [F (7,224)=2.45, p=0.02]. Removal of the endothelium (right) abolished this effect of isolation. Additional effects of endothelial removal on half-maximally effective concentration (EC50) values for NP are described in the text. Note: values are shown as mean ± SEM.