Sphingolipids are believed to cause smooth muscle constriction at least partly via activation of a Rho kinase-mediated pathway, and we have previously reported to the Society that sphingosylphosphorylcholine (SPC) causes a dose dependent contraction of rat intra-pulmonary arteries (IPA) between 3-100μM (Thomas et al., 2003). However, as with other studies the concentration of SPC required to cause constriction is significantly higher than those reported for plasma in vivo (100nM to 1μM) (Meyer Zu et al., 2002). We therefore examined whether a sub-contractile concentration of SPC (1μM) affected the vasoreactivity of small (300-500 μm i.d.) IPA of the rat. Rats were humanely killed by a Home Office approved method, and IPA mounted on a small vessel myograph. Data are given as mean +/- SEM tested for significance using an unpaired Students t test or ANOVA as appropriate. 1μM SPC did not cause constriction of IPA. However, pre-incubation with 1 μM SPC potentiated vasoconstriction induced by 23 mM [K+] by 464 +/- 78% (n=5; p<0.05). This potentiation was not significantly reduced by the Rho kinase antagonist Y27632 (10 μM) (389 +/- 61%, n=5), but was accompanied by an enhanced rise in intracellular calcium as measured with Fura PE3 (n=4). To examine whether the potentiation was related to SPC-induced depolarisation, similar experiments were performed on IPA sub-maximally constricted with PGF2α (5μM). 1 μM SPC also potentiated the response to PGF2α, though to a much smaller extent (40 +/- 9%, n=8), but the potentiation was only partially inhibited by 10μM diltiazem (L-type channel blocker) (21 +/- 3%, n=4). Conversely 10 μM La³⁺, which blocks certain non-selective calcium entry pathways including capacitative calcium entry, reduced the potentiation to 17 +/ 3% (n=7, p<0.05). These results suggest that concentrations of SPC close to those reported in vivo potentiate IPA vasoreactivity by enhancing calcium entry via both L-type channels and non-selective cation channels, but that Rho kinase mediated calcium sensitisation is not involved.