Investigation of the mechanisms of uridine 5' triphosphate induced contraction of rat mesenteric artery smooth muscle

Uridine 5' triphosphate (UTP) is a pyrimidine nucleotide which is released from a variety of cells including platelets and endothelial cells. The release of UTP can lead to vasoconstriction if there is damage to the endothelial layer and hence its actions on the vasculature are important areas of investigation. The research presented in this thesis describes the measurement of isometric contractile responses of rat mesenteric arteries to UTP and of the electrophysiological measurements of ionic currents of smooth muscle cells isolated from these arteries. UTP induced contractions were recorded using various pharmacological tools to investigate the possible signaling mechanisms leading to the contractile response. Particular attention was given to the identification of the ion channels involved in generating the UTP induced contraction. It was clear that the contraction was dependent on Ca2+ influx and my results indicated that this influx was only partly due to an increased activity of voltage-gated Ca2+ channels and that non-selective cation channels were also important. The involvement of PLC is likely as using U73122, a PLC blocker, significantly reduced the UTP induced response. However, the potential involvement of PKC is less convincing as several PKC isoform peptide inhibitors, linked to the carrier peptide Tat(47-57) to render them membrane permeable, failed to affect the contraction. The ionic basis of the UTP induced contraction was studied by measuring the effect of UTP on various ionic currents measured in enzymatically isolated mesenteric artery smooth muscle cells using the whole-cell patch-clamp technique. UTP was found to inhibit both Kv and KATP currents, though in parallel to the contraction results, the UTP induced inhibition of these currents remained even in the presence of PKC block, suggesting a lack of involvement of PKC. Finally, application of UTP resulted in the activation of a non-selective current. An inhibition of K+ currents and the activation of a non-selective cation current would lead to membrane depolarization, the activation of voltage-gated Ca2+ channels and, dependent on the permeability of the non-selective channels, an additional route for Ca2+ influx and hence contraction.