Appropriate control of fetoplacental vascular tone by chorionic plate resistance arteries is necessary for adequate oxygen and nutrient transfer to the growing fetus and a successful pregnancy. Arterial smooth muscle cells (SMCs) express potassium (K+) channels which regulate tone in response to humoral vasoactive agents and oxygen. Previous studies of human placental chorionic plate arteries show alterations in vascular reactivity by oxygen and K+ channel modulators. However, it remains to be determined whether K+ channels are localised to the SMCs of chorionic plate arteries where they directly control excitation-contraction coupling. The overall hypothesis for this thesis is that K+ channels are expressed in chorionic plate arterial SMCs and regulate fetoplacental vascular tone in response to altered oxygenation.Wire myography was used to assess whether chorionic plate arterial tone was modulated (1) by acute (3h) and chronic (24h, 48h culture) exposure to placental hyperoxia, normoxia or hypoxia, or (2) following blockade of voltage-gated K+ channels (Kv) and the oxygen-sensitive Kv channel, Kv1.5. Acute and chronic exposure to different oxygenations did not affect constriction to the thromboxane mimetic U-46619. Acutely, inhibition of Kv channels with 4-AP (1 mM) enhanced basal tone at all oxygenations and U-46619 constriction under hypoxia. Inhibition of Kv1.5 channels with DPO-1 (3 μM) had no effect on basal tone or U-46619 constriction.An in vitro model was developed of freshly isolated chorionic plate arterial SMCs to investigate protein expression and function of K+ channels including Kv1.5. Using immunocytochemistry, the cell isolates were confirmed as SM due to expression of alpha-smooth muscle actin and markers of SMC contractile and synthetic phenotype, in common with native arteries. 4-AP (5 mM) and DPO-1 (3 μM) inhibited a small but significant component of the whole-cell current at negative membrane potentials, and the SMCs expressed Kv1.5 protein. The small contribution to whole-cell currents from 4-AP and DPO-1 sensitive channels supports the myography data suggesting that these channels do not have a marked effect on chorionic plate arterial tone. The majority of whole-cell current was inhibited by TEA (5 mM), charybdotoxin (100 nM) and iberiotoxin (100 nM) and therefore mediated by the Ca2+-activated K+ channel, BKCa. IKCa and SKCa channels are functional in chorionic plate arterial SMCs as activation with 1-EBIO (100 μM) markedly enhanced whole-cell currents; an effect that was abolished by the IKCa inhibitor TRAM-34 and reduced by the SKCa inhibitor apamin (100 nM). BKCa and IKCa protein was expressed in SMCs after isolation and in the native artery.These studies demonstrate expression and function of the oxygen-sensitive K+ channel, Kv1.5, in chorionic plate arterial SMCs. However, Kv channels do not appear to have a prominent role in controlling vascular tone in the whole vessel. Furthermore, these data highlight a more important role for KCa channels in chorionic plate arteries where all three isoforms are expressed in the SMCs and responsible for the majority of whole-cell K+ current. The localisation of diverse K+ channels to chorionic plate arterial SMCs may be related to their phenotype, which displays both contractile and synthetic characteristics, and implicates a dual role for these SMCs to control both fetoplacental vascular resistance and vasculogenesis throughout pregnancy.
|Date of Award||1 Aug 2012|
- The University of Manchester
|Supervisor||Susan Greenwood (Supervisor) & Mark Wareing (Supervisor)|