Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC359
Ca2+-activated K+ channels in human fetoplacental arterial smooth muscle cells
M. F. Brereton1, M. Wareing1, S. L. Greenwood1
1. Maternal and Fetal Health Research Centre, The University of Manchester, Manchester, United Kingdom.
K+ channel expression and function is closely related to vascular smooth muscle cell (SMC) phenotype. Large conductance Ca2+-activated K+ channels (BKCa) control excitation-contraction coupling and predominate in SMCs with a contractile phenotype1. Intermediate conductance Ca2+-activated K+ channels (IKCa) regulate proliferation during vasculogenesis and are expressed in SMCs with an immature or synthetic phenotype1. We previously demonstrated fetoplacental arterial SMCs have a mixed phenotype and express both contractile and synthetic proteins2. Here we test the hypothesis that SMCs isolated from fetoplacental arteries express BKCa and IKCa channels. SMCs were isolated from chorionic plate arteries of normal term placentas using methods reported previously1. Whole-cell patch clamp was used to assess K+ currents, (5mM [K+]e, 140mM [K+]i, 0.5mM EGTAi, 0mM [ATP]i). Cells were clamped at -60mV and depolarised from -70mV to +80mV for 500ms to record current-voltage relationships. KCa channel function was assessed by extracellular application of; TEA (KCa inhibitor; 5mM), charybdotoxin (ChTx; BKCa and IKCa inhibitor 100nM), iberiotoxin (IbTx; BKCa inhibitor; 100nM), TRAM-34 (IKCa inhibitor; 10μM), and 1-EBIO (IKCa opener; 100μM). BKCa and IKCa protein expression was assessed in isolated SMCs with immunocytochemistry. Whole-cell K+ currents (IK) were outwardly-rectifying and displayed spontaneous outward currents (SOCs) at depolarised potentials greater than +50mV (n=56 cells, N=25 placentas). SOCs were abolished by TEA (n=11, N=6), ChTx (n=4, N=2) and IbTx (n=6, N=3). Outward currents were insensitive to TRAM-34 (n=3, N=2). Addition of 1-EBIO significantly increased IK at depolarised potentials greater than +40mV, with a 431±62% (mean±SE) increase observed at +80mV (pA/pF: control 6.6±2.1; 1-EBIO 29.1±6.9; P<0.05; n=22, N=10; Wilcoxon signed rank test). 1-EBIO-sensitive current (pA/pF) was inhibited by TRAM-34 (1-EBIO 34.5±10.0; 1-EBIO+TRAM-34 7.1±2.7; P<0.05; n=7, N=4), but not iberiotoxin (1-EBIO 15.2±11.2; 1-EBIO+IbTx 14.2±9; P>0.05 n=3, N=3). All isolated SMCs (N=3) displayed positive immunostaining for BKCa and IKCa. Outward whole-cell K+ currents in chorionic plate arterial SMCs are mediated by BKCa. IKCa does not contribute to IK under resting conditions; however, 1-EBIO stimulates a large increase in current which is abolished following inhibition of IKCa with TRAM-34. Therefore, fetoplacental arterial SMCs express BKCa and IKCa currents. This, coupled with the expression of BKCa and IKCa protein in SMCs, supports our previous observation that these cells have both contractile and synthetic characteristics. This mixed phenotype may be related to the dual function of fetoplacental SMCs to control both vascular resistance and promote vasculogenesis during pregnancy.
Where applicable, experiments conform with Society ethical requirements