Long-term hypoxia increases calcium affinity of BK channels in ovine fetal and adult cerebral artery smooth muscle

Abstract

Tao X, Lin MT, Thorington GU, Wilson SM, Longo LD, Hessinger DA. Long-term hypoxia increases calcium affinity of BK channels in ovine fetal and adult cerebral artery smooth muscle. Am J Physiol Heart Circ Physiol 308: H707-H722, 2015. First published January 16, 2015; doi:10.1152/ajpheart.00564.2014.-Acclimatization to high-altitude, long-term hypoxia (LTH) reportedly alters cerebral artery contraction-relaxation responses associated with changes in K ϩ channel activity. We hypothesized that to maintain oxygenation during LTH, basilar arteries (BA) in the ovine adult and near-term fetus would show increased large-conductance Ca 2ϩ activated potassium (BK) channel activity. We measured BK channel activity, expression, and cell surface distribution by use of patch-clamp electrophysiology, flow cytometry, and confocal microscopy, respectively, in myocytes from normoxic control and LTH adult and nearterm fetus BA. Electrophysiological data showed that BK channels in LTH myocytes exhibited 1) lowered Ca 2ϩ set points, 2) left-shifted activation voltages, and 3) longer dwell times. BK channels in LTH myocytes also appeared to be more dephosphorylated. These differences collectively make LTH BK channels more sensitive to activation. Studies using flow cytometry showed that the LTH fetus exhibited increased BK ␤1 subunit surface expression. In addition, in both fetal groups confocal microscopy revealed increased BK channel clustering and colocalization to myocyte lipid rafts. We conclude that increased BK channel activity in LTH BA occurred in association with increased channel affinity for Ca 2ϩ and left-shifted voltage activation. Increased cerebrovascular BK channel activity may be a mechanism by which LTH adult and near-term fetal sheep can acclimatize to long-term high altitude hypoxia. Our findings suggest that increasing BK channel activity in cerebral myocytes may be a therapeutic target to ameliorate the adverse effects of high altitude in adults or of intrauterine hypoxia in the fetus. ) channels, which are well known to modulate vascular tone and promote vasorelaxation (3,7,13,28).In contrast with the role of BK channels in mediating cerebrovascular response to short-term hypoxia, their role in acclimatization to long-term hypoxia (LTH) is less well known. During high-altitude LTH, the CBF in adult humans (23, 60) and sheep (34, 63) returns to normal following a period of transitional increased blood flow, as compiled by Brugniaux et al. (8). The ovine near-term fetus diverts an increased fraction of total cardiac output to the brain, when subjected to hypoxia during gestation. This implies that the cerebral vasculature is more dilated than the systemic vasculature (37, 38). Increased vessel dilation was also the major factor contributing to increased CBF and maintenance of oxygenation in human volunteers taken to high altitude or subjected to acute hypoxia at sea level (66). Because increased blood flow correlates directly with increased vessel diameter in mid-cerebral arteries in humans, as reviewed by Ains...

Negative feedback regulation of vasocontraction by potassium channels in 10‐ to 15‐day‐old rats: Dominating role of Kv7 channels

“…However, studies on smooth muscle cells isolated from cerebral arteries demonstrated a decline of BK Ca channels activity during ontogenesis. The BK Ca ‐current in ovine basilar artery myocytes of fetuses was shown to be enlarged in comparison to adult animals because of a higher calcium sensitivity of BK Ca channels, their augmented phosphorylation by protein kinase G and increased clustering in the plasma membrane . Presumably, the high activity of BK Ca channels observed in patch‐clamped fetal cerebrovascular myocytes is not seen at the functional level because of a dramatically decreased frequency of calcium sparks, necessary for the activation of BK Ca channels in cerebral arteries…”

“…A high activity of these types of potassium channels in the circulatory system of 10‐ to 15‐day‐old animals can serve as a mechanism keeping peripheral resistance and blood pressure low in the immature cardiovascular system thereby reducing the risk of vessel injury/remodelling. Of note, the functional role of vascular potassium channels can be altered in early ontogenesis by intrauterine stress and fetal hypoxia . Such developmental abnormalities can have detrimental cardiovascular consequences later in adulthood .…”

Systemic Hypertension at High Altitude

“…74 After the same period of exposure to H-ALT, increased activity (more sensitive to activation and higher affinity for Ca 2+ ) of large-conductance Ca 2+ -activated K + channels (BK) was found in basilar arteries of ovine adults. 75 This behavior of cerebral arteries could favor an increased CBF during H-ALT exposure, 75 since in vascular smooth muscle cells, BK channel opening favors K + efflux and hyperpolarization, with subsequent closure of voltage-dependent Ca 2+ channels, decreased intracellular Ca 2+ , and vasodilation. 76…”

Modulation of BK Channel Function by Auxiliary Beta and Gamma Subunits

“…Native BK channels exhibit a wide range of biophysical, pharmacological, and functional properties that differ among various cell types (Ding, Li, & Lingle, 1998; Gessner et al, 2005; Jones, Gray-Keller, Art, & Fettiplace, 1999; Xia, Ding, & Lingle, 1999), at different stages of development (Carvalho-de-Souza, Varanda, Tostes, & Chignalia, 2013), and in different physiological or pathological conditions (Hu et al, 2011; Manzanares et al, 2015; Tao et al, 2015). BK channels with slightly different biophysical properties can be produced by alternative splicing of KCNMA1 precursor mRNA at several different sites (Glauser, Johnson, Aldrich, & Goodman, 2011; Ramanathan, Michael, Jiang, Hiel, & Fuchs, 1999; Saito, Nelson, Salkoff, & Lingle, 1997; Schreiber, Yuan, & Salkoff, 1999; Yu, Upadhyaya, & Atkinson, 2006).…”