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Mechanisms underlying regional differences in the Ca 2+ sensitivity of BK Ca current in arteriolar smooth muscle
Author(s) -
Yang Yan,
Sohma Yoshiro,
Nourian Zahra,
Ella Srikanth R.,
Li Min,
Stupica Aaron,
Korthuis Ronald J.,
Davis Michael J.,
Braun Andrew P.,
Hill Michael A.
Publication year - 2013
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2012.241562
Subject(s) - cremaster muscle , chemistry , protein subunit , biophysics , gating , vascular smooth muscle , patch clamp , bk channel , membrane potential , anatomy , medicine , microcirculation , biochemistry , smooth muscle , biology , receptor , gene
Key pointsThe plasma membrane large‐conductance Ca 2+ ‐activated, K + channel (BK Ca ) is a major ion channel contributing to the regulation of membrane potential. Activation of large‐conductance Ca 2+ ‐activated K + channel by both depolarization and increased intracellular Ca 2+ results in hyperpolarization that acts to limit agonist and mechanically induced vasoconstriction in small arteries. Using patch‐clamp techniques we demonstrate that regional differences exist in how BK Ca is regulated, particularly with respect to its Ca 2+ sensitivity. Using single‐channel recordings and siRNA to manipulate protein subunit expression, it is argued that the β1‐subunit plays a more dominant role in cerebral blood vessels as compared with small arteries from skeletal muscle. Subtle differences in the regulation of membrane potential in different vascular beds allow local blood flow and pressure to be closely adapted to the tissue's metabolic needs.Abstract β1‐Subunits enhance the gating properties of large‐conductance Ca 2+ ‐activated K + channels (BK Ca ) formed by α‐subunits. In arterial vascular smooth muscle cells (VSMCs), β1‐subunits are vital in coupling SR‐generated Ca 2+ sparks to BK Ca activation, affecting contractility and blood pressure. Studies in cremaster and cerebral VSMCs show heterogeneity of BK Ca activity due to apparent differences in the functional β1‐subunit:α‐subunit ratio. To define these differences, studies were conducted at the single‐channel level while siRNA was used to manipulate specific subunit expression. β1 modulation of the α‐subunit Ca 2+ sensitivity was studied using patch‐clamp techniques. BK Ca channel normalized open probability ( NP o ) versus membrane potential ( V m ) curves were more left‐shifted in cerebral versus cremaster VSMCs as cytoplasmic Ca 2+ was raised from 0.5 to 100 μ m . Calculated V 1/2 values of channel activation decreased from 72.0 ± 6.1 at 0.5 μ m Ca 2+ i to −89 ± 9 mV at 100 μ m Ca 2+ i in cerebral compared with 101 ± 10 to −63 ± 7 mV in cremaster VSMCs. Cremaster BK Ca channels thus demonstrated an ∼2.5‐fold weaker apparent Ca 2+ sensitivity such that at a value of V m of −30 mV, a mean value of [Ca 2+ ] i of 39 μ m was required to open half of the channels in cremaster versus 16 μ m [Ca 2+ ] i in cerebral VSMCs. Further, shortened mean open and longer mean closed times were evident in BK Ca channel events from cremaster VSMCs at either −30 or 30 mV at any given [Ca 2+ ]. β1‐Subunit‐directed siRNA decreased both the apparent Ca 2+ sensitivity of BK Ca in cerebral VSMCs and the appearance of spontaneous transient outward currents. The data are consistent with a higher ratio of β 1 ‐subunit:α‐subunit of BK Ca channels in cerebral compared with cremaster VSMCs. Functionally, this leads both to higher Ca 2+ sensitivity and NP o for BK Ca channels in the cerebral vasculature relative to that of skeletal muscle.