Mechanisms underlying regional differences in the Ca2+ sensitivity of BKCa current in arteriolar smooth muscle

Key points The plasma membrane large‐conductance Ca2+‐activated, K+ channel (BKCa) is a major ion channel contributing to the regulation of membrane potential. Activation of large‐conductance Ca2+‐activated K+ channel by both depolarization and increased intracellular Ca2+ 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 BKCa is regulated, particularly with respect to its Ca2+ 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. β1‐Subunits enhance the gating properties of large‐conductance Ca2+‐activated K+ channels (BKCa) formed by α‐subunits. In arterial vascular smooth muscle cells (VSMCs), β1‐subunits are vital in coupling SR‐generated Ca2+ sparks to BKCa activation, affecting contractility and blood pressure. Studies in cremaster and cerebral VSMCs show heterogeneity of BKCa 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 Ca2+ sensitivity was studied using patch‐clamp techniques. BKCa channel normalized open probability (NPo) versus membrane potential (Vm) curves were more left‐shifted in cerebral versus cremaster VSMCs as cytoplasmic Ca2+ was raised from 0.5 to 100 μm. Calculated V1/2 values of channel activation decreased from 72.0 ± 6.1 at 0.5 μm Ca2+i to −89 ± 9 mV at 100 μm Ca2+i in cerebral compared with 101 ± 10 to −63 ± 7 mV in cremaster VSMCs. Cremaster BKCa channels thus demonstrated an ∼2.5‐fold weaker apparent Ca2+ sensitivity such that at a value of Vm of −30 mV, a mean value of [Ca2+]i of 39 μm was required to open half of the channels in cremaster versus 16 μm[Ca2+]i in cerebral VSMCs. Further, shortened mean open and longer mean closed times were evident in BKCa channel events from cremaster VSMCs at either −30 or 30 mV at any given [Ca2+]. β1‐Subunit‐directed siRNA decreased both the apparent Ca2+ sensitivity of BKCa in cer