Table 1.

K_IR channel Ba²⁺ sensitivity, K⁺ activation and subunit distribution throughout the neurovascular unit. This table is not exhaustive, as some channels are formed from heteromeric subunit assemblies. Molecular studies for many subunits have not been performed for PAs, although pharmacological data are consistent with the expression of K_IR channels that are activated by external K⁺ and blocked by <100 μM Ba²⁺ (i.e., K_IR2 channels). Ba²⁺ blockade is voltage-sensitive, and where data are available we have listed IC₅₀ values for negative potentials (note that the highly positive voltages listed for K_IR1.1 and 6.1 mean that IC₅₀ values at more negative potentials will be lower).

Isoform	Rectification	Ba2+ IC50 (nM)	Voltage (mV)	[K+]o (mM)	K+-activated?*	NVU expression*			Refs
KIR1.1	Weak	4300	0	100	Yes	Neurons	Astrocytes	Parenchymal arteriole	[3,12,23,48,70,110]
						✓	n.d.	n.d
KIR2.1	Strong	3.2	−100	60	Yes	✓	✓	✓	[17,26,39,47,65,67,107,108]
KIR2.2	Strong	0.5	−100	60	Yes	✓	✓	✓	[17,26,39,47,65,67,107,108]
KIR2.3	Strong	10.3	−100	60	Yes	✓	✓	n.d.	[17,39,47,65,107,108]
KIR2.4	Strong	390	−80	96	n.d	✓ - cranial nerve nuclei	x	n.d.	[17,65,123]
KIR3.1	Strong	14 (+3.4) a	−130	90	No	✓	✓	n.d.	[9,47,59,80]
KIR3.2	Strong		n.d. b		No	✓	x	n.d.	[9,47,80]
KIR3.3	Strong		n.d.		No	✓	x	n.d.	[9,47,80]
K|r3.4	Strong	92	−60	100	No	✓	x	n.d.	[9,45,47,80]
K|r4.1	Intermediate	7.1	−130	50	No	x	✓	n.d.	[9,22,38,108]
KIR4.2	Intermediate		n.d. b		Yes	x	x	n.d.	[38,94]
KIR5.1	Strong		n.d. b		n.d	✓ - cell culture	✓	n.d.	[38,108,121]
KIR6.1	Weak	89.3	+60	5.4	No	x	✓	x	[16,41,122]
KIR6.2	Weak	29.3 (+SUR1)	−102	5.6	No	✓	x	x	[16,32,81,117,118,122]
K|r7.1	Weak	1100-1900	−110	150	No	✓	✓	n.d	[19,55,92]

^*K⁺-activation is defined as an increase in channel conductance in the presence of elevated [K⁺]_o at a constant electrical driving force.

^#All brain regions considered, where it is known that expression is highly restricted this is noted. No distinction is made between the smooth muscle and endothelial expression of K_IR subunits in parenchymal arterioles due to the current lack of evidence.

✓ = expression confirmed, x = lack of expression observed. n.d. = not investigated to date, to the authors’ knowledge.

^aK_IR3.1 homomers are non-functional, association with other K_IR3.x subunits is required for functional channels [39].

^bK_IR3.2 [114], 4.2 [94] and 5.1 [121] homomers are all inhibited by Ba²⁺ ions, but IC₅₀ data are currently lacking.