Endothelial KIR channel dysfunction in aged cerebral parenchymal arterioles

Abstract

Aging is associated with cognitive decline via mechanisms that are not fully understood. Endothelial dysfunction is known to occur in parenchymal arterioles, the bottlenecks of the cerebral microcirculation. Dilation of these arterioles is highly dependent on changes in extracellular K+ ([K+]E), caused by activity of inwardly rectifying K+ channels (KIR2) or by opening of Ca2+‐activated small‐ and intermediate‐conductance K+ channels (KCa2.3 / KCa3.1, respectively). The effects of aging on activity of these channels in the endothelium of cerebral parenchymal arterioles remain undetermined, as well as their impact on endothelium‐dependent dilation. Thus, we hypothesized that advanced age will impair both KIR2 and KCa2.3 / KCa3.1 function in parenchymal arterioles of Aged mice (>24 months) when compared to Young (4‐6 months). Data are means ± SEM, Young vs. Aged males and females. No sex differences were observed, thus the data were combined. Using electrophysiology, we observed that freshly‐isolated endothelial cell tubes from Aged mice showed a similar hyperpolarization to the purinergic receptor agonist 2‐methyl‐S‐ATP (1 µM), which purportedly activates the downstream effectors KIR2 and KCa2.3 / KCa3.1, than those from Young mice (Δmembrane potential (Vm): ‐21.0 ± 1.7 vs ‐23.9 ± 5.1 mV, n = 5‐7), although vasodilatory responses of pressurized parenchymal arterioles were smaller (%Vasodilation: 22.4 ± 3.8 vs. 12.7 ± 3.2%, p<0.05, Student’s t‐test, n = 7‐6). Activity of KCa2.3 / KCa3.1 after activation by NS‐309 (1 µM) was unchanged by aging, as evidenced by similar hyperpolarization (ΔVm: ‐36.1 ± 2.4 vs. ‐32.4 ± 2.2 mV, n = 13‐14) and vasodilation (%Vasodilation: 16.5 ± 2.0 vs. 21.3 ± 3.2%, n = 8‐6). Activation of KIR2 by 15 mM [K+]E induced a smaller hyperpolarization in Aged than Young (ΔVm: ‐11.0 ± 1.1 vs ‐6.9 ± 0.7 mV, p<0.05, Student’s t‐test, n = 13‐13), with a paradoxical larger vasodilation that was unchanged by endothelium removal (%Vasodilation: 10.7 ± 2.3 vs. 25.0 ± 3.6% vs. 27.3 ± 4.5, one‐way ANOVA, Young endothelium intact vs. Aged endothelium intact vs Aged endothelium removed, n = 7‐13‐6), suggesting a compensation by smooth muscle KIR2. Lastly, spontaneous myogenic tone was higher in parenchymal arterioles from Aged mice, and endothelium removal did not further enhance myogenic tone (%myogenic tone: 25.4 ± 1.7 vs. 34.3 ± 2.3 vs 36.4 ± 3.2%, p<0.05, one‐way ANOVA, Young endothelium intact vs. Aged endothelium intact vs Aged endothelium removed, n = 22‐23‐6). In conclusion, aging impairs endothelial KIR2 channels and increases resting myogenic tone, which may underlie vascular dysfunction and exacerbate cognitive decline.