A vascular smooth muscle potassium channel dysfunction underlies small vessel disease of the brain in both hypertension and alzheimer's disease

Alzheimer's disease (AD) and hypertension-related vascular dementia share many common clinical and radiological features. Indeed, at post-mortem the majority of patients diagnosed with either disorder show extensive small vessel disease (SVD) of the brain. One phenotypic character of SVD is a reduction in cerebral blood flow (CBF). The large-conductance calcium-activated potassium (BK) channel reduces arterial contractility through hyperpolarisation of the vascular smooth muscle cell membrane. The BK channel is activated via small, localised calcium release events from ryanodine receptors (RyR), known as calcium sparks. We wished to examine whether defects in calcium spark to BK channel coupling contributes to the SVD phenotype in these diseases, which could suggest new avenues for therapy based on improvement of brain vascular health. Male spontaneously hypertensive (BPH/2) mice and normotensive controls (BPN/3) were used at 8 months of age to study hypertension-induced cSVD. To study AD we used 18–20 month old male APP23 mice that have a 7-fold increased expression of amyloid precursor protein, which results in significant amyloid-beta accumulation as is seen in human AD. During this study we used a range of physiological techniques; mainly pressure myography, patch clamp electrophysiology and high-speed spinning disc confocal microscopy, to investigate in depth, vascular ion channel function within the cerebral microvasculature of these mice. Pressure-induced constriction from cerebral pial arteries was significantly greater in both the BPH/2 and APP23 animals compared to controls. Pressure myography studies revealed that in both models there was a reduction in BK channel activity. This was confirmed with electrophysiology data that showed a decrease in spontaneous transient outward currents, which represent BK channel openings in response to calcium sparks. In the APP23 model, this reduction in BK channel function was due to a decrease in calcium spark frequency. However in the BPH/2 mice, spark frequency was equal between hypertensive individuals and controls. In this model separation of the BK channel and RyR resulted in BK channel dysfunction. Overall our data indicates that vascular BK channel dysfunction is a common mechanism underpinning reduced CBF in SVD. Therefore, therapies aimed at improving vascular BK channel function could provide new therapeutic targets for the treatment of both AD and hypertension-induced vascular dementia.