Aldosterone activates cerebrovascular smooth muscle BK channels in absence of β1 subunits, yet these potentiate steroid action and thus facilitate aldosterone-induced dilation of cerebral arteries.

Abstract

Calcium/voltage-gated, big conductance K+ channels (BK α subunits or slo1 proteins) modulate smooth muscle (SM) tone and thus cerebral artery diameter. In cerebral arteries, the BK current phenotype is conditioned by regulatory β1 subunits which fine-tune channel activity to tissue-specific physiology. Several physiological steroids modify BK function and may modify cerebral artery function. Thus, cholesterol and pregnenolone inhibit SM BK and evoke middle cerebral artery (MCA) constriction through recognition by slo1 whereas estradiol and cholanes activate SM BK through recognition by β1. Aldosterone is a steroid with documented vasoactive properties, yet the involvement of BK in aldosterone's cerebrovascular action and the contribution of distinct channel subunits to this action remain unknown. Here, we first used nanoscale differential scanning fluorimetry to test whether aldosterone (0.3-100 μM) could directly bind BK α or β1 subunits. Changes in temperature of onset and inflection point (indicators of protein-steroid binding) were evident at 10 and 1 µM aldosterone for α and β1. Next, MCA SM cells were enzymatically isolated from mice, and cell-free recordings were conducted to determine changes in BK activity by aldosterone. Increased activity by aldosterone was concentration-dependent: EC50 = 3 μM; ECMAX = 10 μM, at which activity increased by 20%. Consistently with this small yet reproducible SM BK activation, 10 μM aldosterone dilated MCA by 2%, independently of circulating and endothelial factors. In SM cells and MCA segments from β1-/- mice, aldosterone activated BK with reduced potency and efficacy, and vasodilation by the steroid was evoked by very high concentrations (>>10 µM). Therefore, aldosterone is able to activate SM BK in absence of β1, yet this regulatory subunit potentiates aldosterone action and thus enables subµM-µM aldosterone to dilate MCA.