Large conductance calcium-activated potassium channels contribute to the reduced myogenic tone of peripheral microvasculature after cardiopulmonary bypass.

BACKGROUND: We investigated the role of calcium-activated potassium (K(Ca)) channel activation in myogenic tone in human peripheral microvasculature after heart surgery.
METHODS: Human skeletal muscle arterioles (90-180microm diameter) were dissected from tissue harvested pre- and post-cardiopulmonary bypass (CPB) during cardiac surgery. Myogenic reactivity in response to stepwise increases in intraluminal pressure was studied between pressure steps. Microvessel tone was determined pre-CPB, post-CPB, and after blockade of K(Ca) channels. Expression and localization of large conductance (BK) K(Ca) channels in the coronary microvasculature was assessed by immunoblot and immunofluorescence photomicroscopy.
RESULTS: Myogenic tone of skeletal muscle arterioles was significantly reduced post-CPB compared with pre-CPB. Decrease in myogenic tone after CPB was reflected by the increase in microvessel internal diameter. Myogenic tone of post-CPB microvessels was significantly increased after treatment with BK(Ca)-blocker iberiotoxin, but unchanged in the combined presence of the blockers of intermediate (IK(Ca)) and small conductance (SK(Ca)) K(Ca) channels, TRAM34/apamin. The increases in myogenic tone after iberiotoxin treatment were demonstrated as a decrease in microvessel internal diameter. No significant differences in BK(Ca) protein levels were noted comparing pre- and post-CPB conditions judged by immunoblot and by immunofluorescence staining of skeletal muscle microvessels. Prominent staining for BK(Ca)-alpha and BK(Ca)-beta(1) subunits localized to the microvascular smooth muscle.
CONCLUSION: CPB-associated decrease in peripheral myogenic reactivity is likely due to activation of BK(Ca), but not IK(Ca) or SK(Ca). CPB may increase BK(Ca) activity without increasing BK polypeptide level.