Matthias Sausbier

Malfunctions of potassium channels are increasingly implicated as causes of neurological disorders. However, the functional roles of the large-conductance voltage- and Ca(2+)-activated K(+) channel (BK channel), a unique calcium, and voltage-activated potassium channel type have remained elusive. Here we report that mice lacking BK channels (BK(-/-)) show cerebellar dysfunction in the form of abnormal conditioned eye-blink reflex, abnormal locomotion and pronounced deficiency in motor coordination, which are likely consequences of cerebellar learning deficiency. At the cellular level, the BK(-/-) mice showed a dramatic reduction in spontaneous activity of the BK(-/-) cerebellar Purkinje neurons, which generate the sole output of the cerebellar cortex and, in addition, enhanced short-term depression at the only output synapses of the cerebellar cortex, in the deep cerebellar nuclei. The impairing cellular effects caused by the lack of postsynaptic BK channels were found to be due to depolarization-induced inactivation of the action potential mechanism. These results identify previously unknown roles of potassium channels in mammalian cerebellar function and motor control. In addition, they provide a previously undescribed animal model of cerebellar ataxia.

Both cGMP-dependent and -independent mechanisms have been implicated in the regulation of vascular tone by NO. We analyzed acetylcholine (ACh)- and NO-induced relaxation in pressurized small arteries and aortic rings from wild-type (wt) and cGMP kinase I-deficient (cGKI(-/-)) mice. Low concentrations of NO and ACh decreased the spontaneous myogenic tone in wt but not in cGKI(-/-) arteries. However, contractions of cGKI(-/-) arteries and aortic rings were reduced by high concentrations (10 micromol/L) of 2-(N:, N-diethylamino)-diazenolate-2-oxide (DEA-NO). Iberiotoxin, a specific blocker of Ca(2+)-activated K(+) (BK(Ca)) channels, only partially prevented the relaxation induced by DEA-NO or ACh in pressurized vessels and aortic rings. DEA-NO increased the activity of BK(Ca) channels only in vascular smooth muscle cells isolated from wt cGKI(+/+) mice. These results suggest that low physiological concentrations of NO decrease vascular tone through activation of cGKI, whereas high concentrations of DEA-NO relax vascular smooth muscle independent of cGKI and BK(Ca). NO-stimulated, cGKI-independent relaxation was antagonized by the inhibition of soluble guanylyl cyclase or cAMP kinase (cAK). DEA-NO increased cGMP to levels that are sufficient to activate cAK. cAMP-dependent relaxation was unperturbed in cGKI(-/-) vessels. In conclusion, low concentrations of NO relax vessels by activation of cGKI, whereas in the absence of cGKI, NO can relax small and large vessels by cGMP-dependent activation of cAK.

BACKGROUND: Abnormally elevated blood pressure is the most prevalent risk factor for cardiovascular disease. The large-conductance, voltage- and Ca2+-dependent K+ (BK) channel has been proposed as an important effector in the control of vascular tone by linking membrane depolarization and local increases in cytosolic Ca2+ to hyperpolarizing K+ outward currents. However, the BK channel may also affect blood pressure by regulating salt and fluid homeostasis, particularly by adjusting the renin-angiotensin-aldosterone system. METHODS AND RESULTS: Here we report that deletion of the pore-forming BK channel alpha subunit leads to a significant blood pressure elevation resulting from hyperaldosteronism accompanied by decreased serum K+ levels as well as increased vascular tone in small arteries. In smooth muscle from small arteries, deletion of the BK channel leads to a depolarized membrane potential, a complete lack of membrane hyperpolarizing spontaneous K+ outward currents, and an attenuated cGMP vasorelaxation associated with a reduced suppression of Ca2+ transients by cGMP. The high level of BK channel expression observed in wild-type adrenal glomerulosa cells, together with unaltered serum renin activities and corticotropin levels in mutant mice, suggests that the hyperaldosteronism results from abnormal adrenal cortical function in BK(-/-) mice. CONCLUSIONS: These results identify previously unknown roles of BK channels in blood pressure regulation and raise the possibility that BK channel dysfunction may underlie specific forms of hyperaldosteronism.

Atherosclerotic vascular lesions are considered to be a major cause of ischemic diseases, including myocardial infarction and stroke. Platelet adhesion and aggregation during ischemia-reperfusion are thought to be the initial steps leading to remodeling and reocclusion of the postischemic vasculature. Nitric oxide (NO) inhibits platelet aggregation and smooth muscle proliferation. A major downstream target of NO is cyclic guanosine 3', 5'-monophosphate kinase I (cGKI). To test the intravascular significance of the NO/cGKI signaling pathway in vivo, we have studied platelet-endothelial cell and platelet-platelet interactions during ischemia/reperfusion using cGKI-deficient (cGKI-/-) mice. Platelet cGKI but not endothelial or smooth muscle cGKI is essential to prevent intravascular adhesion and aggregation of platelets after ischemia. The defect in platelet cGKI is not compensated by the cAMP/cAMP kinase pathway supporting the essential role of cGKI in prevention of ischemia-induced platelet adhesion and aggregation.

The large conductance voltage- and Ca2+-activated potassium (BK) channel has been suggested to play an important role in the signal transduction process of cochlear inner hair cells. BK channels have been shown to be composed of the pore-forming alpha-subunit coexpressed with the auxiliary beta1-subunit. Analyzing the hearing function and cochlear phenotype of BK channel alpha-(BKalpha-/-) and beta1-subunit (BKbeta1-/-) knockout mice, we demonstrate normal hearing function and cochlear structure of BKbeta1-/- mice. During the first 4 postnatal weeks also, BKalpha-/- mice most surprisingly did not show any obvious hearing deficits. High-frequency hearing loss developed in BKalpha-/- mice only from approximately 8 weeks postnatally onward and was accompanied by a lack of distortion product otoacoustic emissions, suggesting outer hair cell (OHC) dysfunction. Hearing loss was linked to a loss of the KCNQ4 potassium channel in membranes of OHCs in the basal and midbasal cochlear turn, preceding hair cell degeneration and leading to a similar phenotype as elicited by pharmacologic blockade of KCNQ4 channels. Although the actual link between BK gene deletion, loss of KCNQ4 in OHCs, and OHC degeneration requires further investigation, data already suggest human BK-coding slo1 gene mutation as a susceptibility factor for progressive deafness, similar to KCNQ4 potassium channel mutations.