Maxi-K channels localize to caveolae in human myometrium: a role for an actin-channel-caveolin complex in the regulation of myometrial smooth muscle K+ current

1 Department of Physiology and Biophysics, Carver College of Medicine, University of Iowa, Iowa City, Iowa; and 2 Department of Pharmacology, University of Michigan, Ann Arbor, Michigan Submitted 15 August 2004 ; accepted in final form 8 February 2005 Multiple cell-signaling pathways converge to modulate large-conductance, voltage- and Ca 2+ -sensitive K + channel (maxi-K channel) activity and buffer cell excitability in human myometrial smooth muscle cells (hMSMCs). Recent evidence indicates that maxi-K channel proteins can target to membrane microdomains; however, their association with other proteins within these macromolecular complexes has not been elucidated. Biochemical isolation of detergent-resistant membrane fractions from human myometrium demonstrates the presence of maxi-K channels in lipid raft microdomains, which cofractionate with caveolins. In both nonpregnant and late-pregnant myometrium, maxi-K channels associate and colocalize with caveolar scaffolding proteins caveolin-1 and caveolin-2, but not caveolin-3. Disruption of cultured hMSMC caveolar complexes by cholesterol depletion with cyclodextrin increases an iberiotoxin-sensitive K + current. Coimmunoprecipitations have indicated that the maxi-K channel also is associated with both - and -actin. Immunocytochemical analysis indicates colocalization of maxi-K channels, actin, and caveolin-1 in primary cultures of hMSMCs. Further experiments using immunoelectron microscopy have shown the proximity of both actin and the maxi-K channel within the same cell surface caveolar structures. Functionally, disruption of the actin cytoskeleton in cultured hMSMCs by cytochalasin D and latrunculin A greatly increased the open-state probability of the channel, while stabilization of actin cytoskeleton with jasplakinolide abolished the effect of latrunculin A. These data indicate that the actin cytoskeleton is involved as part of a caveolar complex in the regulation of myometrial maxi-K channel function. potassium channel; membrane microdomain Address for reprint requests and other correspondence: S. K. England, Dept. of Physiology and Biophysics, Univ. of Iowa Carver College of Medicine, 5-660 Bowen Science Bldg., Iowa City, IA 52242 (e-mail: sarah-england{at}uiowa.edu )