Located within the gastrointestinal (GI) musculature are networks of cells known as interstitial cells of Cajal (ICC). ICC are associated with several functions including pacemaker activity that generates electrical slow waves and neurotransmission regulating GI motility. In this study we identified a voltage‐dependent K+ channel (Kv1.1) expressed in ICC and neurons but not in smooth muscle cells. Transcriptional analyses demonstrated that Kv1.1 was expressed in whole tissue but not in isolated smooth muscle cells. Immunohistochemical co‐localization of Kv1.1 with c‐kit (a specific marker for ICC) and vimentin (a specific marker of neurons and ICC) indicated that Kv1.1‐like immunoreactivity (Kv1.1‐LI) was present in ICC and neurons of GI tissues of the dog, guinea‐pig and mouse. Kv1.1‐LI was not observed in smooth muscle cells of the circular and longitudinal muscle layers. Kv1.1 was cloned from a canine colonic cDNA library and expressed in Xenopus oocytes. Pharmacological investigation of the electrophysiological properties of Kv1.1 demonstrated that the mamba snake toxin dendrotoxin‐K (DTX‐K) blocked the Kv1.1 outward current when expressed as a homotetrameric complex (EC50= 0.34 nm). Other Kv channels were insensitive to DTX‐K. When Kv1.1 was expressed as a heterotetrameric complex with Kv1.5, block by DTX‐K dominated, indicating that one or more subunits of Kv1.1 rendered the heterotetrameric channel sensitive to DTX‐K. In patch‐clamp experiments on cultured murine fundus ICC, DTX‐K blocked a component of the delayed rectifier outward current. The remaining, DTX‐insensitive current (i.e. current in the presence of 10−8m DTX‐K) was outwardly rectifying, rapidly activating, non‐inactivating during 500 ms step depolarizations, and could be blocked by both tetraethylammonium (TEA) and 4‐aminopyridine (4‐AP). In conclusion, Kv1.1 is expressed by ICC of several species. DTX‐K is a specific blocker of Kv1.1 and heterotetrameric channels containing Kv1.1. This information is useful as a means of identifying ICC and in studies of the role of delayed rectifier K+ currents in ICC functions.

The regulation of cardiac delayed rectifier potassium (Kv) currents by cAMP-dependent protein kinase (PKA) contributes to the control of blood pressure and heart rate. We investigated the modulation by PKA and protein phosphatases of cloned Kv1.5 channels expressed in Xenopus laevis oocytes. Exposure of oocytes to activators of PKA (100 nM forskolin, 1 mM 8-bromo-cAMP, or 1 mM 3-isobutyl-1-methylxanthine) had no effect on the amplitude of Kv1.5 currents. Inhibition of PKA by injection of protein kinase A inhibitor peptide or exposure to myristoylated protein kinase A inhibitor peptide (M-PKI; 100 nM) reduced currents mediated by Kv1.5. M-PKI also reduced the amplitude of currents mediated by mutated Kv1.5 channels in which the COOH terminal PKA phosphorylation sites and PSD-95, Disc-large, and ZO-1-binding domain were removed. The reduction of Kv1.5 currents by M-PKI was attenuated by inhibition of actin polymerization by 1 M cytochalasins B and D, but was not affected by 10 M phalloidin (stabilizes actin filaments) or 50 M colchicine (disrupts microtubules). Treatment of oocytes with antisense oligonucleotides against ␣-actinin-2 abolished the reduction in Kv1.5 current by M-PKI. These observations suggest that Kv1.5 currents are activated by endogenous PKA in "resting" oocytes and that inhibition of PKA activity reveals the action of endogenous phosphatases. Indeed, injection of alkaline phosphatase reduced currents mediated by Kv1.5. Further preincubation of oocytes with 1 mM sodium orthovanadate (a protein tyrosine phosphatase inhibitor) abolished the reduction in Kv1.5 currents by M-PKI. We conclude that currents encoded by Kv1.5 are regulated by PKA and protein tyrosine phosphatase and that this regulation requires an intact actin cytoskeleton and ␣-actinin-2.