The patch-clamp recording technique and RNApolymerase chain reaction were used to identify the voltage-dependent K ؉ channels expressed by murine fetal and adult CD4 ؊ CD8 ؊ thymocytes. Two distinct currents, encoded by the genes Kv1.1 and Kv1.3 were identified based upon their biophysical and pharmacologic characteristics and confirmed with RNA-polymerase chain reaction. Peptide blockers of Kv1.1 and Kv1.3 gene products were also applied to a murine fetal thymic organ culture system to

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... ate the developmental role of these K ؉ channels. Dendrotoxin (DTX) and charybdotoxin (CTX), antagonists of Kv1.1 and Kv1.3 channels, respectively, decreased thymocyte yields in organ culture without affecting thymocyte viability. DTX-treated thymi contained 56 ؎ 8% (n ‫؍‬ 8 experiments), and CTX-treated thymi contained 74 ؎ 4% (n ‫؍‬ 7 experiments) as many thymocytes as untreated lobes. DTX and CTX also altered the developmental progression of thymocytes in fetal organ culture. These data provide the first evidence of Kv1.1 expression in a lymphoid cell and indicate that thymocyte voltage-dependent K ؉ channels are critical to thymocyte preclonal expansion and/or maturation.