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Keywords:

  • inwardly rectifying K+ channels;
  • KATP;
  • immunocytochemistry;
  • electrophysiology;
  • frog retina;
  • glial cells

Abstract

The retinae and brains of larval and adult amphibians survive long-lasting anoxia; this finding suggests the presence of functional KATP channels. We have previously shown with immunocytochemistry studies that retinal glial (Müller) cells in adult frogs express the KATP channel and receptor proteins, Kir6.1 and SUR1, while retinal neurons display Kir6.2 and SUR2A/B (Skatchkov et al., 2001a: NeuroReport 12:1437–1441; Eaton et al., in press: NeuroReport). Using both immunocytochemistry and electrophysiology, we demonstrate the expression of Kir6.1/SUR1 (KATP) channels in adult frog and tadpole Müller cells. Using conditions favoring the activation of KATP channels (i.e., ATP- and spermine-free cytoplasm-dialyzing solution containing gluconate) in Müller cells isolated from both adult frogs and tadpoles, we demonstrate the following. First, using the patch-clamp technique in whole-cell recordings, tolbutamide, a blocker of KATP channels, blocks nearly 100% of the transient and about 30% of the steady-state inward currents and depolarizes the cell membrane by 5–12 mV. Second, inside-out membrane patches display a single-channel inward current induced by gluconate (40 mM) and blocked by ATP (200 μM) at the cytoplasmic side. The channels apparently show two sublevels (each of ∼27–32 pS) with a total of 85-pS maximal conductance at −80 mV; the open probability follows a two-exponential mechanism. Thus, functional KATP channels, composed of Kir6.1/SUR1, are present in frog Müller cells and contribute a significant part to the whole-cell K+ inward currents in the absence of ATP. Other inwardly rectifying channels, such as Kir4.1 or Kir2.1, may mediate the remaining currents. KATP channels may help maintain glial cell functions during ATP deficiency. GLIA 38:256–267, 2002. © 2002 Wiley-Liss, Inc.