Functional expression of Kir 6.1/SUR1‐K_ATP channels in frog retinal Müller glial cells

Abstract

The retinae and brains of larval and adult amphibians survive long-lasting anoxia; this finding suggests the presence of functional K(ATP) channels. We have previously shown with immunocytochemistry studies that retinal glial (Müller) cells in adult frogs express the K(ATP) 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 (K(ATP)) channels in adult frog and tadpole Müller cells. Using conditions favoring the activation of K(ATP) 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 K(ATP) 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 microM) at the cytoplasmic side. The channels apparently show two sublevels (each of approximately 27-32 pS) with a total of 85-pS maximal conductance at -80 mV; the open probability follows a two-exponential mechanism. Thus, functional K(ATP) 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. K(ATP) channels may help maintain glial cell functions during ATP deficiency.

Downregulation of Kir4.1 inward rectifying potassium channel subunits by RNAi impairs potassium transfer and glutamate uptake by cultured cortical astrocytes

“…the Kir subfamily) (Biedermann et al, 1998;Brew et al, 1986;Higashi et al, 2001;Kofuji et al, 2000;Li et al, 2001;Skatchkov et al, 1995;Sontheimer, 1994). In the specialized M€ uller glial cells of the retina, the channel proteins underlying their K 1 conductances have been intensely studied (Bringmann et al, 1997;Ishii et al, 1997;Raap et al, 2002;Skatchkov et al, 2001Skatchkov et al, , 2002Skatchkov et al, , 2006a. In the retina, Kir4.1 channels are mainly found in M€ uller glial cells (Dalloz et al, 2003;Ishii et al, 1997;Kofuji et al, 2002;Nagelhus et al, 1999;Skatchkov et al, 2001).…”

Complex rectification of Müller cell Kir currents

“…The presence of long distance (Bykov et al, 1981;Dick et al, 1985) and short distance (Karwoski et al, 1989) spatial K 1 -buffering were demonstrated by simultaneous measurement of potassium ion concentration and electrical potential generated by the glial M€ uller cells of the retina. Theoretical analysis suggests that long distance K 1 -spatial buffering will be efficient if K expressed in glial M€ uller cells (Ishii et al, 1997;Kofuji et al, 2000;Kusaka and Puro, 1997;Skatchkov et al, 2001Skatchkov et al, , 2002. Immunolabel for Kir2.1 was found in these cells by Kofuji et al (2002), but not by Tian et al (2003) and these channels were never functionally observed neither in Kir4.1 knock-out mice (Kofuji et al, 2000) nor in conditional knock-out of Kir4.1 mice (Djukic et al, 2007) as well as in rat M€ uller cells (rev.…”

KR-31378, a Potassium-Channel Opener, Induces the Protection of Retinal Ganglion Cells in Rat Retinal Ischemic Models

“…The disturbed ionic environment is deleterious to retinal neurons and lead to cell death. Retinal Müller cells have K ATP channels and can protect retinal neurons endogenously (8,11).…”

“…ATP-sensitive potassium channels (K ATP channels) in the mitochondrial or plasma membrane of retinal cells may provide protection against retinal ischemia (10,11). Yamauchi and colleagues suggested that opening of mitochondrial K ATP channels can inhibit glutamate.…”

“…Yamauchi and colleagues suggested that opening of mitochondrial K ATP channels can inhibit glutamate. Skatchkov and his colleagues observed K ATP channels in Müller cells and suggested that these channels regulate retinal current and play a key role in retinal protection against ischemic conditions (11).…”