Spatial distribution of spermine/spermidine content and K+-current rectification in frog retinal glial (Muller) cells

Previous studies in retinal glial (Muller) cells have suggested that (1) th e dominant membrane currents are mediated by K+ inward-rectifier (Kir) chan nels (Newman and Reichenbach, Trends Neurosci 19:307-312, 1996), and (2) re ctification of these Kir channels is due largely to a block of outward curr ents by endogenous polyamines such as spermine/spermidine (SPM/SPD) (Lopati n et al., Nature 372:366-369, 1994). In frog Muller cells, the degree of re ctification of Kir-mediated currents is significantly higher in the endfoot than in the somatic membrane (Skatchkov et al., Glia 27:171-181, 1999). Th is article shows that in these cells there is a topographical correlation b etween the local cytoplasmic SPM/SPD immunoreactivity and the ratio of inwa rd to outward K+ currents through the surrounding membrane area. Throughout the retina, Muller cell endfeet display a high SPM/SPD immunolabel (assess ed by densitometry) and a large inward rectification of K+ currents, as mea sured by the ratio of inward to outward current produced by step changes in [K+](o). In the retinal periphery, Muller cell somata are characterized by roughly one-half of the SPM/SPD immunoreactivity and K+-current rectificat ion as the corresponding endfcet. In the retinal center, Muller cell somata are virtually devoid of both SPM/SPD immunolabel and K+-current inward rec tification. Comparing one region of the retina with another, we find an exp onential correlation between the local K+ rectification and the local SPM/S PD content. This finding suggests that the degree of inward rectification i n a given membrane area is determined by the local cytoplasmic polyamine co ncentration. (C) 2000 Wiley-Liss, Inc.