Cell swelling and a nonselective cation channel regulated by internal Ca2+and ATP in native reactive astrocytes from adult rat brain

Hypoxia-ischemia and ATP depletion are associated with glial swelling and b lebbing, but mechanisms involved in these effects remain incompletely chara cterized. We examined morphological and electrophysiological responses of f reshly isolated native reactive astrocytes (NRAs) after exposure to NaN3, w hich depletes cellular ATP. Here we report that NaN3 caused profound and su stained depolarization attributable to activation of a novel 35 pS Ca2+-act ivated, [ATP](i)-sensitive nonselective cation (NCCa-ATP) channel, found in >90% of excised membrane patches. The channel was impermeable to Cl-, was nearly equally permeable to monovalent cations, with permeabilities relativ e to K+ being P-Cs(+)/P-K(+)(1.06) approximate to P-Na(+)/P-K(+)(1.04) appr oximate to P-Rb(+)/P-K(+)(1.02) approximate to P-Li(+)/P-K(+)(0.96), and wa s essentially impermeable to Ca2+ and Mg2+ (P-Ca(2+)/P-K(+) approximate to P-Mg(2+)/P-K(+) < 0.001), with intracellular Mg2+ (100 <mu>M to 1 mM) causi ng inward rectification. Pore radius, estimated by fitting relative permeab ilities of organic cations to the Renkin equation, was 0.41 nm. This channe l exhibited significantly different properties compared with previously rep orted NCCa-ATP channels, including different sensitivity to block by variou s adenine nucleotides (EC50 of 0.79 muM for [ATP](i), with no block by AMP or ADP), and activation by submicromolar [Ca] i. The apparent dissociation constant for Ca2+ was voltage dependent (0.12, 0.31, and 1.5 muM at -40, -8 0, and -120 mV, respectively), with a Hill coefficient of 1.5. Channel open ing by [ATP] i depletion was accompanied by and appeared to precede blebbin g of the cell membrane, suggesting participation of this channel in cation flux involved in cell swelling. We conclude that NRAs from adult rat brain express a 35 pS NCCa-ATP channel that may play an important role in the pat hogenesis of brain swelling.