Cell Swelling and a Nonselective Cation Channel Regulated by Internal Ca ²⁺ and ATP in Native Reactive Astrocytes from Adult Rat Brain

Abstract

Hypoxia–ischemia and ATP depletion are associated with glial swelling and blebbing, but mechanisms involved in these effects remain incompletely characterized. We examined morphological and electrophysiological responses of freshly isolated native reactive astrocytes (NRAs) after exposure to NaN ₃, which depletes cellular ATP. Here we report that NaN ₃ caused profound and sustained depolarization attributable to activation of a novel 35 pS Ca ²⁺-activated, [ATP] _i-sensitive nonselective cation (NC _Ca-ATP) channel, found in >90% of excised membrane patches. The channel was impermeable to Cl ⁻, was nearly equally permeable to monovalent cations, with permeabilities relative to K ⁺ being P _Cs ⁺/P _K ⁺(1.06) ≈ P _Na ⁺/P _K ⁺(1.04) ≈ P _Rb ⁺/P _K ⁺(1.02) ≈ P _Li ⁺/P _K ⁺(0.96), and was essentially impermeable to Ca ²⁺ and Mg ²⁺(P _Ca ²⁺/P _K ⁺≈ P _Mg ²⁺/P _K ⁺< 0.001), with intracellular Mg ²⁺ (100 μ m to 1 m m) causing inward rectification. Pore radius, estimated by fitting relative permeabilities of organic cations to the Renkin equation, was 0.41 nm. This channel exhibited significantly different properties compared with previously reported NC _Ca-ATP channels, including different sensitivity to block by various adenine nucleotides (EC ₅₀of 0.79 μ m for [ATP] _i, with no block by AMP or ADP), and activation by submicromolar [Ca] _i. The apparent dissociation constant for Ca ²⁺ was voltage dependent (0.12, 0.31, and 1.5 μ m at −40, −80, and −120 mV, respectively), with a Hill coefficient of 1.5. Channel opening by [ATP] _i depletion was accompanied by and appeared to precede blebbing 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 NC _Ca-ATP channel that may play an important role in the pathogenesis of brain swelling.