Osmotic Shrinkage Activates Nonselective Cation (NSC) Channels in Various Cell Types

Abstract.

Osmotic cell shrinkage activates a nonselective cation (NSC) channel in M-1 mouse cortical collecting duct cells (Volk, Frömter & Korbmacher, 1995, Proc. Natl. Acad. Sci. USA 92: 8478-8482). To see whether shrinkage-activated NSC channels are an ubiquitous phenomenon, we tested the effect of hypertonic extracellular solution on whole-cell currents of HT₂₉ human colon carcinoma cells, BSC-1 renal epithelial cells, A10 vascular smooth muscle cells, and Neuro-2a neuroblastoma cells. Addition of 100 mm sucrose to an isotonic NaCl bath solution induced cell shrinkage of HT₂₉ cells as evidenced by a decrease in cell diameter from 18 ± 1 μm to 12 ± 1 μm (n= 13). Upon cell shrinkage whole-cell currents of HT₂₉ cells increased within 8 ± 1 min by about 30-fold (n= 13). Cell shrinkage and current activation were reversible upon return to isotonic solution. Replacement of bath Na⁺ by K⁺ or Li⁺ had almost no effect on the stimulated inward current. In contrast, replacement by N-methyl-d-glucamine (NMDG) completely abolished it and shifted the reversal potential from −4.5 ± 0.7 mV to −57 ± 4.1 mV (n= 10). Thus, the stimulated conductance is nonselective for alkali cations but highly selective for cations over anions with a cation-to-anion permeability ratio of about 13. Flufenamic acid (100 μm) inhibited the stimulated current by 84 ± 4.7% (n= 8). During the early phase of hypertonic stimulation single-channel transitions could be detected in whole-cell current recordings, and a gradual activation of 12 and more individual channels with a single-channel conductance of 17.6 ± 0.9 pS (n= 4) could be resolved. In analogous experiments similar shrinkage-activated NSC channels were also observed in BSC-1 renal epithelial cells, A10 vascular smooth muscle cells, and Neuro-2a neuroblastoma cells. These findings indicate that shrinkage-activated NSC channels are an ubiquitous phenomenon and may play a role in volume regulation.