Properties of osmolyte fluxes activated during regulatory volume decrease in cultured cerebellar granule neurons

Abstract

Efflux pathways for amino acids, K, and CI activated during regulatory volume decrease (RVD) were characterized in cultured cerebellar granule neurons exposed to hyposmotic conditions. Results of this study favor diffusion pores (presumably channels) over energy‐dependent transporters as the mechanisms responsible for the efflux of these osmolytes. The selectivity of osmolyte pathways activated by RVD was assessed by increasing the extracellular concentrations of cations, anions, and amino acids to such an extent that upon opening of the pathway, a permeable compound will enter the cell and block RVD by reducing the efflux of water carried by the exit of intracellular osmolytes. The cationic pathway was found selective for K (and Rb), whereas the anionic pathway was rather unselective being permeable to CI, nitrate, iodine, benzoate, thiocyanate, and sulfate but impermeable to gluconate. Glutamate and aspartate as K but not as Na salts were permeable through the anion channel. RVD was slightly inhibited by quinidine but otherwise was insensitive to known K channel blockers. RVD was inhibited by 4,4′‐ diisothiocyanostilbene‐2‐2′‐disulfonic acid (DIDS), niflumic acid, and dipridamole. Gramicidin did not affect cell volume in isosmotic conditions but greatly accelerated RVD, suggesting that cell permeability to CI is low in isosmotic conditions but increase markedly during RVD making K permeability the rate limit of the process. The permeability pathway for amino acids activated during RVD was permeable to short chain α‐ and β‐amino acids, but excluded glutamine and basic amino acids. Wiley‐Liss, Inc.