The Role of Na⁺ Ions in the Transport of α-Aminoisobutyric Acid and Other Amino Acids into the Lens

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Abstract

The transport systems for α-AIB and neutral, acid, and basic amino acids into lens were found to comprise a Na⁺-dependent and a Na⁺-independent component. Na⁺ ions, required for 60 to 70 per cent of ¹⁴C-α-AIB transported into rabbit lens, were combined with α-AIB molecules in a 1:1 ratio while transported by a common "carrier" system. However, Na⁺ ions did not affect the diffusion constant nor the ¹⁴C-α-AIB efflux rates from the lens. The in vitro lens incubations showed accumulation by the lens and against the concentration gradient of ¹⁴C-α-AIB and other ¹⁴C-amino acids. Throughout the incubation periods, Na⁺ ions transported with the amino acid continued to be actively extruded from the lens. The Na⁺ deprivation effects closely resembled ouabain inhibitory effects on α-AIB transport, and were both exerted in the lens epithelium. These restdts, and the independence of the lens α-AIB and K⁺ or Rb⁺ carriers, established the presence of a Na⁺ ions link between the Na⁺-dependent fraction of α-AIB and other amino acid transport systems and the ouabain-sensitive Na⁺-K⁺ "pump."