Sugar transport in giant axons of Loligo

1. The transport of glucose and a number of other sugars has been investigated in the giant axons of Loligo forbesi. 2. Glucose and 2-deoxy-D-glucose are phosphorylated by squid axons, alpha-methyl-D-glucopyranoside and 3-O-methylglucose are not metabolized. All four sugars can diffuse freely in axoplasm. 3. Sugar uptake in squid axons is a passive, saturable process. The maximum rate of sugar uptake increases in the order 3-O-methylglucose less than 2-deoxy-D-glucose less than D-glucose. Competition between these sugars suggests a common uptake mechanism. 4. The uptake of D-glucose but not 3-O-methylglucose or 2-deoxy-D-glucose is reduced when the external Na concentration is lowered. 5. Glucose uptake is sensitive to temperature with a Q10 for saturated uptake of 1.9 between 14.5 and 5 degrees C. Uptake is unaffected by external pH in the range 5-10 but is reduced by cyanide (2 mM). 6. Glucose and 2-deoxy-D-glucose uptake and metabolism are increased by electrical stimulation. These effects are prevented by ouabain. The uptake of 3-O-methylglucose is unaffected by stimulation. 7. The maximum rate of 3-O-methylglucose efflux is higher than the maximum rate of uptake of the sugar suggesting that transport of this sugar is asymmetric. 8. 3-O-methylglucose efflux is reduced by external sugars with order of potency 3-O-methylglucose greater than 2-deoxy-D-glucose greater than D-glucose. These effects persist when the internal 3-O-methylglucose concentration is as high as 50 mM. 9. 3-O-methylglucose efflux is inhibited reversibly by cytochalasin B and phloridzin but irreversibly by phloretin. Efflux is reduced reversibly by cyanide (2 mM). 10. 3-O-methylglucose efflux is sensitive to temperature with a Q10 of 3.2 over the range 10-20 degrees C. Efflux is unaffected by external pH in the range 6-9 but is reduced reversibly by internal acidification.