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.