A role for membrane transport in modulation of intramuscular free glutamine turnover in streptozotocin diabetic rats

Abstract

We wished to examine the effects of diabetes on muscle glutamine kinetics. Accordingly, female Wistar rats (200 g) were made diabetic by a single injection of streptozotocin (85 mg/kg) and studied 4 days later; control rats received saline. In diabetic rats, glutamine concentration of gastrocnemius muscle was 33% less than in control rats: 2.60 ± 0.06 μmol/g vs. 3.84 ± 0.13 μmol/g ( P < 0.001). In gastrocnemius muscle, glutamine synthetase activity ( V max) was unaltered by diabetes (approx. 235 nmol/min per g) but glutaminase V max increased from 146 ± 29 to 401 ± 94 nmol/min per g; substrate K m values of neither enzyme were affected by diabetes. Net glutamine efflux (AZ concentration difference × blood flow) from hindlimbs of diabetic rats in vivo was greater than control values (−30.0 ± 3.2 vs. −1.9 ± 2.6 nmol/min per g ( P < 0.001) and hindlimb NH 3 uptake was concomitantly greater (about 27 nmol/min per g). The glutamine transport capacity ( V max) of the Na-dependent System N m in perfused hindlimb muscle was 29% lower in diabetic rats than in controls (820 ± 50 vs. 1160 ± 80 nmol/min per g ( P < 0.01)), but transporter K m was the same in both groups (9.2 ± 0.5 nM). The difference between inward and net glutamine fluxes indicated that glutamine efflux in perfused hindlimbs was stimulated in diabetes at physiological perfusate glutamine (0.5 mM); ammonia (1 mM in perfusate) had little effect on net glutamine flux in control and diabetic muscles. In Intramuscular Na + was 26% greater in diabetic (13.2 μmol/g) than control muscle, but muscle K + (100 μmol/g) was similar. The accelerated rate of glutamine release from skeletal muscle and the lower muscle free glutamine concentration observed in diabetes may result from a combination of; (i), a diminished Na + electrochemical gradient (i.e., the net driving force for glutamine accrual in muscle falls); (ii), a faster turnover of glutamine in muscle and (iii), an increased V max/ K m for sarcolemmal glutamine efflux.