Rapid stimulatory effect of bradykinin on glucose transport across the brush-border and basolateral membranes of rat jejunal enterocytes

Mechanisms of bradykinin-induced glucagon release in clonal alpha-cells In-R1-G9: involvement of Ca(2+)-dependent and -independent pathways

The mechanisms by which bradykinin (BK) increases glucagon release were investigated. BK (0.1-10 microM) increased [Ca(2+)](i) and glucagon release in clonal alpha-cells In-R1-G9. BK-induced glucagon release was lower in the absence than in the presence of extracellular Ca(2+), but it still increased glucagon release while [Ca(2+)](i) was stringently deprived. Depletion of intracellular Ca(2+) store with thapsigargin abolished both the BK-induced Ca(2+) peak and sustained plateau. Microinjection of heparin abolished BK-induced Ca(2+) release. Pertussis toxin (PTX) did not block BK-induced [Ca(2+)](i) increase or glucagon release. U-73122 (8 microM), a phospholipase C (PLC) inhibitor, abolished BK-induced increases in [Ca(2+)](i), but only reduced BK-induced glucagon release by 40%. A phospholipase D (PLD) inhibitor zLYCK reduced BK-induced glucagon release by 60%. The combination of U-73122 and zLYCK abolished BK-induced glucagon release. Both SK&F 96365, a receptor-operated Ca(2+) channel (ROC) blocker and nimodipine, an L-type Ca(2+) channel blocker, reduced BK-induced [Ca(2+)](i) increase and glucagon release. These findings suggest that BK increase glucagon release through a PTX-insensitive G protein and both Ca(2+)-dependent and -independent pathways. The Ca(2+)-dependent pathway is attributable to PLC activation. PLC catalyzes IP(3) formation, inducing Ca(2+) release from the endoplasmic reticulum, which, in turn, triggers Ca(2+) influx via both ROCs and L-type channels. PLD activation may be involved in Ca(2+)-dependent and/or -independent pathway.

Vagal involvement in dietary regulation of nutrient transport

In omnivores, gradual alterations in dietary nutrient composition are observed. To efficiently absorb dietary nutrients these animals alter intestinal nutrient transporter expression to match the pattern of nutrient intake. This often involves reprogramming the crypt cell to express greater numbers of the relevant transport system. The aim of this study was to determine whether vagal afferents are involved in this adaptive process. Guinea pigs were habituated to a low-carbohydrate diet and then switched to a high-carbohydrate diet. The resultant increase in glucose transporter expression was assessed by determining rates of glucose transport in jejunal brush-border membrane vesicles. Ablation of vagal afferents was accomplished by application of capsaicin to exposed cervical vagi and confirmed using Fast blue tracer studies. We found that animals in which vagal afferents were ablated with capsaicin were unable to alter rates of glucose transport in response to an increase in dietary carbohydrate. This suggests that vagal afferents are involved in this adaptive process. These findings support a role for the vagus nerve in regulating intestinal transport function, which may be important to consider in clinical disease that involves the vagus nerve.

Rapid enhancement of brush border glucose uptake after exposure of rat jejunal mucosa to glucose

BACKGROUND

Increased jejunal glucose transport after ingestion of carbohydrate rich diets may reflect higher concentrations of lumenal glucose. Normal processing of carbohydrate causes wide fluctuations in glucose concentration in the jejunal lumen and this raises the question of whether the high lumenal concentrations seen at peak digestion affect glucose uptake.

AIMS

To study the effects of 30 minute exposure of rat jejunal mucosa to glucose on sodium-glucose transporter (SGLT1) mediated glucose transport across the brush border membrane.

METHODS

Jejunal mucosa was exposed in vitro or in vivo to 25 mM glucose or 25 mM mannitol for 30 minutes. In addition, isolated villus enterocytes were incubated with mannitol or glucose for the same time. Brush border membrane vesicles were isolated from these preparations and phlorizin sensitive 3H-D-glucose accumulation was measured.

RESULTS

Lumenal glucose in vivo significantly enhanced SGLT1 mediated glucose uptake by 49.2-57.2%. For jejunal loops in vitro, the increase was 32.0-85.2%. Kinetic analysis disclosed a 50% greater Vmax for glucose uptake in each preparation. The facilitated and passive components of uptake were, however, unaffected by prior exposure to glucose. Incubation of villus enterocytes with 25 mM glucose did not influence glucose uptake by brush border membranes. Finally, exposure of intact mucosa to 20 mM galactose, a nonmetabolised sugar also transported by SGLT1, did not alter glucose transport.

CONCLUSIONS

Lumenal glucose promotes glucose transport by brush border membrane within 30 minutes. An intact mucosa is necessary for upregulation and evidence suggests that the response is mediated by locally acting mechanisms.

Rapid regulation of rat jejunal glucose transport by insulin in a luminally and vascularly perfused preparation

1. The regulation of glucose transport by physiological concentrations of insulin was investigated using a preparation of rat jejunum perfused in situ with 5 mM glucose on both sides. 2. Luminal uptake was 87% inhibited (P < 0.001) by 0.2 mM phlorizin, indicating that it occurred by means of the Na(+)-D-glucose cotransporter. Vascular uptake was completely abolished by 0.2 mM phloretin, indicating that it was facilitated in nature. 3. When infused into the vascular circuit, insulin (10(-11) to 10(-7) M) stimulated vascular, and inhibited luminal, glucose uptake to a similar extent. Maximal stimulation of vascular uptake was increased by 40% compared with control infusions (P < 0.01) and occurred at 10(-10) M insulin. These effects were independent of changes in metabolism and vascular glucose concentration. 4. The time taken for half-maximal stimulation of vascular uptake was 6.3 +/- 0.7 min and preceded that for inhibition of luminal uptake by 6.5 +/- 1.3 min (P < 0.02). 5. The rapid inhibition of luminal glucose uptake by the acute administration of insulin was also detected by perfusion of jejunal loops in vivo. 6. It is concluded that the transport steps involved in intestinal glucose uptake are subject to rapid regulation by physiological concentrations of insulin and that the initial site of action is on the vascular side.

Insulin induced hypoglycaemia and sugar transport across the brush border and basolateral membranes of rat jejunal enterocytes

Acute hypoglycaemia enhances intestinal sugar uptake but the mechanisms involved are unknown. Results from the present study show increased galactose movement across the brush border and basolateral membranes of isolated upper, but not mid-villus, jejunal enterocytes 45 min after intravenous administration of insulin to rats at a level which reduced by half plasma glucose concentration. Incubation of upper villus cells from uninjected animals with insulin (100 mU ml-1) for 40 min was without effect on brush border or basolateral sugar transport. Insulin treatment of rats did not affect glucose uptake by brush border vesicles prepared from upper villus cells when the process was driven by an inwardly directed 100 mM sodium thiocyanate gradient. In contrast, glucose uptake using a 100 mM inwardly directed sodium chloride gradient was reduced by 49% following hypoglycaemia. It is concluded that the enhanced sugar uptake following insulin hypoglycaemia involves both brush border and basolateral membranes of only the most mature villus cells at the villus tip. Upregulation of Na(+)-sugar cotransport at the brush border is best explained by an increased electrochemical driving force for Na(+)-sugar cotransport rather than increased numbers of transporters. The transport response is not due to a direct effect of insulin on the enterocyte and the possible systemic factors involved are discussed.