The role of cyclic AMP in the control of sugar transport across the brush‐border and basolateral membranes of rat jejunal enterocytes

Abstract

SUMMARYAlthough the involvement of the adenylate cyclase system with glucose transport in the small intestine is poorly understood, there is increasing evidence that cyclic AMP stimulates sugar uptake. In order to study further the effects of cyclic AMP on this process, we have measured glucose accumulation by brush-border and basolateral membrane vesicles prepared from villus enterocytes following exposure of these cells to cyclic AMP and theophylline. Brush-border vesicles derived from enterocytes incubated with cyclic AMP and theophylline accumulated significantly more glucose over a wide range of sugar concentrations, suggesting a change in maximum velocity of the transport system. Glucose uptake by basolateral vesicles was increased at low, but not at high sugar concentrations. Incubation of isolated enterocytes with pancreatic glucagon at concentrations known to stimulate sugar transport by these cells significantly increased enterocyte levels of cyclic AMP. Treatment with glucagon or cyclic AMP resulted in significant hyperpolarization of the potential difference across the brushborder membrane, an important driving force for Na+-sugar cotransport. The response to glucagon and cyclic AMP appears to be caused by a decrease in Na+ permeability of the mucosal membrane. Taken together, these results suggest that cyclic AMP is a mediator of the actions of glucagon on enterocytes and provide further evidence for a role of cyclic AMP in the modulation of sugar transport across the intestinal enterocyte.

GLUT2 protein at the rat proximal tubule brush border membrane correlates with protein kinase C (PKC)-βl and plasma glucose concentration

“…Although most studies have used the small intestine, the mechanisms controlling glucose uptake by proximal tubular cells display striking similarity to those occurring in enterocytes. In both cell types, activation of the protein kinase A signalling pathway increases intracellular cAMP levels and promotes SGLT1-mediated glucose transport [16,34]. Short-term modulation of SGLT1-mediated transport occurs via apical membrane insertion of additional SGLT1 protein [35] and through changes in the membrane electrochemical gradient [16].…”

“…In both cell types, activation of the protein kinase A signalling pathway increases intracellular cAMP levels and promotes SGLT1-mediated glucose transport [16,34]. Short-term modulation of SGLT1-mediated transport occurs via apical membrane insertion of additional SGLT1 protein [35] and through changes in the membrane electrochemical gradient [16]. In enterocytes, activation of the PKC signalling pathway by PMA and the influx of Ca 2+ through L-type channels have been shown to induce cytoskeletal re-arrangement, leading to insertion of GLUT2 protein at the BBM [6-8, 18, 36].…”

“…Brush border membrane uptake studies Uptake studies were carried out at 20°C on freshly prepared vesicles as described for intestinal BBM [16]. In brief, the transport process was initiated by mixing equal volumes of vesicle suspension and uptake buffer consisting of (in mmol/l) 200 NaSCN, 20 HEPES, 0.1 MgSO 4 containing D-[ 3 H]glucose, such that the final concentration of glucose was 30 to 960 μmol/l.…”

Streptozotocin diabetes and the expression of GLUT1 at the brush border and basolateral membranes of intestinal enterocytes

“…A possible humoral mediator is pancreatic glucagon which is known to increase BBM and BLM sugar transport [9] and which is secreted in increased amounts during diabetes. The hormone may act by increasing the production of cAMP within enterocytes and in this context it is of interest that experimentally raised cytosolic levels of cAMP promote enterocyte sugar transport [23]. cAMP has also been implicated in the regulation of glucose transporter expression in other cell types [24].…”

Transport

“…The role which diabetes may play in the modulation of the a very high affinity L-arginine transport system for L-arginine in the human fetal endothelium remains to be investigated. Regional effects of cyclic AMP on expression of the glucose transporters SGLT1, GLUT2 and GLUTS in isolated rat intestinal brush-border membrane Exposure of jejunal enterocytes to cAMP increases brushborder (BB) expression of SGLTl (Brown et al 1997) and SGLT1 -mediated sugar transport (Sharp & Debnam, 1994). Our present study has examined regional changes in BB levels of SGLT1 , as well as the facilitative transporters GLUT2 and GLUT5 , following mucosal exposure to cAMP.…”