Fig. 6.

GIP secretion in the presence of elevated cAMP. a Mixed intestinal cultures from the upper small intestine were incubated for 2-4 h in bath solution containing fsk/IBMX (10 μmol/l of each) plus different glucose concentrations, as indicated. GIP was measured in the supernatant fraction and cell extracts, and is expressed relative to the secretion in wells containing fsk/IBMX measured in parallel on the same day. The data were fitted with a logistic equation [y=A+(1−A)/(1+(x/ED₅₀)ⁿ)], with a maximal amplitude A=2.1, an ED₅₀=0.6 mmol/l and slope factor n of 1.3. Error bars represent 1 SE, and significance is shown relative to fsk+IBMX, analysed by single-factor t test; *p<0.05, **p<0.01. b GIP responses to agonists in the presence of fsk/IBMX (10 μmol/l of each). Experiments were carried out as in a, with additions as indicated: control (Con), glucose (Gluc, 10 mmol/l), glutamine (Gln, 10 mmol/l), tolbutamide (Tolb, 500 μmol/l), sucralose (Sucrl, 1 mmol/l), αMG (10 mmol/l), phloridzin (Phlorid, 5 μmol/l), diazoxide (Diazox, 340 μmol/l). Number of wells: Gluc, 17; Gln, 6; Tolb, 20; Sucrl, 3; αMG+Tolb, 13; αMG, 24; αMG+Phlorid, 6; aMG+Diazox, 6. Secretion was normalised to that measured in fsk/IBMX alone, measured in parallel on the same day (indicated by the dashed line). Error bars represent 1 SE, and significance is shown relative to fsk/IBMX, analysed by a single-factor t test; *p<0.05, **p<0.01, ***p<0.001. Significance of differences between different combinations that included αMG were tested by Student’s t test; ns non significant, †††p<0.001