Responses and molecular heterogeneity of IR-GIP after intraduodenal glucose and fat

The plasma responses and molecular heterogeneity of human immunoreactive gastric inhibitory polypeptide (IR-GIP) after intraduodenal administration of equicaloric amounts of glucose (25 g) and fat (10 g) were determined with five different antisera. Two of these do not cross-react with 8 kdaltons (kDa) IR-GIP. In the fasting state plasma IR-GIP concentrations were highest measured with antisera, which detected both 5 and 8 kDa IR-GIP. The total integrated or incremental IR-GIP areas were similar after glucose and fat when determined with the same antiserum. When antisera with similar cross-reactivity with human 5 kDa IR-GIP were compared, no difference in incremental IR-GIP areas after either stimulus was found between antisera that did and did not measure 8 kDa IR-GIP. Gel filtration of plasma showed a consistent increase in 5 kDa IR-GIP 60 min after both glucose and fat but small and inconsistent changes in 8 kDa IR-GIP. Thus fat on a weight and molar basis is more potent than glucose in releasing IR-GIP. Both fat and glucose release predominantly the 5 kDa IR-GIP. Although of importance for the absolute IR-GIP level in plasma, 8 kDa IR-GIP contributes little to the increase in IR-GIP after both stimuli.