Synthesis-secretion coupling of insulin: effect of pregnancy and lactation

Synthesis-secretion coupling of insulin was determined using perfused pancreata taken from either control, pregnant, or lactating female Sprague-Dawley rats. The pancreata were stimulated for 3 h with one of the two glucose concentrations used: either 150 or 300 mg/dl. In the case of pregnancy, the pancreata exhibited a twofold hypersecretory activity in response to the physiological glucose level of 150 mg/dl. Net insulinogenesis did not occur in response to the normoglycemic glucose concentration, and, as with the controls, there was an overall depletion of insulin stores. There was no insulin hypersecretion at the hyperglycemic level of 300 mg/dl, but net insulinogenesis did occur; however, it matched that of the controls. Therefore, the hypersecretion of insulin ascribed to pregnancy appears to be a function of the secretory process only, to be most demonstrable under normoglycemic conditions, and not to be due to enhanced synthesis-secretion coupling. In the case of pancreata from lactating rats, just the converse was observed. Compared with controls, there was no substantial difference in insulin secretion by pancreata from lactating rats at either glucose level or of insulinogenesis under normoglycemia conditions. However, when glucose was at a hyperglycemic load in the lactating group, not only was there no net insulinogenesis, but this condition actually resulted in a depletion of intracellular insulin stores (as opposed to both the control and the pregnant groups). This suggests that the hypoinsulinemia noted during lactation may be related to a reduced capacity of those beta-cells to synthesize insulin the face of prolonged hyperglycemic stimulation.