Distribution of neurotransmitters and their effects on glucagon secretion from the in vitro normal and diabetic pancreatic tissues

The distribution of adrenergic, cholinergic and amino acid neurotransmitters and/or their enzymes were examined in both the normal and diabetic pancreatic tissues in rat using immunohistochemistry to determine whether changes in the pattern of distribution of nerves containing these neurotransmitters will occur as a result of diabetes mellitus. In addition to this, the effect of noradrenaline (NA), adrenaline (ADR), acetylcholine (ACh) and γ-amino butyric acid (GABA) on glucagon secretion from the isolated normal and diabetic pancreatic tissues was also investigated. Pancreatic fragments from the tail end of normal and diabetic rats were removed and incubated with different concentrations (10^-8-10^-4 M) of these neurotransmitters. Glucagon secretion into the supernatant was later determined by radioimmunoassay. NA at 10^-6 M evoked a three-fold increase in glucagon secretion from normal pancreatic tissue fragments. In diabetic pancreatic tissue, NA at 10^-6 M was able to increase glucagon secretion 1.5 times the value obtained from diabetic basal. ADR (10^-8 M) increased glucagon secretion slightly but not significantly in normal pancreatic tissue. ADR inhibited glucagon secretion from diabetic pancreas at all concentrations. ACh (10^-8 M) induced a five-fold increase in glucagon secretion from normal pancreatic tissue. In a similar way, ACh evoked a two-fold increase in glucagon secretion from diabetic pancreas at 10^-4 M. In normal pancreatic tissue, GABA produced a slight but not significant increase in glucagon secretion at 10^-4 M. In contrast to this it inhibited glucagon secretion from diabetic pancreatic tissue fragments at all concentrations. In summary, tyrosine hydroxylase- and choline acetyltransferase-positive nerves are equally well distributed in both normal and diabetic rat pancreas. There was an increase in the number of glucagon positive cells and a decrease in the number of GABA-positive cells in diabetic pancreas. NA and ACh have a potent stimulatory effect on glucagon secretion from normal pancreatic tissue fragments, whereas ADR and GABA produced a small but not significant increase in glucagon secretion from normal pancreas. NA and GABA stimulated glucagon secretion from diabetic pancreas. In contrast, ADR and ACh inhibited glucagon secretion from diabetic pancreas. Neurotransmitters vary in their ability to provoke glucagon secretion from either normal or diabetic pancreas. (C) 2000 Harcourt Publishers Ltd.