Mechanism of exocrine pancreatic insufficiency during diabetes mellitus

Abstract

The interaction between endocrine and exocrine pancreas has not been properly and thoroughly investigated especially during diabetic conditions. This study was designed to investigate the interaction of the islet hormone insulin with the
cholinergic neurotransmitter acetylcholine (ACh) and the gut hormone cholecystokinin-octapeptide (CCK-8), measuring the amylase output, intracellular free calcium concentration ([Ca2i), intracellular free magnesium concentration ([Mg2 ]) and pancreatic juice secretion in the isolated pancreatic segments and the anaesthetised rat preparations of diabetic and age-matched control rats. Alter 6-8 weeks of diabetes induction, diabetic rats in all the experimental groups weighed significantly (pc0.05) less (236.88 ± 7.2 g) compared to age-matched controls (388.68 ± 30.8 g). Similarly, blood glucose levels were significantly (pc0.05) higher (>>500 mg dF') in diabetic rats compared to age-matched controls (92.67 ± 2.41 mg
Basal values for pancreatic juice flow, [Ca 2 ']1, and [Mg2 ] in diabetic pancreas were 0.45 ± 0.046 p1 mm 4 , 53.8 ± 11.5 nM, 1.07 ± 0.023 mlvi, respectively. Basal values for pancreatic juice flow, [Ca 2 ]; and [Mg2 ']1 in age-matched controls were 0.65 ± 0.021 p1 min', 76.8 ± 0.85 nM and 1.21 ± 0.026 mM, respectively. Stimulation of isolated pancreatic segments from normal and diabetic rats with different concentrations of either ACh or CCK-8 resulted in marked increases of amylase
output. The effects of the secretagogues were less pronounced on the pancreatic segments from diabetic animals compared to age-matched controls. Amylase output in the presence of either 10 5M ACh or 10 8M CCK-8 from control healthy animals
was 10.75 ± 0.82 U mI 1 (100 mg oftissue' and 25.6 ± 2.5 U ml' (100 mg oftissue, respectively. The effects of the two secretagogues were less pronounced in pancreatic segments taken from diabetic animals when compared to age-matched control animals. Amylase output in the presence of 10 5M ACh and 10 8M CCK-8 from diabetic segments was 8.25 ± 0.76 U mY' (100 mg of tissue) -' and 17.1 ± 1.9 U mY' (100 mg of tissue)- ', respectively. Combining the islet hormone insulin with
either ACh or CCK-8 resulted in marked potentiation of secretagogues-evoked pancreatic amylase output in healthy animals but not in the diabetic rats. Typically, the ACh (1 0 5M) and CCK-8 (1 0 8M)-evoked amylase secretion in the presence of 10
6M insulin in normal and diabetic rats was 19.7 ± 3.4 U mY' (100 mg of tissue', 36.3 ± 4.1 U mY' (100mg oftissue' and 8.1 ± 0.53 U mY' (100mg of tissue) 4 , 13.2 ± 1.7 U mY' (100 mg of tissue) 1 , respectively.
Stimulation of fijra-2-AM loaded acinar cells with CCK-8 resulted in marked increases in peak [Ca2 ]a in both control (517.4 ± 12.09 nM) and diabetic rats (582 ± 16.05 nM) . The Ca2 transient composed of an initial peak followed by a plateau phase. The effects of CCK-8 in the plateau phase of the Ca 2 transient was reduced in diabetic acinar cells compared to age-matched control acinar cells. CCK-8 evoked a marked decrease in [Mg 2 ]1 in Magfura-2-AJvI loaded acinar cells of normal (0.72 ± 0.22 mM) and diabetic rats (0.61 ± 0.15 m M). The decrease in [Mg2 +] i was larger in diabetic acinar cells compared to age-matched control cells. Intravenous infusion of CCK-8 resulted in marked increases of pancreatic juice flow in both diabetic and agematched control anaesthetised rats. Typically the CCK-8 evoked plateau phase in control and diabetic acinar cells was 065 ± 0.054 mM and 0.59 ± 0.036 mM, respectively. The CCK-8 evoked pancreatic juice flow was significantly (p<0.05) reduced in diabetic rats compared to age matched controls. Typically, CCK-8 evoked pancreatic juice flow during the first 40 min of infusion was 1.25 ± 0.063 p1 min 1 and 0.97 ± 0.071 p1 min i , in normal and diabetic rats, respectively. The results suggest that diabetes mellitus is associated with decreased pancreatic amylase output and juice flow. This reduction in pancreatic secretion may be associated with changes in cellular Ca2 and Mg2 transport in diabetic pancreatic acinar cells.