The influence of nitric oxide and cholecystokinin on tissue homeostasis in exocrine pancreas : an experimental study in rats
Sammanfattning: Growth of the pancreas is stimulated by cholecystokinin (CCK) in rats. Nitric oxide (NO), which is synthesized from the amino acid L-arginine by NO-synthases (NOS), interferes with CCK in modulating the pancreatic secretion. Both pro- and anti-apoptotic influences of NO have been observed in other tissues, but its importance for pancreatic tissue homeostasis has not been studied.Cell proliferation and death rates were studied in the rat pancreas during different experimental conditions. CCK-octapeptide (CCK-8) was given in different doses either intermittently or continuously for three days and the pancreatic growth response was studied. Exogenous CCK-8 caused dose-dependent pancreatic atrophy when given intermittently and hyperplasia when given continuously.The influence of NO on cell death and proliferation was studied during: 1) basal conditions, 2) CCK-8 induced hyperplasia, and 3) CCK-8 induced atrophy. The NO level was manipulated either by NOS inhibition (L-NNA) or by exogenous NO supply (SNAP). NO-metabolism was assessed in the basal situation by analysis of nitrite/nitrate excretion in urine (which was decreased by L-NNA and increased by SNAP), and L-arginine in serum (which increased by L-NNA) and L-citrulline in serum.1) During basal conditions NOS inhibition (NO↓) increased apoptosis and decreased cell proliferation.2) During CCK-8 induced hyperplasia NOS inhibition (NO↓) increased both apoptosis and cell proliferation. The apoptosis dominated as indicated by decreased DNA content. SNAP administration (NO↑) did neither influence apoptosis nor cell proliferation.3) During CCK-8 induced atrophy (DNA↓, apoptosis↑, cell proliferation↑, and cytoplasmic vacuolization) NOS inhibition further increased apoptosis, reduced cell proliferation and abolished vacuole formation. SNAP administration (NO↑) decreased the DNA content, and increased both apoptosis and cell proliferation. The vacuole formation was still present. Hence, NO influences both the basal and the disturbed homeostasis in hyperplastic and atrophic rat pancreatic tissue.Early events of a load of L-arginine, known to induce pancreatitis within 48 hrs, was studied at 8, 16 and 24 hrs by analysis of serum L-arginine and L-citrulline, pancreatic tissue ATP, apoptosis and cell proliferation. The initially increased serum L-arginine and L-citrulline decreased to levels below control at 24 hrs. Administration of L-arginine was correlated to a biphasic ATP production and formation of small vacuoles (mitochondrial swelling) in the acinar cells, most prominent at 8 hrs and followed by a gradually increased apoptosis rate. The cell proliferation decreased. At 24 hrs there was pronounced cell degeneration, but no evident necrosis. Another 20 amino acids in serum were also analysed at 24 hrs. Twelve amino acids (including the 'glutamate family') were significantly reduced. After an L-arginine load the augmented A TP production correlates to the initiation of pancreatic cell death. The disturbed amino acid metabolism seems to be of importance for development of experimental pancreatitis.
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