Tyrosine hydroxylase activity in the endocrine pancreas: changes induced by short-term dietary manipulation

BACKGROUND: Tyrosine hydroxylase (TH) activity and its possible participation in the control of insulin secretion were studied in pancreatic islets of adult Wistar rats fed a standard commercial diet (SD) or carbohydrates alone (CHD) for one week. TH activity, norepinephrine (NE) content, and glucose-induced insulin secretion were assessed. Blood glucose and insulin levels were measured at the time of sacrifice. RESULTS: CHD rats had significantly higher blood glucose and lower insulin levels than SD rats (114.5 PlusMinus; 6.7 vs 80.7 PlusMinus; 7.25 mg/dl, p < 0.001; 20.25 PlusMinus; 2.45 vs 42.5 PlusMinus; 4.99 &mgr;U/ml, p < 0.01, respectively). Whereas TH activity was significantly higher in CHD isolated islets (600 PlusMinus; 60 vs 330 PlusMinus; 40 pmol/mg protein/h; p < 0.001), NE content was significantly lower (18 PlusMinus; 1 vs 31 PlusMinus; 5 pmol/mg protein), suggesting that TH activity would be inhibited by the end-products of catecholamines (CAs) biosynthetic pathway. A similar TH activity was found in control and solarectomized rats (330 PlusMinus; 40 vs 300 PlusMinus; 80 pmol/mg protein/h), suggesting an endogenous rather than a neural origin of TH activity. CHD islets released significantly less insulin in response to glucose than SD islets (7.4 PlusMinus; 0.9 vs 11.4 PlusMinus; 1.1 ng/islet/h; p < 0.02). CONCLUSIONS: TH activity is present in islet cells; dietary manipulation simultaneously induces an increase in this activity together with a decrease in glucose-induced insulin secretion in rat islets. TH activity - and the consequent endogenous CAs turnover - would participate in the paracrine control of insulin secretion.