| Abstrakt: |
The effects of the phorbol ester, 12‐tetradecanoyl‐phorbol‐13‐acetate (TPA) and related compounds on acetylcholine (ACh)‐evoked [3H]leucine‐labelled protein release (secretion) were tested in isolated permeabilized rat pancreatic acini. The aim was to determine whether the diacylglycerol‐like compounds can still potentiate the actions of ACh during unfluctuating supramaximal elevation of cytosolic free calcium (Ca2+) levels. TPA and R59022, an inhibitor of diacylglycerol kinase, evoked marked biphasic dose‐dependent increases in 3H‐labelled protein secretion from permeabilized rat pancreatic acini. Synthetic diacylglycerol (OAG) and 8‐bromo cyclic GMP elicited small increases in 3H‐labelled protein release while an inactive phorbol ester (4 alpha‐phorbol‐12,13‐didecanoate; 4 alpha‐PDD) and polymyxin B, an inhibitor of protein kinase C, were unable to stimulate secretion. Combining polymyxin B with TPA resulted in an inhibition of 3H‐labelled protein secretion. Acetylcholine also induced a dose‐dependent increase in 3H‐labelled protein output, but when TPA or R59022 was combined with ACh, there was a marked potentiation of the ACh‐evoked secretory response, particularly at higher concentrations of ACh. This synergism was unaffected by the protein kinase C inhibitor, polymyxin B. The results show that cytosolic free Ca2+ and protein kinase C may not be the only mediators of ACh‐evoked secretion. Moreover, they indicate that protein kinase C may not be involved in the potentiation by TPA of ACh‐evoked 3H‐labelled protein release. |
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