Effects of phorbol ester and related compounds on acetylcholine‐evoked [3H]leucine‐labelled protein secretion in permeabilized rat pancreatic acini

Abstract

SUMMARYThe 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 ofcytosolic free calcium (Ca2+) levels. TPA and R59 022, 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 (4a.-phorbol-12,13-didecanoate; 4a-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.

Interaction of Islet Hormones with Cholecystokinin Octapeptide‐Evoked Secretory Responses in the Isolated Pancreas of Normal and Diabetic Rats

“…The pancreas of male rats was dissociated into acinar cells with collagenase in two stages totalling 75 min by established methods (Francis & Singh, 1990). Amylase release and amylase activity were determined using the Phadebas reagent (Ceska et al 1979).…”

“…The pancreas of either normal or diabetic female rats was dissociated into acinar cells with collagenase in two stages totalling 75 min by established methods (Francis et al 1990b;Francis & Singh, 1990). Pancreatic acinar cells were loaded with 2 ìÒ fura_2-acetomethylester (AM) by methods described previously (Francis et al 1990a).…”

“…For example the phorbol esters can potentiate the secretory actions of ACh and noradrenaline (NA) (Singh, 1985a;Francis & Singh, 1990) and attenuate enzyme secretion evoked by CCK-8 (Ansah et al 1986;Francis et al 1990a). Similarly, secretin can attenuate the secretagogue effect of CCK-8 and activation of intrinsic nerves in the rat pancreas (Singh et al 1992;Wisdom et al 1994).…”

“…It is now known that two or more secretagogues acting via the different and distinct stimulussecretion coupling mechanisms can either potentiate or attenuate one another (Jensen & Gardner, 1981;Singh, 1985a;Ansah et al 1986;Francis & Singh, 1990). For example the phorbol esters can potentiate the secretory actions of ACh and noradrenaline (NA) (Singh, 1985a;Francis & Singh, 1990) and attenuate enzyme secretion evoked by CCK-8 (Ansah et al 1986;Francis et al 1990a).…”

Interaction of islet hormones with cholecystokinin octapeptide-evoked secretory responses in the isolated pancreas of normal and diabetic rats

“…It is now known that two or more secretagogues acting via the different and distinct stimulussecretion coupling mechanisms can either potentiate or attenuate one another (Jensen & Gardner, 1981;Singh, 1985a;Ansah et al 1986;Francis & Singh, 1990). For example the phorbol esters can potentiate the secretory actions of ACh and noradrenaline (NA) (Singh, 1985a;Francis & Singh, 1990) and attenuate enzyme secretion evoked by CCK-8 (Ansah et al 1986;Francis et al 1990a). Similarly, secretin can attenuate the secretagogue effect of CCK-8 and activation of intrinsic nerves in the rat pancreas (Singh et al 1992;Wisdom et al 1994).…”