Process of re‐establishment of β‐adrenergic induced protein secretion after cytochalasin D inhibition in rat parotid gland. Effect of cholinergic agonist, phorbol ester and calcium

Summary— In rat parotid gland, 3 H‐protein secretion is stimulated by β‐adrenergic receptor activation (via cAMP) and also by cholinergic receptor activation (via IP 3 , calcium and diacylglycerol). The disorganization of microfilament system by cytochalasin D induced an inhibition of β‐adrenergic induced 3 H‐protein secretion whereas it did not modify the cholinergic muscarinic one. Cytochalasin D induced the formation of vacuoles in the parotid cell. In this work we show that the activation of muscarinic receptors (with carbachol) partially abolished the inhibitory effect of cytochalasin D on β‐adrenergic induced secretion. Since carbachol induced both intracellular calcium increase and protein kinase C activation, we decided to test separately the effect of calcium (using the calcium ionophore A23187) and protein kinase C activation (using phorbol ester) on the inhibitory effect of cytochalasin D on β‐adrenergic induced secretion. A23187, in the presence of calcium in the external medium was able to partially abolish cytochalasin D effect ( ie re‐establishing protein secretion) whereas activation of protein kinase C by phorbol 12–13 di‐butyrate had no effect. These results suggest that protein kinase C is not involved in re‐establishing a ‘normal’ secretion phenomenom whereas calcium does interfere. Furthermore, our fluorescence study shows that, when cytochalasin D is present in the incubation medium, the actin network is disturbed even in the presence of arbachol. This indicates that a calcium entry in the cell is not sufficient to restore a ‘normal’ actin network. Taken altogether, these results show that the total reorganization of the previously disorganized actin network is not necessary to restore β‐adrenergic secretion since carbachol is able to partially abolish the inhibitory effect of cytochalasin D on protein secretion whereas it has no effect on the effect of cytochalasin D on the actin network. However, this study confirms that, in rat parotid gland, the actin network plays an important role in β‐adrenergic transduction mechanisms.