Activation of protein kinase C does not cause desensitization in rat and rabbit mandibular acinar cells

Abstract

We have examined whether activation of protein kinase C by phorbol esters decreases the responsiveness of rat and rabbit mandibular, and rat lacrimal, acinar cells to muscarinic stimulation. Intracellular free calcium concentration ([Ca²⁺]_i) was measured in isolated single acini and cell clusters by fura-2 microspectrofluorimetry. Accumulation of inositol phosphates was measured in acinar cell suspensions. All three cell types showed very similar changes in [Ca²⁺]_i in response to acetylcholine (ACh), although mobilization of Ca²⁺ required somewhat higher ACh concentrations in rat lacrimal acinar cells than in mandibular acinar cells. There was no evidence for different dose dependencies of the peak and plateau phases of the [Ca²⁺]_i response. The ACh-evoked [Ca²⁺]_i increase in rabbit mandibular acinar cells exhibited desensitization, since it declined in magnitude when cells were stimulated repeatedly with a maximal dose of agonist. The phorbol ester 12-O-tetradecanoylphorbo-l13-acetate (TPA) rapidly and irreversibly decreased the ACh-evoked [Ca²⁺]_i signals in rat lacrimal acinar cells and reduced ACh-stimulated inositol phosphate accumulation. This inhibitory effect of TPA was most marked in cells stimulated with low doses of ACh, implying that TPA treatment shifted the ACh dose response curve to higher ACh concentrations. In contrast to the results obtained with lacrimal acinar cells, TPA had no effect on the [Ca²⁺]i and inositol phosphate responses to ACh in either rat or rabbit mandibular acinar cells. These results suggest that, although ACh-evoked [Ca²⁺]_i signals, and hence presumably the stimulus-response coupling machinery, are very similar between different acinar cell types, acinar cells show marked differences in their sensitivity to phorbol esters. The insensitivity of mandibular acinar cell [Ca²⁺]_i signals to TPA also suggests that the secretory tachyphylaxis observed in perfused rat and rabbit mandibular salivary glands is unlikely to be a consequence of negative feedback mediated by protein kinase C.