Agonist-induced activation of Na+/H+ exchange in rat parotid acinar cells is dependent on calcium but not on protein kinase C.

We demonstrated previously that Na+/H+ exchange activity was enhanced in rat parotid acinar cells following muscarinic (carbachol) or alpha-adrenergic (epinephrine) stimulation and that this activation of the exchanger was preserved in plasma membrane vesicles prepared from these cells (Manganel, M., and Turner, R. J. (1989) J. Membr. Biol. 111, 191-198). The carbachol dose dependence of this effect as determined here in intact acini is in good agreement with the carbachol dose dependence of the fluid secretory response in this tissue (K0.5 approximately 3 microM). In addition, we show here that the stimulation of the exchanger by secretagogues cannot be mimicked by active diacylglycerol analogues, nor can it be prevented by the protein kinase inhibitors H7 and HA1004, arguing strongly against the involvement of protein kinase C in this effect. However, stimulation of the exchanger is observed in both intact acini treated with the calcium ionophore A23187 and in vesicles prepared from these acini. Moreover, carbachol, epinephrine, and A23187 are without effect when extracellular calcium is not present during acinar pretreatment. These results indicate that the stimulation of the Na+/H+ exchanger studied here is a consequence of agonist (or A23187)-induced increases in intracellular calcium levels due to calcium influx from the extracellular solution. The calmodulin inhibitors trifluoperazine and W7 (10(-4) M) prevented the stimulation of the exchanger induced by carbachol or epinephrine, but W7 could not block the stimulation produced by A23187 arguing against the involvement of calmodulin in this effect. Taken together with previous data from this and other laboratories, these results strongly indicate that the Na+/H+ exchanger and its regulation are intimately involved in the fluid secretory response of the rat parotid.