One of the most important functions of saliva is to buffer plaque acid. Bicarbonate (HCO3-) is the acid buffer secreted in saliva. In short, HCO3- can pass through Cl- channels and so will be secreted when ACh triggers the signal transduction cascade that results in increased [Ca2+]i. The question here is, "How does HCO3- get concentrated inside the cells?".
And the Answer is.....
CO2 diffuses into cells where it is combined with water into carbonic acid which then disassociates into H+ and HCO3-. In a resting cell, there is nowhere for the HCO3- to go.... In an activated, secreting cell, it can leave via the apical membrane anion channel.
This process also produces H+ which get passed back into the blood by Na+/H+ exchange, a process energised by the inwardly directed Na+ gradient, which is created by the Na+ pump.
Acinar cells may be the source of HCO3- in saliva, but so may the intercalated duct cells. These are the ducts closest to the acinar cells. They look like acinar cells without secretory vesicles.
Unstimulated Saliva has a Low HCO3- concentration
At low salivary flow rates, HCO3- is reabsorbed by the striated duct cells and so very little reaches the mouth. At high flow rates, the striated ducts can't keep up and so HCO3- reaches the mouth at high concentration (<25mM)).
This makes perfect sense, from a physiological point of view. HCO3- is valuable to the body and not something to throw into the gut for no reason. The main purpose of unstimulated saliva is to keep the mouth lubricated, help you to talk and so on. Unless you are feeding the plaque bacteria, there is no great amount of acid to neutralise and therefore no need for a high HCO3- saliva. When you feed yourelf, and also your plaque bacteria, you stimulate salivary flow and produce a high HCO3- saliva, with a high buffering capacity, just when you need it.