Figure 16-4 A classic transmitter, acetylcholine (ACh), coexists with vasoactive intestinal polypeptide (VIP) in parasympathetic nerves supplying the cat salivary gland. ACh and VIP are stored in separate vesicles; they can be released differentially at different stimulation frequencies to act on acinar cells and glandular blood vessels. Cooperation is achieved by the selected release of ACh at low impulse frequencies and of VIP at high frequencies. Prejunctional and postjunctional modulation is indicated. (From Burnstock G: Local mechanisms of blood flow control by perivascular nerves and endothelium. J Hypertens Suppl 8:S95, 1990.)