Dopamine D2 receptors both acutely and chronically inhibit high-voltage-activated Ca2+ channels (HVA-CCs). Two alternatively spliced isoforms, D2L (long) and D2S (short), are expressed at high levels in rat pituitary intermediate lobe melanotropes but are lacking in anterior lobe corticotropes. We stably transfected D2L and D2S into corticotrope-derived AtT20 cells. Both isoforms coupled to inhibition of Q-type calcium channels through pertussis toxin-sensitive G proteins. Thus, we have created a model system in which to study the kinetics of D2-receptor regulation of Ca2+ channels. Rapid inhibition of HVA-CCs was characterized using a novel fluorescence video imaging technique for the measurement of millisecond kinetic events. We measured the time elapsed (lag time) between the arrival of depolarizing isotonic 66 mM K+, sensed by fluorescence from included carboxy-X-rhodamine (CXR), and the beginning of increased intracellular Ca2+ levels (sensed by changes in indo 1 fluorescence ratio). The lag time averaged 350-550 ms, with no significant differences among cell types. Addition of the D2-agonist quinpirole (250 microM) to the K+/CXR solution significantly increased the lag times for D2-expressing cells but did not alter the lag time for AtT20 controls. The increased lag times for D2L- and D2S-transfected cells suggest that at least a fraction of the Ca2+ channels was inhibited within the initial 350-550 ms. As this inhibition time is too fast for a multistep second messenger pathway, we conclude that inhibition occurs via a membrane-delimited diffusion mechanism.