Dopamine D₂-receptor isoforms expressed in AtT20 cells inhibit Q-type high-voltage-activated Ca²⁺ channels via a membrane-delimited pathway

Dopamine D₂ receptors both acutely and chronically inhibit high- voltage-activated Ca²⁺ channels (HVA-CCs). Two alternatively spliced isoforms, D(2L) (long) and D(2S) (short), are expressed at high levels in rat pituitary intermediate lobe melanotropes but are lacking in anterior lobe corticotropes. We stably transfected D(2L) and D(2S) 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 μM) 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 D(2L)- and D(2S)-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.