REDUCTION OF DOPAMINE D-2 RECEPTOR TRANSDUCTION BY ACTIVATION OF ADENOSINE A(2A) RECEPTORS IN STABLY A(2A) D-2 (LONG-FORM) RECEPTOR CO-TRANSFECTED MOUSE FIBROBLAST CELL-LINES - STUDIES ON INTRACELLULAR CALCIUMLEVELS/

A stably co-transfected mouse fibroblast cell line, which expresses th e long form of the human dopamine D-2 receptor and the dog adenosine A (2a) receptor, was used to analyse the mechanism underlying changes in the cytosolic free calcium concentration ([Ca2+](i)) induced by activ ation of D-2 (long-form) receptors and its modulation by the A(2a) rec eptor agonist CGS 21680 by means of fura-2 imaging. Quinpirole (1-1000 nM), a D-2 receptor agonist, caused a concentration-dependent increas e in [Ca2+](i). Haloperidol, a preferential D-2 receptor antagonist, c ompletely blocked this [Ca2+](i) response to quinpirole. Preincubation with Ca2+-free medium containing 2 mM EGTA or a medium containing 320 mM ethanol, an inositol 1,4,5-triphosphate receptor antagonist, subst antially diminished the increase in [Ca2+](i) evoked by quinpirole. Fu rthermore, quinpirole totally failed to elevate [Ca2+](i) in a medium containing both 2 mM EGTA and 320 mM ethanol. CGS 21680 (1-500 nM) did not, by itself, exert any significant effect on [Ca2+](i). However, 1 00 nM of CGS 21680 substantially counteracted the [Ca2+](i) responses to quinpirole (10-1000 nM). Moreover, this counteraction still occurre d after blocking Ca2+ mobilization from intracellular stores with etha nol, but disappeared when the cells were pretreated with the Ca2+-free medium containing 2 mM EGTA. Our findings imply that the D-2 (long-fo rm) receptors in the present fibroblast cell line can raise [Ca2+](i) both via Ca2+ influx from the extracellular medium and Ca2+ release fr om intracellular stores via activation of inositol 1,4,5-trisphosphate receptors. The co-transfected A(2a) receptors cannot directly control [Ca2+](i), but can modulate the [Ca2+](i) responses to quinpirole thr ough an antagonistic A(2a)/D-2 receptor interaction, operating at the membrane level to control Ca2+ influx.