Calcium alters the sensitivity of intact horizontal cells to dopamine antagonists

Horizontal cells of the carp retina possess dopamine receptors linked to adenylate cyclase. Isolated, intact horizontal cells respond to micromolar concentrations of dopamine, whereas nanomolar concentrations of haloperidol, (+)-butaclamol, and flupenthixol block the dopamine response. Preincubation in Ringer's solution containing increased levels of Ca2+ (5-110 mM) decreases the sensitivity of the cells to these antagonists by 1,000-10,000 times. Dopamine sensitivity of the cells is not affected by Ca2+ levels in the preincubation medium. Preincubation of the cells in Ringer's solution containing 500 microM L-glutamate, an agent that increases intracellular Ca2+ levels in intact horizontal cells, also decreases the sensitivity of the cells to haloperidol. These data suggest that antagonist sensitivity of intact horizontal cells may be regulated by intracellular Ca2+.