CHOLERA TOXIN-SENSITIVE 3',5'-CYCLIC ADENOSINE-MONOPHOSPHATE AND CALCIUM SIGNALS OF THE HUMAN DOPAMINE-D1 RECEPTOR - SELECTIVE POTENTIATION BY PROTEIN KINASE-A

The signal transduction pathways of the dopamine-D1 receptor were investigated in two cell types stably transfected with the human D1 receptor cDNA, rat pituitary GH4C1 cells (GH4-hD1), and mouse Ltk-fibroblast cells (L-hD1). In both GH4-hD1 and L-hD1 cell lines, stimulation of the dopamine-D1 receptor induced a marked increase in cAMP accumulation. In addition, dopamine potentiated activation of L-type voltage-dependent calcium channels in a cAMP-dependent manner in GH4-hD1 cells. However, in L-hD1 cells, dopamine increased cytosolic free calcium concentrations ([Ca++]i) by mobilization of intracellular calcium rather than by calcium influx. This effect was correlated with a dopamine-induced enhancement of phospholipase C activity in L-hD1 cells. Pretreatment (24 h) with cholera toxin (CTX) was used to maximally activate the GTP-binding protein (G protein) Gs, causing a maximal elevation of cAMP levels and uncoupling the D1 receptor from Gs. The described actions of dopamine in both cell lines were abolished by pretreatment with CTX, indicating that CTX substrates (e.g. Gs) may mediate these actions. The blockade by CTX was not due to CTX-induced elevation of cAMP, since pretreatment with forskolin or 8-bromo-cAMP to activate cAMP-dependent protein kinase did not inhibit dopamine actions nor alter basal [Ca++]i. Pretreatment (1-3 h) of L-hD1 cells with forskolin (10 muM) or 8-bromo-cAMP (5 mM) altered neither the basal activity of phospholipase C nor basal [Ca++]i in L-hD1 cells but greatly enhanced the dopamine-induced increase of phosphatidyl inositol turnover and [Ca++]i. From these results we conclude that: 1) the dopamine-D1 receptor induces multiple and cell-specific signals, including elevation of cAMP levels in both GH and L cells, cAMP-dependent activation and potentiation of opening of L-type voltage-dependent calcium channel in GH cells, and a novel phosphatidyl inositol-linked mobilization of cellular calcium in L cells; 2) coupling of the D1 receptor to these responses involves CTX-sensitive proteins, possibly Gs; and 3) acute preactivation of cAMP-dependent protein kinase can markedly enhance, rather than attenuate, certain pathways of dopamine-D1 transmembrane signaling.