Regulation of L‐Type Calcium Channels in GH₄ Cells via A₁ Adenosine Receptors

Abstract

Identification of A1 adenosine receptors (A1 Ps) in a tumor cell line derived from rat pituitary (GH4 ce~is) was performed by ligand binding and immunological experiments. Subsequently, the involvement of A1Rs in the regulation of calcium conductance was studied in these cells. The agonist N 6-(R) -(2-phenylisopropyl) adenosine (R-PIA) did not modify the intracellular calcium basal levels, whereas it inhibited the increase produced by 15mM KCI depolarization. The antagonist 1 ,3-dipropyl-8-cyclopentylxanthine led to the opening of voltage-dependent cell surface calcium channels in the absence of exogenous KCI. The channels were of the L type because the effect was abolished by calciseptine and by verapamil. These results suggest that endogenous adenosine exerts a tonic inhibitory effect on calcium transport. This was confirmed by the high adenosine concentration found in cell supernatants (up to 1 pM) and by the calcium mobilization produced by exogenously added adenosine deaminase. In depolarizing conditions, the calcium peak in the presence of adenosine deaminase was reduced when cells were preincubated with R-PIA, thus suggesting that A 1R activation regulates the intensity of depolarization. These results demonstrate that adenosine is an important regulator of the physiological state of pituitary tumor cells by modulating, in an autocrine manner, the activity of Ltype voltage-dependent calcium channels.

Activation of Adensine A1 and A2A Receptors Modulates Dopamine D2 Receptor‐Induced Responses in Stably Transfected Human Neuroblastoma Cells

“…We thus decided to investigate further the reason why the expression of dopamine D 2 receptors was not accompanied by physiological responses. Previous studies in other cell lines have shown that endogenous adenosine, through its action on adenosine A 1 receptors, exerts a tonic inhibitory effect on calcium transport (Zapata et al, 1997). Because responses induced by both KCl and dopamine were small under our standard culture conditions, we hypothesized that activation of adenosine receptors by endogenous adenosine surrounding the cells could have both decreased the excitability of the cells and their sensitivity to dopamine.…”

Adenosine A1 Receptor in Cultured Neurons from Rat Cerebral Cortex

“…Binding assays in intact cells were performed on 24 multiwell plates at a density of 250,000 cells/well, using [ 3 H]‐DPCPX as radioligand, as described by Zapata et al (1997) with some modifications. Cells were preincubated with 2 U/ml ADA at 37°C for 30 min in DM to eliminate the endogenous adenosine.…”

Purinergic signaling pathways in endocrine system

“…Consistent with the expression of A 1 R signaling through heterotrimeric G i/o proteins, adenosine inhibited cAMP production in rat pituitary cells (Schettini et al, 1990). Numerous experiments with immortalized lacto-somatotrophs (GH 3 and GH 4 C 1 cells) indicated the functional expression of A 1 Rs (Chung et al, 2000; Delahunty et al, 1988; Mollard et al, 1991; Navarro et al, 1997; Navarro et al, 1999; Zapata et al, 1997). It also appears that the A 2 Rs are expressed in rat pituitary cells and that their activation leads to enhanced PRL and TRH release (Kumari et al, 1999).…”