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

Abstract: Identification of A₁ adenosine receptors (A₁Rs) in a tumor cell line derived from rat pituitary (GH₄ cells) 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 15 m M KCl depolarization. The antagonist 1,3-dipropyl-8-cyclopentylxanthine led to the opening of voltage-dependent cell surface calcium channels in the absence of exogenous KCl. 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 µ M ) 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 A₁R 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 L-type voltage-dependent calcium channels.