REGULATION OF L-TYPE CALCIUM CHANNELS IN GH(4) CELLS VIA A(1) ADENOSINE RECEPTORS

Identification of A(1) adenosine receptors (A(1)Rs) in a tumor cell li ne derived from rat pituitary (GH(4) cells) was performed by ligand bi nding and immunological experiments. Subsequently, the involvement of A(1)Rs in the regulation of calcium conductance was studied in these c ells. The agonist N-6-(R)-(2-phenylisopropyl) adenosine (R-PIA) did no t modify the intracellular calcium basal levels, whereas it inhibited the increase produced by 15 mM 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 ch annels were of the L type because the effect was abolished by calcisep tine and by verapamil. These results suggest that endogenous adenosine exerts a tonic inhibitory effect on calcium transport. This was confi rmed by the high adenosine concentration found in cell supernatants (u p to 1 mu M) and by the calcium mobilization produced by exogenously a dded adenosine deaminase. in depolarizing conditions, the calcium peak in the presence of adenosine deaminase was reduced when cells were pr eincubated with R-PIA, thus suggesting that A(1)R activation regulates the intensity of depolarization. These results demonstrate that adeno sine 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.