Adenosine inhibits the renal plasma-membrane (Ca2++Mg2+)-ATPase through a pathway sensitive to cholera toxin and sphingosine

Adenosine, a potent autacoid produced and released in kidneys, affects nearly all aspects of renal function, and an increase in cytosolic calcium has been implicated in adenosine effects. The aim of this work was to investigate whether adenosine modifies the calcium pump present in basolateral membranes of kidney proximal tubule cells. Adenosine exerts a biphasic influence on (Ca2+ + Mg2+)-ATPase activity. Inhibition occurs up to 0.1 mu M and then gradually disappears as the adenosine concentration increases to 100 mu M, an effect mimicked by the adenosine analog N-6-cyclohexyladenosine, which preferentially binds to A(1)-type receptors. In contrast, the A(2) receptor agonist 5',N-ethylcarboxamideadenosine is ineffective. The A(1) receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine blocks the inhibitory effect of 0.1 mu M adenosine and stimulates (Ca2+ + Mg2+)-ATPase activity in the presence of 1 mM adenosine, a concentration high enough to occupy the low-affinity A(2) receptors. Inhibition by adenosine increases as medium ATP is lowered to micromolar concentrations, is maintained in the presence of pertussis toxin, and is completely abolished with 0.1 mu M cholera toxin or 1 mu M sphingosine. The inhibitory effect of adenosine can be reproduced by guanosine 5'-[mu-thio]triphosphate, inositol 1,4,5-trisphosphate or the diacylglycerol analog 12-O-tetradecanoylphorbol 13-acetate. In conjunction with the selectivity for its analogs and for its receptor agonist, the concentration profile of adenosine effects indicates that both inhibitory (A(1)) and stimulatory (A(2)) receptors are involved. The results obtained with the toxins indicate that a pathway that is modulated by G-proteins, involves a phospholipase C and a protein kinase C, and is affected by local variations in adenosine concentrations participates in the regulation of the (Ca2+ + Mg2+)-ATPase resident in basolateral membranes of kidney proximal tubules.