ADENOSINE STIMULATES CA2+ FLUXES AND INCREASES CYTOSOLIC FREE CA2+ IN CULTURED RAT MESANGIAL CELLS

Adenosine has been associated with cellular Ca2+ metabolism in some cell types. Since adenosine is able to contract glomerular mesangial cells in culture, and since Ca2+ is the main messenger mediating contractile responses, we studied the effect of adenosine on Ca-45(2+) movements into and out of mesangial cells and on the cytosolic free Ca2+ concentration ([Ca2+]i). Adenosine at 0.1 mM increased Ca-45(2+) uptake (basal, 9993 +/- 216; + adenosine, 14823 +/- 410 d.p.m./mg; P < 0.01) through verapamil-sensitive Ca2+ channels. These channels seem to be of the A1-adenosine receptor subtype. Adenosine also stimulated Ca-45(2+) efflux from Ca-45(2+)-loaded mesangial cells. This effect was accompanied by a net depletion of intracellular Ca-45(2+) content under isotopic equilibrium conditions (basal, 24213 +/- 978; + adenosine. 18622 +/- 885 d.p.m./mg; P < 0.05). The increase in Ca-45(2+) efflux was inhibited by a Ca2+-free medium or in the presence of 10-mu-M-verapamil. However. the intracellular Ca2+-release blocker TMB-8 (10-mu-M) only partially inhibited the adenosine-stimulated Ca-45(2+) efflux. In addition, adenosine induced an elevation in [Ca2+]i in mesangial cells with an initial transient peak within 15 s (basal, 113 +/- 7; adenosine, 345 +/- 46 nM), and a secondary increase which was slower (3-4 min) and of lower magnitude than the initial peak (250 +/- 21 nM). In summary, adenosine elevates [Ca2+]i and stimulates both Ca2+ uptake from the extracellular pool and Ca2+ efflux from intracellular pools in mesangial cells. The Ca2+ release from internal stores is produced by a combination of a TMB-8-inhibitable and a non-TMB-8-inhibitable mechanism, and seems to be dependent on Ca2+ influx.