Slow spontaneous [Ca2+]i oscillations reflect nucleotide release from renal epithelia

Renal epithelia can be provoked mechanically to release nucleotides, which subsequently increases the intracellular Ca2+ concentration [Ca2+](i) through activation of purinergic (P2) receptors. Cultured cells often show spontaneous [Ca2+](i) oscillations, a feature suggested to involve nucleotide signalling. In this study, fluo-4 loaded Madin-Darby canine kidney (MDCK) cells are used as a model for quantification and characterisation of spontaneous [Ca2+](i) increases in renal epithelia. Spontaneous [Ca2+](i) increases occurred randomly as single cell events. During an observation period of 1 min, 10.9 +/- 6.7% (n = 23) of the cells showed spontaneous [Ca2+](i) increases. Spontaneous adenosine triphosphate (ATP) release from MDCK cells was detected directly by luciferin/luciferase. Scavenging of ATP by apyrase or hexokinase markedly reduced the [Ca2+](i) oscillatory activity, whereas inhibition of ecto-ATPases (ARL67156) enhanced the [Ca2+](i) oscillatory activity. The association between spontaneous [Ca2+](i) increases and nucleotide signalling was further tested in 132-1N1 cells lacking P2 receptors. These cells hardly showed any spontaneous [Ca2+](i) increases. Transfection with either hP2Y(6) or hP2Y(2) receptors revealed a striking degree of oscillations. Similar spontaneous [Ca2+](i) increases were observed in freshly isolated, perfused mouse medullary thick ascending limb (mTAL). The oscillatory activity was reduced by basolateral apyrase and substantially lower in mTAL from P2Y(2) knock out mice (0.050 +/- 0.020 events per second, n = 8) compared to the wild type (0.147 +/- 0.018 events per second, n = 9). These findings indicate that renal epithelia spontaneously release nucleotides leading to P2-receptor-dependent [Ca2+](i) oscillations. Thus, tonic nucleotide release is likely to modify steady state renal function.