Flow-induced [Ca2+]i increase depends on nucleoticle release and subsequent purinergic signaling in the intact nephron

Flow induces cytosolic Ca2+ increases ([Ca2+](i)) in intact renal tubules, but the mechanism is elusive. Mechanical stimulation in general is known to promote release of nucleotides (ATP/UTP) and trigger auto- and paracrine activation of P2 receptors in renal epithelia. It was hypothesized that the flow-induced [Ca2+](i) response in the renal tubule involves mechanically stimulated nucleotide release. This study investigated (1) the expression of P2 receptors in mouse medullary thick ascending limb (mTAL) using P2Y(2) receptor knockout (KO) mice, (2) whether flow increases induce [Ca2+](i) elevations in mTAL, and (3) whether this flow response is affected in mice that are deplete of the main purinergic receptor. [Ca2+](i) was imaged in perfused mTAL with fura-2 or fluo-4. It is shown that luminal and basolateral P2Y(2) receptors are the main purinergic receptor in this segment. Moreover, the data suggest presence of basolateral P2X receptors. Increases of tubular flow were imposed by promptly rising the inflow pressure, which triggered a marked increase of [Ca2+](i).This [Ca2+](i) response was significantly reduced in P2Y(2) receptor KO tubules (fura-2 ratio increase WT 0.44 +/- 0.09 [n = 28] versus KO 0.16 +/- 0.04 [n = 13]). Furthermore, the flow response was greatly inhibited with luminal and basolateral scavenging of extracellular ATP (apyrase 7.5 U/ml) or blockage of P2 receptors (suramin 300 mu M). The flow response could still be elicited in the absence of extracellular Ca2+. These results strongly suggest that increase of tubular flow elevates [Ca2+](i) in intact renal epithelia. This flow response is caused by release of bilateral nucleotides and subsequent activation of P2 receptors.