CHARACTERIZATION AND FUNCTION OF ATP RECEPTORS AN HEPATOCYTES FROM THE LITTLE SKATE RAJA-ERINACEA

Hormonal regulation of hepatocytes, via cytosolic Ca2+ signaling, is w ell. established in higher life forms but has not been investigated in elasmobranchs. We therefore examined Ca2+ signaling in hepatocytes is olated from the little skate, Raja erinacea. In hepatocyte populations , ATP induced a rapid, biphasic increase in Ca2+, as it does in mammal ian hepatocytes. Other hormones that act on mammalian hepatocytes, suc h as vasopressin, angiotensin, and phenylephrine, induced no such Ca2 increase. The initial phase of the ATP-induced Ca2+ increase was seen even in Ca2+-free medium, whereas the late sustained phase of the inc rease was not. Similar dose-response curves were obtained by stimulati on with ATP, ADP, UTP, and 2-methylthio-ATP. In contrast, AMP, adenosi ne, beta,gamma-methyl-ATP, CTP, and GTP induced little or no Ca2+ incr ease. In single hepatocytes, ATP, ADP, UTP, and 2-methylthio-ATP each induced a sustained increase in Ca2+ at high concentrations, but at lo w concentrations induced Ca2+ oscillations. A maximal concentration of ATP (100 mu M) caused a marked, transient increase in bile flow in th e isolated perfused skate liver, whereas 100 mu M adenosine had no suc h effect. These findings demonstrate that skate hepatocytes possess P- 2 nucleotide receptors that link to intracellular plus extracellular C a2+ mobilization, which in turn regulates bile secretion. The broad sp ecificity of the response to ATP and related compounds suggests either that multiple types of P-2 receptors are expressed by skate hepatocyt es or else that these cells possess a single primitive nucleotide rece ptor from which other P-2 subtypes subsequently evolved.