Different purinergic receptors lead to intracellular calcium increases in pancreatic ducts

Extracellular adenosine 5'-triphosphate (ATP) has been described to act as a regulator in many cells and tissues, including epithelia, and in the gastrointestinal tract ATP is one of the substances involved in non-cholinergic non-adrenergic control. However, very little is known about the effect of ATP on pancreatic ducts, which normally secrete bicarbonate-rich fluid in response to secretin. Hence, the aim of our present study was to test the effect of ATP and other nucleotides on intracellular Ca2+ activity ([Ca2+](i)) of pancreatic ducts, and thereby get information about purinergic receptors that might play a role in the regulation of pancreatic bicarbonate transport. Native intralobular ducts were obtained from rat pancreas and [Ca2+](i) in 10-20 cells was measured using the fura-2 method. ATP (10(-4) mol/l) evoked a characteristic biphasic Ca2+ transient in duct cells. Nucleotides, used to classify the P-2 receptors, acted with the following potency on the peak Ca2+ in many ducts: uridine 5'-triphosphate (UTP) greater than or equal to ATP > inosine 5'-triphosphate greater than or equal to 2-methylthio-ATP > beta,gamma-methyl-ATP > adenosine. However, although the peak [Ca2+](i) responses to ATP and UTP were similar, the plateau [Ca2+](i) was nearly doubled with UTP. Moreover, in about one-third of the ducts studied, UTP had no effect on cell Ca2+, while the response to ATP was normal. In further experiments we found that removal of extracellular Mg2+ increased the peak [Ca2+](i) evoked in response to ATP. 2'&3'-O-(4-benzoylbenzoyl) ATP (BzATP) evoked a monophasic and slower increase in [Ca2+](i), which was inhibited by removal of extracellular Ca2+, or by addition of 4, 4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS), Taken together, our data indicate that there are two types of purinergic receptors on pancreatic ducts through which ATP can act. These are pharmacologically known as P-2U and P-2Z receptors and may correspond to P2Y(2) and P2X(7) receptors.