Intracellular Ca2+ signals elicited by nucleotide agonists were investigated in primary cultures of rat cardiac microvascular endothelial cells using the fura-2 technique. UTP increased the intracellular [Ca2+] in 94% of the cells, whereas 2MeSATP was active in 32%. The rank order of potency was ATP = UTP > 2MeSATP and the maximal response to 2MeSATP was lower compared to UTP and ATP. ATP and UTP showed strong homologous and heterologous desensitization. ATP fully inhibited the 2MeSATP response, while UTP abolished 2MeSATP-elicited transients in 25% of cells. 2MeSATP did not desensitize the UTP or ATP response. Adenosine 2',5'-diphosphate inhibited the response to 2MeSATP, while it did not modify the response to ATP and UTP. 2MeSATP was more sensitive to suramin than UTP and ATP. These results indicate that P(2Y1) and P(2Y2) receptors may be coexpressed in CMEC. Nucleotide-induced Ca2+ signals lacked a sustained plateau and were almost independent from extracellular Ca2+. ATP and UTP elicited Ca2+ transients longer than 2MeSATP-evoked transients. The kinetics of Ca2+ responses was not affected by bath solution stirring or ectonucleotidase inhibition. Furthermore, the nonhydrolyzable ATP analogue AMP-PNP induced Ca2+ signals similar to those elicited by ATP and UTP. These results suggest that the distinct kinetics of nucleotide-evoked Ca2+ responses do not depend on the activity of ectonucleotidases or ATP autocrine stimulation. The possibility that Ca2+ signals with different time courses may modulate different cellular responses is discussed.
Copyright 2001 Academic Press.