Calcium response to adenosine and ATP in rabbit afferent arterioles

The effects of purine compounds in the renal vasculature are almost exclusi vely restricted to pre-glomerular vessels. Although their physiological rol e as extracellular messengers is not clear, there are extensive data indica ting the importance of adenosine and ATP in the regulation of renal haemody namics. This study was undertaken to characterize the calcium response of r abbit efferent arteriole to adenosine, ATP and other nucleotides. Experimen ts were performed in isolated afferent arterioles, microdissected from rabb it kidneys and loaded with fura-2, Intracellular calcium concentration ([Ca 2+](i)) was measured by Video in proximal and distal parts of the afferent arteriole. Application of 100 mu M adenosine or ATP increased [Ca2+](i) in both arteriolar regions. In all cases the response had two well distinguish able phases: a quick peak increase and a plateau phase that equilibrated at a [Ca2+](i) significantly higher than the basal level. UTP (100 mu M) had no effect on the arteriole. Removal of extracellular calcium (2.5 mM EGTA) abolished only the plateau phase in response to adenosine, without signific antly changing the peak increase. In contrast, the response to ATP was comp letely abolished in both arteriolar regions, where [Ca2+](i) decreased upon application of the agonist and rapidly increased after restoration of calc ium concentration to plasma level. We conclude that P1 and P2X receptors ar e present along the rabbit afferent arteriole and mediate calcium mobilizat ion, with the same distribution in the proximal and distal segments.