Calcium response to adenosine and ATP in rabbit afferent arterioles

The effects of purine compounds in the renal vasculature are almost exclusively restricted to pre-glomerular vessels. Although their physiological role as extracellular messengers is not clear, there are extensive data indicating the importance of adenosine and ATP in the regulation of renal haemodynamics. This study was undertaken to characterize the calcium response of rabbit afferent arteriole to adenosine, ATP and other nucleotides. Experiments were performed in isolated afferent arterioles, microdissected from rabbit kidneys and loaded with fura-2. Intracellular calcium concentration ([Ca2+]i) was measured by video in proximal and distal parts of the afferent arteriole. Application of 100 microM adenosine or ATP increased [Ca2+]i in both arteriolar regions. In all cases the response had two well distinguishable phases: a quick peak increase and a plateau phase that equilibrated at a [Ca2+]i significantly higher than the basal level. UTP (100 microM) had no effect on the arteriole. Removal of extracellular calcium (2.5 mM EGTA) abolished only the plateau phase in response to adenosine, without significantly changing the peak increase. In contrast, the response to ATP was completely abolished in both arteriolar regions, where [Ca2+]i decreased upon application of the agonist and rapidly increased after restoration of calcium concentration to plasma level. We conclude that P1 and P2X receptors are present along the rabbit afferent arteriole and mediate calcium mobilization, with the same distribution in the proximal and distal segments.