Studies of nucleotide receptors-induced calcium response in glomerular mesangial cells and afferent arterioles

Abstract: It is known that intracellular calcium concentration ([Ca2+]i) changes in rat glomerular mesangial cells (GMC) and rabbit afferent arterioles (AA) in response to extracellular nucleotides. We studied the nucleotide-induced calcium response in cultured GMC and AA. We characterized the response of GMC to extracellular nucleotides at the single cell level and in first passage cells. GMC basal [Ca2+]i was always between 70-90 nM. The cells responded to 0.1 mM ATP or UTP with a biphasic [Ca2+]i increase. The order of efficacy of nucleotide agonists was UTP = ATP > ATPγS > ADP = 2MeS-ATP, Adenosine, AMP and β,γ-Me-ATP had no effect. Consecutive ATP challenges induced receptor desensitization. Stimulation of PKC (PMA) abolished responses to ATP or UTP. Inhibition of PKC (chelerythrine) in contrast prevented desensitization. The desensitization was reversed by 4-min incubation with PDGF-BB or insulin. Theinfluence of protein phosphatases (PP) on receptor desensitization and on insulin-induce resensitization was studied using the PP inhibitors okadaic acid and cypermethrin. Internalization of nucleotides receptors was blocked with hyperosmotic sucrose solution. The calcium response of freshly dissected AA to adenosine, ATP and UTP was characterized in defined proximal and distal AA regions.The results show that: (i) GMC express P2Y2 receptor subtype. (ii) Desensitization is mediated by PKC and resensitization is modulated by growth factors. (iii) PP2B antagonizes desensitization. PP1 is involved in the insulin-induced resensitization. Internalization is a prerequisite for resensitization. (iv) P1 and P2X receptors are present along AA and both, mediate calcium mobilization with no difference in distribution in the proximal and distal AA segments.