CYCLIC ADP-RIBOSE METABOLISM IN RAT-KIDNEY - HIGH-CAPACITY FOR SYNTHESIS IN GLOMERULI

Recent discovery of cyclic ADP-ribose (cADPR) as an agent that trigger s Ca2+ release from intracellular stores, through ryanodine receptor c hannel, is an important new development in the investigation of intrac ellular signaling mechanisms. We determined the capacity of kidney and its components for synthesis of cADPR from beta-NAD, that is catalyze d by enzyme ADP-ribosyl cyclase, and enzymatic inactivation that is ca talyzed by cADPR-glycohydrolase. Little or no activity of ADP-ribosyl cyclase was found in extracts from the whole rat kidney, renal cortex, outer and inner medulla. On the other hand. incubation of beta-NAD wi th similar extracts from rat liver, spleen, heart, and brain resulted in biosynthesis of cADPR. In addition. extracts from suspension of pro ximal tubules or microdissected proximal convoluted tubules virtually lacked ADP-ribosyl cyclase activity. In sharp contrast to proximal tub ules and cortex, extracts from glomeruli had high ADP-ribosyl cyclase activity, similar to that found in non-renal tissues. Authenticity of cADPR biosynthesized in glomeruli was documented by several criteria s uch as HPLC analysis, effect of inhibitors and homologous desensitizat ion of Ca2+-release bioassay. On the other hand, the activity of cADPR -glycohydrolase was similar in extracts from glomeruli and in extracts from kidney cortex. Mesangial cells and vascular smooth muscle cells grown in primary culture displayed considerable ADPR-ribose cyclase ac tivity. Our results show that extracts from glomeruli, unlike extracts from renal tissue zones and proximal tubules, have a singularly high capacity for synthesis of cADPR. We surmise that cADPR-triggered Ca2+- releasing system can serve as an intracellular signaling pathway that may be operant in regulations of glomerular cell functions.