ABSTRACT

ADP-ribosyl cyclase catalyzes the synthesis of two structurally and functionally different Ca 2؉ releasing molecules, cyclic ADP-ribose (cADPR) from ␤-NAD and nicotinic acid-adenine dinucleotide phosphate (NAADP) from ␤-NADP. Their Ca 2؉ -mobilizing effects in ascidian oocytes were characterized in connection with that induced by inositol 1,4,5-trisphosphate (InsP 3 ). Fertilization of the oocyte is accompanied by a decrease in the oocyte Ca 2؉ current and an increase in membrane capacitance due to the addition of membrane to the cell surface. Both of these electrical changes could be induced by perfusion, through a patch pipette, of nanomolar concentrations of cADPR or its precursor, ␤-NAD, into unfertilized oocytes. The changes induced by ␤-NAD showed a distinctive delay consistent with its enzymatic conversion to cADPR. The cADPR-induced changes were inhibited by preloading the oocytes with a Ca 2؉ chelator, indicating the effects were due to Ca