RYANODINE-INDUCED CALCIUM-RELEASE FROM HEPATIC MICROSOMES AND PERMEABILIZED HEPATOCYTES

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3] is a second messenger tha t releases Ca2+ from hepatocyte microsomes. The toxic alkaloid ryanodi ne modulates Ca2+ release via a receptor (RyR) identified in a variety of cell systems, but its regulation and functional significance in li ver are undefined. Similarly, the role in hepatocyte Ca2+ regulation o f adenosine 5'-cyclic diphosphate-ribose (cADPR), which is the putativ e endogenous ligand for RyR in other cell systems, has not been define d. Utilizing microsomes and permeabilized cells, we have investigated Ca2+ regulation in hepatocytes and, in particular, effects of ryanodin e, cADPR, and other putative modulators on Ca2+ release and compared t hese with Ins(1,4,5)P3-induced Ca2+ release. Ryanodine at greater than or equal to 50 mu M released 20% of microsomal Ca2+, and, in contrast to Ins(1,4,5)P-3, no potentiation was observed with guanosine 5'-trip hosphate and polyethylene glycol. Ins(1,4,5)P-3-induced Ca2+ release w as demonstrable after maximal ryanodine-induced Ca2+ release, suggesti ng that distinct Ca2+ stores are involved. cADPR (5 mu M) did not indu ce Ca2+ release, alone or in combination with calmodulin or hepatic cy tosol, nor did it influence ryanodine-induced release, in microsomes o r permeabilized hepatocytes (in which ryanodine released 25% of the se questered Ca2+). Ryanodine-induced Ca2+ release in microsomes was not influenced by 20 mM caffeine, which itself did not mobilize Ca2+, but was prevented by 500 mu M tetracaine, which was shown to induce Ca2+ r elease. We conclude that ryanodine is capable of mobilizing Ca2+ in th e hepatocyte from microsomal stores that are distinct from those that can be regulated by Ins(1,4,5)P-3 but that cADPR has no such effect. T hese data suggest that cADPR does not serve as the endogenous ligand f or RyR in liver cells or that the site of action of ryanodine in hepat ocyte microsomes is distinct from that in other cell types.