INVOLVEMENT OF INTRACELLULAR CA2-1 AND CGMP( STORES IN INHIBITORY EFFECTS OF NO DONOR SIN)

We investigated the role of K+ channels and intracellular Ca2+ stores in the relaxations induced by the NO donor 3-morpholinosydnonimine (SI N-1) and 8-bromo-cGMP (8-BrcGMP), 8-(4-chlorophenylthio)-cGMP (pCPT-cG MP), and alpha,beta-methylene-ATP in isolated segments of rat ileum. T he inhibitory responses to SIN-1 and the cGMP analogs were not influen ced by the K+ blockers apamin, charybdotoxin, iberiotoxin, or glibencl amide, whereas relaxations induced by alpha,beta-methylene-ATP were ab olished by apamin and tetraethylammonium. The NO-donor SIN-1 and the c GMP analogs were able to inhibit contractions induced by activation of L-type Ca2+ channels (BAY-K-8644), by carbachol (CCh), and by cyclopi azonic acid (CPA), a blocker of sarcoplasmic Ca2+-ATPase. However, the inhibition of the combined CPA and CCh response was reduced and the d ose-response curve of SIN-1 shifted to the right. Intracellular Ca2+ s tores were emptied by incubation in Ca2+-free buffer and repetitive st imulation with CCh or BAY-K-8644. After restoration of extracellular C a2+, the inhibitory effect of SIN-1 and pCPT-cGMP was only attenuated, whereas in the additional presence of CPA, the inhibitory effect of S IN-1 was blocked and the effect of 8-BrcGMP reduced. Thus depleting in tracellular Ca2+ stores attenuated the effect of SIN-1 and 8-BrcGMP, s uggesting an involvement of functional Ca2+ stores.