Major difference between rat and guinea-pig ureter in the ability of agonists and caffeine to release Ca2+ and influence force

1. We have investigated the internal Ca2+ store and its ability to affect contraction by simultaneously measuring force and Ca2+ in the ureter from guinea-pig and rat. Both species responded in a similar manner to electrical stimulation and depolarization with high-K+, generating plateau-type action potentials and increasing intracellular calcium([Ca2+](i)) and force. 2. In the guinea-pig, carbachol had no effect on [Ca2+](i) and force in the resting ureter. In contrast, resting rat ureter always responded with a large [Ca2+](i) rise and maintained force to carbachol in Ca2+-containing solution, and in Ca2+-free solution it showed a transient increase in [Ca2+](i) and force. This Ca2+ release and force development was also present in both polarized and high-K+-depolarized preparations and was insensitive to nifedipine,suggesting the presence of a receptor-coupled pathway of Ca2+ release in rat ureter. 3. Caffeine was able to produce a release of Ca2+ from the internal store of guinea-pig ureter and elicit contraction. However, rat ureter failed to respond to caffeine. In the presence of La3+, the caffeine response in the guinea-pig ureter and carbachol response in the rat ureter, elicited in Ca2+-free solutions, were always increased and prolonged and could be repeatedly evoked, suggesting similarity in Ca2+ uptake behaviour of the store in both species. 4. Ryanodine blocked the caffeine responses of the guinea-pig ureter elicited both in Ca2+-containing and Ca2+-free solutions, both in the absence and presence of La3+. However, ryanodine failed to prevent the rat ureter responding to carbachol, suggesting that carbachol was releasing Ca2+ from a ryanodine-insensitive channel in the sarcoplasmic reticulum (SR). 5. Cyclopiazonic acid, which inhibits the SR Ca2+-ATPase, abolished the effects of both caffeine and carbachol in Ca2+-free solutions in guinea-pig and rat, respectively. 6. We conclude that there is a major difference in the mechanisms of Ca2(+) release in the internal Ca2+ store of smooth muscle from guinea-pig and rat ureter. The data suggest that the guinea-pig store is purely a calcium-induced calcium release (CICR)-type store and that the rat store is a pure receptor-operated Ca2+ store.