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 Ca(2+)-containing solution, and in Ca(2+)-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 Ca(2+)-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 Ca(2+)-containing and Ca(2+)-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 Ca(2+)-ATPase, abolished the effects of both caffeine and carbachol in Ca(2+)-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.
Burdyga
T V
TV
Physiological Laboratory, University of Liverpool, UK.
Taggart
M J
MJ
Wray
S
S
D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology*; D006168:Guinea Pigs; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction; D009130:Muscle, Smooth / Q000187:drug effects*; D009543:Nifedipine / Q000494:pharmacology; D010121:Oxytocin / Q000494:pharmacology; D051381:Rats; D014513:Ureter / Q000187:drug effects*
D002110:Caffeine; D010121:Oxytocin; D002217:Carbachol; D009543:Nifedipine; D002118:Calcium