The mechanism of Ca2+ release from the SR of permeabilised guinea-pig and rat ureteric smooth muscle

AbstractRecent work has indicated that there is a major difference in the Ca2+ store of smooth muscle from rat and guinea-pig ureter; with the rat store being agonist-sensitive but ryanodine insensitive and the guinea-pig store being ryanodine sensitive but agonist insensitive [Th.V. Burdyga, M.J. Taggart, S. Wray, J. Physiol. 489 (1995) 327–335]. We have therefore examined directly the mechanism of Ca2+ release from the internal Ca2+ store (SR). Following permeabilisation with α-toxin or β-escin the SR was Ca2+-loaded before application of carbachol or caffeine. Only carbachol evoked a transient contraction in rat ureter. The carbachol-induced contraction was blocked by heparin and cyclopiazonic acid (CPA) but not ryanodine. Only caffeine produced contraction in guinea-pig ureter, and this was blocked by ryanodine. Direct application of IP3 caused a small transient contraction in rat but not guinea-pig ureter. We conclude that rat ureter possesses only an IP3 sensitive store while guinea-pig ureter only has a ryanodine sensitive store