Different Pathways for Ca2+ Influx and Intracellular Release of Ca2+ Mediated by Muscarinic Receptors in Ileal Longitudinal Smooth Muscle.

Abstract

Muscarinic receptor-mediated elevations in intracellular Ca 2-1 concen tration ([Ca 21]i) in the longitudinal smooth muscle of guinea pig ileum were studied by the use of fura-2 fluorescence. Dose-response analysis indicated a difference in the potencies of carbachol (CCh) to increase [Ca 2 +]; in the presence and absence of ex tracellular Ca 2+. For the increase in [Ca 2+]; due to Ca 2+ release from intracellular stores in the absence of extracellular Ca 2+' the ED50 value of CCh was 3 X 105 M. On the other hand, in the presence of Ca 2+' the ED50 value was 2.5 X 10-7 M, in dicating that a low concentration of CCh (< 10-7 M) caused influx of extracellular Ca 2+ without Ca 2+ release. Oxotremorine and pilocarpine induced Ca 2+ influx, but were less potent inducers of Ca 2+ release. CCh also stimulated the formation of inosi tol trisphosphates (IP3) with an ED50 value of (4.5 X 10-5 M), which was similar to that for Ca2+ release from intracellular stores. Treatment of the smooth muscle with neomycin (1 mM), a phospholipase C inhibitor, abolished both CCh-induced IP3 formation and Ca 2+ release from intracellular stores, but did not affect CCh-induced Ca 2+ influx. These results suggest that the pathway for muscarinic stimulation of Ca 2+ influx through plasma membranes is different from that for Ca 2+ release from intracellular stores, which seems to be coupled with IP3 formation.Stimulation of smooth muscles with various receptor agonists including muscarinic agonists results in a contractile response by triggering an increase in cytoplasmic free calcium ([Ca 2+]i). IP3, a product of receptor-activated phospholipase C, plays an important role in this elevation of [Ca 2+]; by inducing the re lease of Ca 2+ from intracellular stores (1-4). Many studies have demonstrated that mus carinic agonists are effective activators of IP3 and diacylglycerol syntheses in smooth muscle (5, 6). However, the dose-response curve for IP3 formation does not coincide with that for muscle contraction, the latter being in a lower concentration range of CCh (7). Moreover, contraction of ileal longitudinal muscle is known mainly to depend on the extracellular Ca 2+ concentration (8). These facts indicate that not only release from intracellular stores, but also influx of extracellular Ca 2+ through

Muscarinic agonist potencies at three different effector systems linked to the M2 or M3 receptor in longitudinal smooth muscle of guinea‐pig small intestine

“…In the present study, pilocarpine and McN‐A343 failed to evoke I K‐Ca , but were able to inhibit the I K‐Ca ‐evoking activity of carbachol, indicating their antagonist behaviour at M 3 receptors. Such properties are also indicated from other studies in which their effects on phosphoinositide turnover and Ca 2+ store‐dependent changes in [Ca 2+ ] i or tension were examined (Gardner & Mitchelson, 1988; Hishinuma et al ., 1997; Morel et al ., 1997; Wang et al ., 1992). Both agonists also shifted the carbachol concentration‐effect curve for I cat activation to the right with depression of the maximum response (Figure 5).…”

Origin of Ca2+ Necessary for Carbachol-Induced Contraction in Longitudinal Muscle of the Proximal Colon of Rats

“…CCh was suggested to elicit Ca 2+ release from SR only at a higher concentration than that necessary for an increase in the Ca 2+ influx in the guinea pig taenia coli (22). It was also suggested that Ca 2+ release from SR was activated by high concentrations of CCh but not by other muscarinic agonists, pilocarpine and oxotremorine that increase Ca 2+ influx from outside of the cells in longitudinal muscle of the guinea pig ileum (23). Although all these results indicate the importance of Ca 2+ influx in smooth muscle contraction, the results on the proximal colon in the present study is conspicuous among them.…”

Contrasting effects of carbachol, McN‐A‐343 and AHR‐602 on Ca2+‐mobilization and Ca2+‐influx pathways in taenia caeci

“…The rise in cytoplasmic Ca 2+ concentration via activation of muscarinic M 3 receptors could be via a Ca 2+ release pathway from intracellular stores and/or a Ca 2+ influx pathway from the extracellular space (Brading & Sneddon, 1980; Parekh & Brading, 1992; Wang et al , 1992). In the absence of extracellular Ca 2+ , and in contrast to the effect of carbachol, neither McN‐A‐343 nor AHR‐602 induced a contractile response.…”