Chloride removal and excitation–contraction coupling in guinea pig ileal smooth muscle
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abstract
The effect of extracellular Cl [Formula: see text] removal on contractions evoked by a selective muscarinic agonist, cis-2-methyl-4-dimethylaminomethyl 1,3-dioxolane methiodide (CD), and high K+ depolarizations in the isolated guinea pig ileal longitudinal muscle was studied. The replacement of [Formula: see text] with impermeant anions, such as isethionate (Ise−), was found to selectively inhibit a portion of the initial phasic response to K+ and CD, leaving the secondary and sustained tonic responses unchanged. In Ca2+-free solutions, the loss of contractile responses to high K+ was faster and more pronounced in Cl−-free compared with Cl−-containing solutions. Furthermore, the uptake of Ca2+, as represented by 45Ca2+, from the saline solution was delayed and reduced in Ise−-containing [Formula: see text]-free solutions. Replacement of [Formula: see text] with other impermeant anions, such as gluconate and methylsulphate, had a similar action on contractile activity as for Ise− replacement. [Formula: see text] replacement with permeant anions, such as nitrate, however, did not significantly inhibit the phasic response and sometimes increased the tonic response to K+. These results indicate that there is a [Formula: see text]-dependent Ca2+ pool in the guinea pig ileal longitudinal muscle and we speculate that this [Formula: see text]-dependent Ca2+ pool is associated with membrane structures, such as calveolae, which would thus offer a degree of protection to depletion by removal of extracellular Ca2+. Overall, these data suggest that agonist responses in guinea pig ileal longitudinal muscle preparations utilize at least three Ca2+ pools: a [Formula: see text]-sensitive pool supplying Ca2+ primarily for phasic contractions, a [Formula: see text]-insensitive pool supplying Ca2+ for tonic and phasic contractions, and an extracellular pool that is most sensitive to organic Ca2+ channel antagonists such as D-600 and supplies Ca2+, presumably entering the cell through voltage-operated Ca2+ channels, for the tonic contractions.
