Effects of acetylcholine and histamine on mechanical activity of rabbit taenia coli, Ca-incorporation and Ca-release in its microsomal fraction.

Abstract

In Ca-free Lock Ringer solution, the contractile responses (mean .+-. SE of 8 experiments) induced by acetylcholine [Ach] (5.5 .times. 10-5 M) and histamine (5.4 .times. 10-4 M) are, respectively, 47.7 .+-. 9.0 and 29.2 .+-. 9.9 (%) of those obtained in normal Locke Ringer solution. Ca-blocker, D-600 [.alpha.-isopropyl-.alpha.-[N-methyl-N-homoveratryl-.gamma.-aminopropyl]-3,4,5-trimethoxyphenylacetonitrile] (10-7 M) caused a more significant reduction of histamine-induced contraction than that of contraction induced by Ach. Ca incorporation in a microsomal fraction prepared from the rabbit tenia coli was also studied. Ca incorporation was increased in the presence of both ATP and Mg2+, unaffected by azide and enhanced by oxalate. Ach (5.5 .times. 10-5 M), H (5.4 .times. 10-5 M) and atropine (3.5 .times. 10-6 M) were without any effect on Ca incorporation in the presence and absence of oxalate. The rate of Ca release from microsomal vesicles was significantly increased by Ach (5.5 .times. 10-5 M), but not by histamine (5.4 .times. 10-5 M). Atropine (3.5 .times. 10-6 M) inhibited this effect due to Ach. Ca release induced by Ach seems to be due specifically to Ach receptors. The present observations suggest that in rabbit tenia coli, the mechanisms by which Ach and histamine interact with Ca to induce contraction are different.