EFFECTS OF ACETYLCHOLINE AND HISTAMINE ON MECHANICAL ACTIVITY OF RABBIT TAENIA COLI, Ca-INCORPORATION AND Ca-RELEASE IN ITS MICROSOMAL FRACTION

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In Ca-free Locke Ringer solution, the contractile responses (mean±S.E. of 8 experiments) induced by acetylcholine (5.5×10^<-5> M) and histamine (5.4×10^<-4> M) are, respectively, 47.7±9.0 and 29.2±9.9 (%) of those obtained in normal Locke Ringer solution. Furthermore, Ca-blocker, D-600 (10^<-7> M) caused a more significant reduction of histamine-induced contraction than that of contraction induced by acetylcholine. Ca-incorporation in a microsomal fraction prepared from the rabbit taenia coli was also studied. Ca-incorporation was increased in the presence of both adenosine triphosphate (ATP) and Mg^<2+>, unaffected by azide and enhanced by oxalate. Acetylcholine (5.5×10^<-5> M), histamine (5.4×10^<-5> M) and atropine (3.5×10^<-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 acetylcholine (5.5×10^<-5> M), but not by histamine (5.4×10^<-5> M). Atropine (3.5×10^<-6> M) inhibited this effect due to acetylcholine. Thus Ca-release induced by acetylcholine seems to be due specifically to acetylcholine receptors. The present observations suggest that in rabbit taenia coli, the mechanisms by which acetylcholine and histamine interact with Ca to induce contraction are different.
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