CALCIUM SOURCE FOR CONTRACTILE RESPONSE OF GUINEA PIG TAENIA CAECUM TO CARBACHOL IN A CALCIUM DEFICIENT, POTASSIUM RICH SOLUTION

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The Ca storing site for the carbachol-induced contraction in a high-K medium without adding CaCl2 was further investigated in the guinea pig taenia caecum. La3+ (0.05-0.5 mM) caused an increase in peak tension and relaxation rate in the carbachol contraction but reduced or abolished the response to CaCl2. Simultaneous application of carbachol and CaCl2 produced tension development the trace of which was comparable to the curve obtained by adding graphically the respective traces of the carbachol-induced and Ca-induced tensions. La3+ (0.5 mM) abolished the late, sustained component without appreciable change in the initial, transient contraction. The carbachol contraction persisted after 5 min perfusion of solution containing 0.25 mM EGTA (estimated free Ca2+ concentration less than 10-8 M). The intracellular Ca content was little changed before and after exposure to carbachol. Histamine induced a transient contraction with a dose-dependent amplitude and resulted in a decrease in tension development produced by subsequently applied carbachol. There was an inversely proportional relationship in amplitude between the histamine contraction and the following carbachol contraction, while the sum of tensions raised by both drugs showed little variation. These results favour an intracellular site for the carbachol-sensitive Ca store. Histamine appears to release Ca2+ from the carbachol-sensitive store.
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