The calcium content of the smooth muscle of the guinea‐pig taenia coli

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Abstract
The in vitro Ca-content of the smooth muscle of the guinea pig taenia coli was 3.0 m-M Ca/kg wet wt. [weight] when phosphate was omitted from the bathing medium, and was almost independent of pH changes in the range 6.7-7.6. The Ca content was not changed when 1 mM phosphate was included in the medium, if the pH was 6.7 or 7.0. However, when the pH was 7.6, the Ca content increased by 1.5 mM Ca/kg wet wt. in the presence of phosphate. The Ca content rose by 1.1 m-M Ca/kg wt wt. when NaCl in the bathing medium was replaced by isotonic sucrose, and rose by 0.7 m-M Ca/kg wet wt. when MgCl2 in the bathing medium was replaced. These increases may reflect a competition between Ca2+ and other cations for fixed negative sites in the tissue. The initial rapid phase of K42 exchange corresponded to an extracellular K42-space of 470 ml/kg fresh wt. in normal solution, rising to 560 ml/kg fresh wt. in low Na solution and to 760 ml/kg fresh wt. in Ca-free low-Na solution. In this last medium the extracellular [Cl4]sorbitol space was only 390 ml/kg fresh wt., so that there was a large excess of rapidly-exchanging K which may have been competing at fixed negative sites.