The calcium content of the smooth muscle of the guinea-pig taenia coli.

1. The in vitro calcium content of the smooth muscle of the guinea-pig taenia coli was 3.0 m-mole Ca/kg wet wt. when phosphate was omitted from the bathing medium, and was almost independent of pH changes in the range 6.7-7.6.2. The calcium 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 calcium content increased by 1.5 m-mole Ca/kg wet wt. in the presence of phosphate.3. The calcium content rose by 1.1 m-mole Ca/kg wet wt. when NaCl in the bathing medium was replaced by isotonic sucrose, and rose by 0.7 m-mole Ca/kg wet wt. when MgCl(2) in the bathing medium was replaced. These increases may reflect a competition between Ca(2+) and other cations for fixed negative sites in the tissue.4. The initial rapid phase of (42)K exchange corresponded to an ;extra-cellular (42)K-space' of 470 ml./kg fresh wt. in normal solution, rising to 560 ml./kg. fresh wt. in low-sodium solution and to 760 ml./kg fresh wt. in calcium-free low-sodium solution. In this last medium the extra-cellular [(14)C]sorbitol space was only 390 ml./kg fresh wt., so that there was a large excess of rapidly-exchanging potassium which may have been competing at fixed negative sites.