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Department of Pharmacology, Tohoku University School of Medicine, Seiryo-machi, Sendai 980, Japan

1. Thin fibre bundles (about 100 µm in width and 1.5 mm in length) were dissected from taenia caeci of the guinea-pig, and were mounted in an apparatus to record isometric tension mainly at 25 °C.

2. Fibre bundles were chemically skinned by treating with 50 µg saponin/ml. for 20 min in a relaxing solution. Upon application of an activating solution containing 80 µM-Ca²⁺ the fibre bundles rapidly developed a tension which was comparable in magnitude to that elicited with 10 µM-carbachol before the saponin treatment.

3. It was confirmed that skinned fibre bundles develop active tension slowly in the virtual absence of Ca²⁺ when Mg²⁺ concentration was raised above 2 mM in the presence of MgATP. However, this tension was not developed in the presence of an ATP regenerating system.

4. Maximum Ca²⁺ activated tension and Ca sensitivity of skinned smooth muscle fibre bundles were examined using solutions in which pH, ionic strength, and the concentrations of Mg²⁺ and MgATP^2- were varied around supposedly physiological values. An ATP regenerating system was used when necessary.

5. Maximum Ca²⁺ activated tension changed only slightly with alteration in ionic strength (0.15-0.3 M). It decreased by 40% with increase in pH from 6.4 to 7.2, and decreased by 10-30% when Mg²⁺ concentration was either lowered to 0.2 mM or raised to 2 and 10 mM from 1 mM. The tension increased by about 10-20% as MgATP^2- concentration was raised from 0.4 to 4 or 8 mM.

6. Changes in pH, ionic strength, and MgATP^2- concentration in the above range had no important effects on the relative isometric tension—pCa relation. Decrease of Mg²⁺ concentration from 1 to 0.2 mM had little effect, but increase to 2 and 10 mM shifted the curve some 0.2 and 0.6 pCa units, respectively, toward a lower pCa. There was not large difference between the relative isometric tension—pCa relation at 25 °C and that at 38 °C.

7. The Ca²⁺ concentration in the smooth muscle cell of taenia during physiological contraction can be estimated from the results obtained and the values for the relevant in vivo ionic constituents given in the literature; the lowest Ca²⁺ concentration for detectable tension development is between 0.2 and 0.5 µM, and tension tends to level off above 10 µM-Ca²⁺.