Tension responses of chemically skinned fibre bundles of the guinea‐pig taenia caeci under varied ionic environments

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Abstract
Thin fiber bundles (.apprx. 100 .mu.m in width and 1.5 mm in length) were dissected from tenia ceci of the guinea pig and were mounted in an apparatus to record isometric tension at 25.degree. C. Fiber bundles were chemically skinned by treatment with 50 .mu.g saponin/ml for 20 min in a relaxing solution. Upon application of an activating solution containing 80 .mu.M Ca2+ the fiber bundles rapidly developed a tension which was comparable in magnitude to that elicited with 10 .mu.M carbachol before the saponin treatment. Skinned fiber bundles develop active tension slowly in the virtual absence of Ca2+ when Mg2+ concentration was raised above 2 mM in the presence of MgATP. This tension was not developed in the presence of an ATP regenerating system. Maximum Ca2+ activated tension and Ca sensitivity of skinned smooth muscle fiber bundles were examined using solutions in which pH, ionic strength and the concentrations of Mg2+ and MgATP2- were varied around supposedly physiological values. An ATP regenerating system was used when necessary. Maximum Ca2+ activated tension changed only slightly with alteration in ionic strength (0.15-0.3 M). It decreased by 40% with an increase in pH from 6.4 to 7.2 and decreased by 10-30% when Mg2+ concentration was lowered to 0.2 mM or raised to 2 and 10 mM from 1 mM. The tension increased by about 10-20% as MgATP2- concentration was raised from 0.4 to 4 or 8 mM. Changes in pH, ionic strength and MgATP2- concentration in the above range had no important effects on the relative isometric tension-pCa relation. A decrease of Mg2+ concentration from 1 to 0.2 mM had little effect, but an increase to 2 and 10 mM shifted the curve 0.2 and 0.6 pCa units, respectively, toward a lower pCa. There was not a large difference between the relative isometric tension-pCa relation at 25.degree. C and that at 38.degree. C. The Ca2+ concentration in the smooth muscle cell of tenia 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 .mu.M and tension tends to level off above 10 .mu.M Ca2+.