Increase in membrane permeability in the absence of Ca and Mg in the smooth muscle of guinea-pig taenia coli.

In Ca-deficient smooth muscle of guinea-pig taenia coli, repeated application of high-K solution (45.4 mM) containing Ca induced contractions of similar shape and magnitude. In muscle treated by a Ca- and Mg-deficient solution, however, addition of Ca and K either did not induce contraction or induced only a delayed contraction. Ouabain (1 X 10(-3) M) also inhibited Ca- and K-induced contraction. Na content of taenia coli smooth muscle increased and K content decreased during incubation in Ca- and Mg-deficient solution. Ouabain produced similar, but smaller, changes in Na and K contents. Contractility of Ca- and Mg-deficient taenia partially recovered if the muscle was treated with Na-deficient solution which resulted in a large decrease in Na content. Similar treatment produced only a small decrease in Na content in ouabain-treated taenia, and contractility did not recover in these muscles. Application of hyperosmotic NaCl (160 mM) decreased tissue weight in both control and ouabain-treated taenia. In muscle treated with Ca- and Mg-deficient solution, however, hyperosmotic NaCl application had little effect on tissue weight. Following pretreatment of muscle with Ca- and Mg-deficient solution (containing 2 mM EDTA) for 2 hr, graded contractions were induced by cumulative application of Ca between 10(-7) to 10(-6) M in the presence of Mg and ATP. It is concluded that Ca- and Mg-deficient solution increases membrane permeability and also abolishes transmembrane gradients of Na and K in guinea pig taenia coli.