Mechanisms involved in contraction of smooth muscles of the rat portal vein as induced by sodium depletion.

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Abstract
Responses to external Na depletion were investigated in the rat portal vein, using a microelectrode and an isometric force transducer. Mechanical response to Na depletion was characterized by a large tonic contraction with phasic contractions in a choline solution, and by phasic contractions without a tonic contraction in a Li solution. An identical depolarization of the membrane occurred in either choline or Li solution. After the tonic contraction was established in the Na-free choline-solution, the vein all but completely relaxed with partial re-admission of Na, while maintaining constant the concentration of choline. The Na-free choline-solution at 13.degree. C did not induce the tonic contraction. In nominal Ca-free solution, no contraction occurred with a depletion of Na. A tonic contraction was also induced by Na depletion in the presence of Mn but not Ca. It is concluded that with depletion of both external and internal Na, Ca and Mn may enter the cell through channels usually occupied by Na.

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