Sodium Gradient, Smooth Muscle Tone, and Blood Pressure Regulation

Rat colon strips respond to an acute reduction of sodium concentration in the medium (Nao) by an immediate increase in tension, followed by relaxation to the basal tension as the tissue equilibrates. Following equilibration in low Nao the responsiveness to drug-induced contraction is increased, while in high Nao it is decreased. A contraction cycle, variously induced, can be aborted by the acute addition of sodium to the medium. The applicability of these findings to vascular smooth muscle in the whole animal can be demonstrated by appropriate acute sodium infusions. The effects in all cases can be referred to sodium, although modified by the anionic component of the salts used. The results are considered to support the view that smooth muscle tension is governed in part by the concentration gradient Nao/Nai and the theoretic implication of this for chronic hypertensive and hypotensive states is developed.