Similarities between the Vascular Smooth Muscle Responses to Sudden Changes in External Potassium, Sodium and Chloride Ion Concentrations

The effects of graded reductions in extracellular potassium, sodium and chloride concentrations on the spontaneous electrical and mechanical activity of the isolated rat portal vein have been studied. The immediate responses obtained on marked reduction in these ion concentrations and on readministration of normal solution were common to the three ion species and differed from the long‐term effect of the ionic environment. Thus abrupt large reductions in the concentration of any of the three ions resulted in a transient excitation whereas return to normal Krebs solution led to a temporary complete inhibition of muscle activity. These initial, transient responses could in many cases not be predicted on the basis of the “constant field equation” by merely inserting the actual ion concentration values. However, if the concentrations of chloride and potassium ions (or chloride and sodium ions) were changed in a reciprocal manner no such transient responses occurred. Some possible explanations for this difference between the immediate and long‐term effects of a changed ionic environment are discussed.