Sodium, Potassium, and Peripheral Resistance in the Rat Tail

The early effects of systematic changes of environmental Na+ on vascular resistance and on cationic equilibration were studied using a closed circuit perfusion of the rat tail. The recirculating medium was monitored for Na+ and K+ with glass electrodes operating through a computer. The composition of a given medium perfusing the vascular bed is very rapidly altered as it comes into equilibrium with cells and interstitial tissue. Because exchanges begin quickly, only limited conclusions can be drawn from manipulation of a perfusing medium without close monitoring of the equilibration process. Reduction of sodium in the medium, using sucrose as osmotic replacement, is followed by a rapid efflux of Na+ , some of which apparently comes from cells. This is associated with a fall of vascular resistance. Neither efflux nor fall in resistance occurs if choline or K+ is used as replacement for Na+ . Elevation of potassium in the medium is followed by a gradual fall during equilibration of the ion and, if K+ is raised sufficiently, an increase in vascular resistance occurs. These findings support the view that the transcellular distribution of Na+ and K+ is causally involved in the maintenance of vascular tone.