Potassium relaxation of vascular smooth muscle from spontaneously hypertensive rats

Helical strips of tail artery from spontaneously hypertensive (SHR) and Kyoto Wistar normotensive rats (WKY) were observed to relax in response to potassium after contraction induced by norepinephrine in potassium-free solution. Helical strips from SHR consistently showed greater relaxation in response to the addition of potassium than did those from WKY. The amplitude of potassium-induced relaxation is believed to be an index of the activity of electrogenic sodium-potassium transport and hence of sodium-potassium ATPase. Thus, the sodium-potassium ATPase activity of SHR vascular smooth muscle is increased as compared to WKY. This interpretation is supported by the observation that ouabain eliminated potassium-induced relaxation in both SHR and WKY strips. Potassium-induced relaxation in SHR was more sensitive to ouabain inhibition than in WKY. Relaxation induced by potassium in SHR and WKY strips was also shown to vary with: (1) the length of incubation in potassium-free solution, and (2) the concentration of potassium added back. The results of these experiments on potassium-induced relaxation serve as evidence that SHR have either an increased intrinsic sodium-potassium ATPase activity, or an enzyme activity that has been stimulated to a greater degree by an elevated intracellular sodium concentration which resulted from the incubation in potassium-free solution.