Functional evidence for increased sodium permeability in aortas from DOCA hypertensive rats

We studied the role of increased Na⁺ permeability on the increased responsiveness to ouabain and to K⁺-free solution in aortas from DOCA hypertensive rats. Helically cut strips from DOCA hypertensive and normotensive control rats were mounted in a muscle bath for recording isometric force. In response to ouabain, aortas from DOCA hypertensive rats were significantly more sensitive and developed a greater maximal force than aortas from control rats. The rate of force development in response to K⁺-free solution was significantly faster in aortas from DOCA hypertensive rats as compared to those from control rats. Monensin (10^-5 M), a Na⁺ ionophore, increased the contractile response to ouabain and the rate of force development in response to K⁺-free solution in both DOCA hypertensive and control aortas. Amiloride (3 X 10^-5 M), a Na⁺ channel blocker, decreased the contractile response to ouabain and the rate of force development to a K⁺-free solution in both the DOCA hypertensive and control aortas, but the magnitude of decrease was greater in aortas from DOCA hypertensive rats. Thus, a Na⁺ ionophore causes the control aortas to perform like those from DOCA hypertensive rats, and a Na⁺ channel blocker causes aortas from DOCA hypertensive rats to perform like those from control rats. It is concluded that the difference between the two is that the smooth muscle of aortas from DOCA hypertensive rats is more permeable to Na⁺ than is that from control rats.