Effects of different stages of aortic coarctation hypertension on aortic contraction and relaxation in rats.

Contractile responses to norepinephrine, serotonin and potassium (K+) and relaxant responses to isoproterenol and papaverine were studied in vitro with spirally cut thoracic aortic strips from aortic coarcted hypertensive rats (AHR)2, 6, 14 and 28 days postoperatively and compared to time-matched, sham-operated normotensive controls. At every stage after coarctation, the rats developed hypertension with elevated plasma renin activity. In response to stimulation by norepinephrine and serotonin, aortic strips from 2 to 28 day AHR developed the same tension as controls, whereas aortas of 6 and 14 day AHR had reduced maximal responses. For K+-stimulated aortic strips, maximal contractile force was decreased at 6 day AHR only. Relaxation by isoproterenol and papaverine in serotonin-contracted aortas was the same in AHR and normotensive controls 2 and 28 days postoperatively but was reduced at 6 and 14 days. The demonstrated changes of vascular contractility and relaxation in AHR is a hypertensive stage-dependent phenomenon. It is speculated that 6 and 14 days after coarctation the diminished relaxant ability of the aortas helps to maintain the elevated blood pressure and the diminished sensitivity to contractile stimulants is a protective mechanism in response to the elevated blood pressure. The return of normal contraction and relaxation to the agonists in the chronic stage of hypertension may possibly reflect an adaptive change to the prolonged stimulus of the elevated blood pressure that aortic tissue had undergone in order to maintain normal physiologic functions.