Calcium and the impairment of contractions to norepinephrine in aorta isolated from the spontaneously hypertensive rat.

Helical strip contractility from hypertensive (SHR) and normotensive (WKY) rat aortas assessed in the presence of varying calcium concentrations indicated that SHR strips exhibit a higher intrinsic myogenic tone and contract less to norepinephrine (NE) in a physiological calcium concentration compared to controls. Relatively higher isometric tension was developed in the SHR in low calcium (0-0. 27 mM). While control responses were blunted by LaCl3, EGTA, and nifedipine, the SHR strips were unaffected. Addition of procaine significantly enhanced SHR contractility to NE with no effect on control strips. These data suggest that abnormal cytosolic calcium provokes an increase in myogenic tone and an impaired contractile response of aortic smooth muscle cells to NE.