Calcium fractions in aortic segments from hypertensive rabbits

The rate constants (-k) of Ca2+ from cellular compartments of intact smooth muscle cells of thoracic aorta of normotensive, renal hypertensive, and cadmium hypertensive rabbits were studied by 45Ca efflux and by employing norepinephrine (NE, 5.9 X 10(-7) M). Tissues from hypertensive animals showed an increased rate of release of 45Ca over a 5-h unloading period when compared to normotensive controls. Three exchangeable compartments were found to contribute to the total kinetic efflux curve for all groups. The Ca2+ fraction in compartments I and II showed an increased rate of Ca2+ release for the hypertensive subgroups. Although the rates were slowed for tissues treated with NE prior to efflux, the rates of the hypertensive subgroups were increased compared to the normotensive group. Pretreatment of aortic segments with NE caused an overall increase in 45Ca activity of the incubation medium and may explain the slowed rates attained in the efflux analyses. Influx studies showed that tissues from hypertensive animals had a greater uptake of 45Ca than did the normotensive controls. Such data may represent a larger exchangeable component in aortae from the hypertensive subgroups. These findings indicate differences in the rates of Ca2+ release from intact smooth muscle cells of hypertensive rabbits.