Regional variation in norepinephrine-stimulated calcium uptake in rabbit aorta

This study was conducted to determine if there is a difference in the calcium handling mechanism of the thoracic and abdominal aortic regions of the rabbit aorta. Isolated segments of aorta from both regions were studied in HEPES buffer at 37 degrees C and bubbled with 100% oxygen using identical procedures. Intracellular calcium levels were determined after segments had been exposed to 45Ca-labeled buffer during experimental procedures and subsequently washed with a 'quench' solution of zero calcium, 2 mM EGTA, and HEPES buffer, maintained at 0 degree C. The results suggest that the thoracic region is capable of taking up more calcium under resting conditions compared to the abdominal aorta. During stimulation with norepinephrine, both regions show significantly increased calcium influx rates after 10 min of exposure to the agonist, but only in the abdominal region is this effect maintained after 60 min of norepinephrine stimulation. Net (summation of calcium influx and efflux) intracellular calcium levels are significantly increased by norepinephrine in the thoracic but not in the abdominal aorta. Taken together, these data suggest that, compared to the thoracic aorta, the abdominal aorta may possess a more effective calcium efflux mechanism which may be able to fully compensate for the norepinephrine-stimulated increase in calcium influx.