Influence of parathyroidectomy on aortic responsiveness to noradrenaline in spontaneously hypertensive rats

The development of hypertension was delayed and attenuated after removal of the parathyroid glands in 6 weeks-old spontaneously hypertensive rats (SHR). The influence of parathyroidectomy on vascular contractile responses to noradrenaline was investigated on isolated aortic rings. An increased sensitivity to cumulative concentrations of noradrenaline in physiological solution containing 1.25 mmol/l calcium was observed 22 weeks after surgery in aorta from parathyroidectomized rats as compared to control rats. In response to a single concentration of noradrenaline (10(-5) mol/l), aortae from parathyroidectomized SHR rats compared to controls displayed an increased tonic component. No difference in contractile responses to calcium was observed between aortae from control and parathyroidectomized rats. These data are not specific for hypertension in SHR since after parathyroidectomy similar results were also obtained on normotensive Wistar rats. In addition, 8 weeks-old Wistar rats contractile responses evoked by cumulative concentrations of noradrenaline in solution containing 0.6 mmol/l calcium were decreased in aortae from both control and parathyroidectomized rats. Moreover, calculated contractile responses were similar in parathyroidectomized and control rats for the in vivo levels of ionized calcium. From these results it appears that removal of parathyroid glands in both spontaneously hypertensive and normotensive rats, leads to an increase in the in vitro vascular reactivity to noradrenaline, and the extracellular calcium concentration seems to play a key role in this mechanism.