The thyroid and hypoxic moderation of systemic hypertension in the spontaneously hypertensive rat

Surgically thyroidectomized (TX), sham-operated euthyroid (EU), or thyroidectomized with dietary hormone replacement (RPL), 8-week-old male spontaneously hypertensive rats were subjected to 4 weeks of either of two altitude treatments: normoxia (N; lab altitude = 1520 m) or hypobaric hypoxia (H; simulated altitude = 3658 m). Systolic blood pressure (SBP) was attenuated in all hypoxic and in TX-N rats (p less than 0.05). Thyroidectomy reduced oxygen consumption, rectal temperature, and hormonal indices of thyroid function, as well as attenuating hypoxia-induced polycythemia and right ventricular hypertrophy. Thyroidectomy decreased the sensitivity of aortic rings to KCl and isoproterenol with no differences between EU-N and EU-H or between TX-N and TX-H apparent. Vessel responsiveness in RPL-H was consistent with the hypothyroid status indicated by hormonal measurement, while RPL-N vessel responsiveness was characteristic of euthyroid vessels. Since EU-H rats were euthyroid, with similar vascular responses to EU-N, hypoxia-induced attenuation of SBP does not require hypothyroid-like vascular alterations. Thus, hypoxia and thyroidectomy appear to mitigate systemic hypertension by different mechanisms.