Role of the thyroid gland in the development and maintenance of spontaneous hypertension in rats.

We studied the vascular mechanisms involved in the prevention of the development of hypertension following thyroidectomy. Ablation of the thyroid gland of 4-week-old spontaneously hypertensive (SH) rats inhibited the development of hypertension and reduced the sensitivity of aortic strips to the vasoconstrictors phenylephrine, norepinephrine, and potassium chloride and the vasodilator isoproterenol. Daily injections of a replacement dose of L-thyroxine caused a complete recurrence of hypertension in these rats. This was accompanied by complete recovery of the aortic sensitivity to the vasconstrictors and isoproterenol. In 10-week-old SH rats thyroidectomy prevented a further increase of blood pressure but did not reverse the hypertention. Here, the aortic sensitivity to vasoactive substances also was reduced but to a lesser extent than in the SH rats thyroidectomized at 4 weeks of age. Hypertension was not obviously associated with hyperfunction of the thyroid gland. Furthermore, we found that at 4 weeks of age, during the prehypertensive period, SH rats have a significantly lower (42%) serum thyroxine level than age-matched normotensive Kyoto Wistar, American Wistar, and Sprague-Dawley rats. However, at 6 and 9 weeks, the serum thyroxine levels of SH rats are similar to those of the normotensive rats. In conclusion, we propose that the reduced sensitivity to endogenous vasoconstrictors in arteries of SH rats following juvenile ablation of the thyroid gland may prevent the development of hypertension in these rats. Moreover, the low serum thyroxine level in SH rats during the prehypertensive period may explain why young SH rats do not develop hypertension before 6 weeks of age.