Key result
In cats and dogs, raising blood pressure caused an inverse linear relationship with sympathetic spike frequency in females, whereas in males the discharge rate remained steady or increased at high pressures.
This preclinical study demonstrates a sex difference in sympathetic nerve activity response to elevated blood pressure, likely mediated by testosterone sensitizing chemoreceptor blood vessels to vasoconstrictors.
Sex-specific sympathetic responses in animal models should not alter clinical BP management; leaves open sex hormone effects on human baroreflex function.
In cats and dogs impulse frequency was studied in single and few-fibre preparations of the cervical sympathetic nerves when blood pressure was raised in steps to about 200 mm Hg by infusions of noradrenaline and angiotensin. Two forms of frequency/blood pressure relationships were found which differed in their distribution between the sexes.2. In females, both normal and ovariectomized, there was an inverse linear relationship between spike frequency and blood pressure over the whole range of blood pressure studied. The same was true in castrated males and normal males pretreated with oestrogens.3. In normal males the inverse linear relationship held to a pressure of about 150 mm Hg. At higher pressures the sympathetic discharge rate remained steady or increased. This was also found in castrated males pretreated with testosterone. There was a statistically significant difference between the responses of these animals and those of normal females and castrated or oestrogen treated males.4. The sex difference originates peripherally since the responses of both sexes were similar following baroreceptor and chemoreceptor denervation. During ventilation with oxygen instead of air the male pattern of response becomes like that of the female. The latter observation implicated the chemoreceptors.5. alpha adrenoceptor blockade with phenoxybenzamine and phentolamine localized to the region of the carotid bifurcation in male cats abolished the increase in sympathetic discharge rate at high blood pressure induced by noradrenaline, but did not alter the response to angiotensin.6. Records of spike frequency in single and few-fibre preparations of chemoreceptor units in the sinus nerves of cats also showed an inverse relationship between frequency and blood pressure in females. In males at the higher pressures, the discharge rate was steady or increased. The increased chemoreceptor discharge at high pressures in males was abolished by ventilation with oxygen instead of air.7. The sex difference in behaviour of chemoreceptor units could not be explained by differences in either total carotid body blood flow or arterial oxygen tension.8. It is tentatively concluded that the observed sex difference is related to the state of the blood supply to the chemoreceptors and that, in males, testosterone sensitizes the blood vessels to vasoconstrictors, so that at high pressures blood is diverted through shunts and the chemoreceptors are subjected to a stagnant hypoxia.
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Morrison et al. (1971) studied this question. Infusion of angiotensin and noradrenaline vs. Sex differences (males vs females) was evaluated on Changes in sympathetic nerve activity (spike frequency) when blood pressure is raised. In cats and dogs, raising blood pressure caused an inverse linear relationship with sympathetic spike frequency in females, whereas in males the discharge rate remained steady or increased at high pressures.