Sex differences in the changes in sympathetic nerve activity when arterial pressure is raised by infusion of angiotensin and noradrenaline.

1. 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.