Sex differences in reflex tachycardia induced by hypotensive drugs in unanesthetized rats

Possible sex differences in autonomic regulation of the heart were studied by injecting standard hypotensive doses of histamine or bradykinin to induce reflex tachycardia in awake rats. Although depressor responses to both drugs were unaltered, the tachycardia was greatly reduced by beta-adrenergic blockade with propranolol and abolished almost completely after combined blockade with propranolol and atropine. These findings indicate that the tachycardia was mostly due to increased sympathetic stimulation wiht a minor contribution from parasympathetic withdrawal. With large doses of bradykinin, direct myocardial stimulation also seemed likely, since some residual tachycardia remained even after combined blockade. Reflex tachycardia was generally more pronounced in male than in female rats. Because males had lower base-line heart rates and more tachycardia that was resistant to beta-adrenergic blockade than females, autonomic regulation was concluded to be predominantly parasympathetic in males and sympathetic in females. Whereas the exact cause of these variations in chronotropic regulation is unknown, it was considered possible that differences in sex hormones may have affected sensitivity of some component of the reflex arc.