SEX DIFFERENCE IN PRESYNAPTIC ADRENERGIC INHIBITION OF NOREPINEPHRINE RELEASE DURING NORMOXIA AND ISCHEMIA IN THE RAT-HEART

Using a perfused innervated rat heart model, we studied the sex difference in the sympathetic nerve stimulation-induced norepinephrine release and its presynaptic alpha-2-adrenergic inhibition in normoxic and ischemic conditions. During normoxic perfusion, the alpha-2-adrenoceptor antagonist rauwolscine resulted in a higher overflow of norepinephrine during nerve stimulation in females than in males (p < 0.05). This more marked potentiation of norepinephrine overflow in females was accompanied by an increased chronotropic and inotropic response (p < 0.01). During early stop-flow ischemia neural norepinephrine overflow was lower in female than in male hearts (p < 0.005). Rauwolscine enhanced norepinephrine overflow more in females than in males (p < 0.05), thereby eliminating the initial difference in norepinephrine overflow during ischemia between the two sexes. Ovariectomy attenuated the presynaptic alpha-2-adrenergic inhibition of norepinephrine release compared with sham-operated females (p < 0.02). No sex difference was found in either cardiac norepinephrine content or nonexocytotic norepinephrine overflow induced by a 40-minute period of stop-flow ischemia. Thus, presynaptic alpha-2-adrenergic inhibition of myocardial norepinephrine release is greater in female than in male rats. This difference persists into the early phase of ischemia and is largely responsible for the lower neural norepinephrine release in the female heart. Female hormones may increase presynaptic alpha-2-adrenergic activity in the heart.