Role of the sympathetic nervous system in the ischemic and reperfused heart.

Norepinephrine, that has been released from sympathetic nerve endings in response to myocardial ischemia, may have either a beneficial or a harmful effect on the ischemic heart. If the duration of ischemia is short, the release of norepinephrine may be favorable for the production of energy and for protection of the heart against ischemic damage. If the duration of ischemia is prolonged, there is a marked increase in number of both alpha 1 and beta-adrenoceptors located in the sarcolemmal membrane, as well as an excessive increase in release of norepinephrine. These events during the prolonged period of ischemia can produce an imbalance between oxygen supply and demand, which is harmful to the heart. The anti-ischemic effect of alpha 1- and beta-adrenoceptor antagonists is not attributed merely to improvement of oxygen balance, but reduction of phospholipase activity or stabilization of membrane may also be important as an underlying mechanism.