Presynaptic beta-adrenoceptors.

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.