Adrenaline Mediates A Positive Feedback Loop in Noradrenergic Transmission: Its Possible Role in Development of Hypertension

Abstract

Adrenaline activates prejunctional β-adrenoceptors of the β₂-subtype on sympathetic nerve terminals and enhances noradrenergic transmission. Adrenaline can be incorporated in transmitter stores of noradrenergic nerves and, when released as a cotransmitter, activates the prejunctional β₂-adrenoceptors, thereby mediating autofacilitation of noradrenergic transmission. Adrenaline released from the adrenal medulla in stress may be incorporated in noradrenergic transmitter stores and reach a sufficient concentration as a cotransmitter to activate the autofacilitatory feedback loop involving prejunctional β₂-adrenoceptors, resulting in prolongation of the increases in vasomotor tone and cardiac activity that occur acutely: with frequent repetition of stress, there may be progression into a hypertensive state. In accord with this hypothesis, adrenaline administration produces persistent increases in blood pressure in rats, and plasma levels of adrenaline are elevated in a proportion of hypertensive patients; furthermore, repeated stress produces prolonged increases in blood pressure in animals and man.

Key Words:

• Adrenaline
• β-Adrenoceptors
• Hypertension
• Noradrenergic Transmission
• Prejunctional β-Adrenoceptors
• Stress