By definition, beta-blockers block beta-receptors. Structur¬ally they resemble the catecholamines (adrenaline and noradrenaline) and block the action of these catechol-amines at their receptor sites. The beta-receptors are situated on the cell membrane and are believed to be a part of the adenyl cyclase system. An agonist acting on its receptor site activates adenyl cyclase to produce cyclic adenosine-5-monophosphate, which is believed to be the intracellular messenger of beta stimulation. There are two types of beta-receptors, beta-1 and beta-2.
A. Beta-1 Receptors
The beta-1 receptors are present mainly in the heart, renin-secreting tissues of the kidney, parts of the eye responsible for the production of aqueous humor, and to a limited degree in bronchial tissue of the lung. Beta-1-adrenergic receptors regulate heart rate and myocardial contractility, but in situations of stress with the provo¬cation of epinephrine release stimulation of cardiac beta-2 receptors contribute to additional increases in heart rate and contractility.
B. Beta-2 Receptors
These are predominant in the bronchial tissues of the lung, vascular smooth muscle, insulin-secreting tissues of pan¬creas, gastrointestinal tract, and to a limited degree in the heart and coronary arteries. None of these tissues exclusively contains one subgroup of receptor. The popula¬tion density of receptors decreases with age. In addition, the beta-receptor population is not static, and during long-term therapy with beta-adrenergic blocking agents the number of receptors is increased.

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