Effects of selective alpha 1‐, alpha 2‐, beta 1‐and beta 2‐adrenoceptor stimulation on potentials and contractions in the rabbit heart.

Selective adrenoceptor agonists and antagonists have been used to analyse the effects of stimulation of individual types of adrenoceptor in various parts of the rabbit heart. The selective alpha 1- and alpha 2-adrenoceptor agonists used were St 587 and BHT 933 respectively, and the antagonists were prazosin (alpha 1) and WY 25309 (alpha 2). The selective beta 1- and beta 2-adrenoceptor antagonists were atenolol and ICI 118551, respectively. Pirbuterol was a highly selective beta 2-adrenoceptor agonist. The non-selective agonists noradrenaline, adrenaline and isoprenaline were also employed with various combinations of antagonists. Phenylephrine was found to stimulate beta- as well as alpha-adrenoceptors. Rimiterol was a beta-adrenoceptor agonist, partially selective for beta 2-adrenoceptors. In the sinus node beta 1-, but not beta 2-adrenoceptor stimulation increased the fast phase of depolarization (Vmax). Both beta 1- and beta 2-adrenoceptor stimulation increased the slope of slow diastolic depolarization, accelerated repolarization and increased maximum diastolic potential. After blockade of both beta 1- and beta 2-adrenoceptors alpha 1-adrenoceptor stimulation caused bradycardia, due exclusively to delayed repolarization. alpha 2-adrenoceptor stimulation had no effect. In Purkinje cells and papillary muscle both beta 1- and beta 2-adrenoceptor stimulation accelerated repolarization. Stimulation of alpha 2-adrenoceptors had no effect. Beta 1-, not beta 2-adrenoceptor stimulation augmented peak contractions 3-5-fold, and greatly increased rate of development of tension. After beta-blockade alpha 1-adrenoceptor stimulation moderately increased peak contractions (up to 47%), but increased time-to-peak and duration of contractions. These patterns of adrenoceptor-mediated effects were unchanged in animals pre-treated with sufficient 6-hydroxydopamine to eliminate responses to sympathetic nerve stimulation. The results would be consistent with beta 1-, and beta 2-adrenoceptor stimulation increasing inward calcium current, and with stimulation of alpha 1-adrenoceptors delaying its inactivation, rather than increasing its magnitude.