Evidence for beta 2-adrenoceptor-mediated facilitation of 3H-noradrenaline release from isolated guinea pig papillary muscles.

The effects of beta-adrenoceptor agonists and antagonists on field-stimulated release of radioactivity from superfused guinea pig papillary muscles preincubated with 3H-noradrenaline were studied. Stimulation-evoked overflow of tritium was abolished in the absence of Ca2+ or the presence of tetrodotoxin. Isoprenaline (1 mumol/L) caused a slight facilitation of evoked overflow, whereas phentolamine (1 mumol/L) exerted a strong facilitatory action. However, when phentolamine (1 mumol/L) was present throughout superfusion, isoprenaline and the selective beta 2-adrenoceptor agonist, zinterol, caused concentration-dependent increases (half-maximal effects at 1 nmol/L). The effects of the agonists were inversely related to stimulation frequency. Furthermore, the concentration-response curve of isoprenaline was shifted to the right by the selective beta 2-adrenoceptor antagonist, ICI 118,551, but not by the selective beta 1-adrenoceptor antagonist, ICI 89,406. Schild-plot analysis revealed competitive antagonism and a pA2 value of 9.04 for ICI 118,551. Both ICI 118,551 and ICI 89,406, as well as beta-adrenoceptor antagonists with intrinsic sympathomimetic activity (pindolol and celiprolol; 1 mumol/L), had no effect on stimulation-evoked overflow of tritium (phentolamine present). It is concluded that guinea pig papillary muscles are endowed with prejunctional beta 2 adrenoceptors facilitating impulse-evoked noradrenaline release. The facilitation is markedly promoted by blockade of prejunctional alpha adrenoceptors.