Response of failing canine and human heart cells to beta 2-adrenergic stimulation. 
Failing human hearts lose beta 1- but not beta 2-adrenergic receptors.  In canine hearts with tachypacing failure, the ratio of beta 2- to beta 1-adrenergic receptors is increased.  The present study was designed to determine whether heart failure increases sensitivity to beta 2-adrenergic stimulation in isolated canine ventricular cardiomyocytes and to verify that myocytes from failing human ventricles contain functional beta 2-adrenergic receptors.  Myocytes from healthy dogs, dogs with tachypacing failure, and human transplant recipients were loaded with fura 2-AM and subjected to electric field stimulation in the presence of zinterol, a highly selective beta 2-adrenergic agonist.  Zinterol significantly increased [Ca2+]i transient amplitudes in all three groups.  The failing canine myocytes were significantly more responsive than normal to beta 2-adrenergic stimulation.  We also measured isotonic twitches, indo-1 fluorescence transients, and L-type Ca2+ currents in healthy canine myocytes.  Zinterol (10(-5) mol/L) elicited large increases in the amplitudes of simultaneously recorded twitches and [Ca2+]i transients.  Zinterol also increased L-type Ca2+ currents in the normal canine myocytes; this augmentation was abolished by 10(-7) mol/L ICI 118,551.  cAMP production by suspensions of healthy and failing canine myocytes was not increased by zinterol (10(-9) to 10(-5) mol/L), nor did 10(-5) mol/L zinterol elicit phospholamban phosphorylation.  Failing human ventricular cardiomyocytes contain functional beta 2-adrenergic receptors.  Canine myocytes also contain functional beta 2-adrenergic receptors.  The canine ventricular response to beta 2-agonists is increased in tachypacing failure.  Positive inotropic responses to beta 2-stimulation are not mediated by increases in cAMP or cAMP-dependent phosphorylation of phospholamban.