Fig 2. Autoantibodies directed against the β1-adrenergic receptor (β1-AAB) of Doberman pinschers (n = 6) target the second extracellular receptor loop.

Using the bioassay of spontaneously beating cultured neonatal rat cardiomyocytes, the chronotropic activities of the Doberman pinschers’ β1-AAB is demonstrated by the absence (-) or presence (L1/L2) of the peptides (L1 = first loop; L2 = second loop) competing with the first and second extracellular receptor loops. To rule out any interference with M2-AAB, if present in the sample, the bioassay was performed in the presence of atropine to block the M2-AAB activity. The control experiment was performed in the presence of bisoprolol (Biso). Values are expressed as x±SD; values below the low limit of detection (LLD) were displayed as half range values. LLD = 4 beats/min. * p<0.001.