Cardiac β-adrenoceptor-effector coupling in portal vein-stenosed rats

Cardiac β-adrenergic signal transduction was examined in chronic portal vein-stenosed rats. Basal tension and maximum rate of tension development were significantly depressed in left ventricular papillary muscles (0.21 ± 0.03 N/cm² and 8.2 ± 1.7 N · s^-1 · cm^-2, respectively) compared with sham-operated controls (0.51 ± 0.05 N/cm² and 19.9 ± 4.4 N · s^-1 · cm^-2, respectively). The positive inotropic response to isoproterenol was also attenuated. Adenosine 3',5'-cyclic monophosphate formation was decreased significantly when GTP (-41.9%), isoproterenol with GTP (-45.3%), or guanosine 5'-O-(3-thiotriphosphate) (-52.4%) was used to stimulate adenylyl cyclase, but not when Mn²⁺ or forskolin was used. β-Adrenoceptor density (sham operated 24.6 ± 2.0 fmol/mg; portal vein stenosed 26.4 ± 2.1 fmol/mg) and the apparent dissociation constant (sham operated 0.26 ± 0.04 nM; portal vein stenosed 0.29 ± 0.04 nM) were unaffected. Portal venous hypertension did not alter β-adrenergic receptor affinity for isoproterenol. However, it was necessary for isoproterenol to occupy three times the number of receptors in papillary muscles from stenosed animals to produce an equal increase in force generation. These data suggest that although portal vein stenosis does not alter cardiac β-adrenoceptor density or affinity for ligands, transduction of the signal between the receptor and adenylyl cyclase is adversely influenced and may be responsible for the diminished responsiveness of β-adrenoceptors in the myocardium.