Abstract 770: Coordinated Upregulation of Cardiomyocyte Urotensin II (U-II) and its Receptor (UT) Exhibits Enhanced Inhibition of L-Type Calcium Current in Heart Failure
Background. Heart failure has been linked to urotensin II (U-II), the most potent mammalian vasoconstrictor. However, the alteration and functional significance of U-II in HF are uncertain.
Methods. We compared LV myocyte U-II and its receptor (UT) protein levels and ICa,L response to U-II stimulation in 10 normal (N) adult rats and 10 age-matched rats with isoproterenol (ISO)-induced HF (4 months after 340 mg/kg, sq, for 2 days). ICa,L was measured using whole-cell voltage clamp technique. In 3 subgroups, U-II-mediated actions were determined after pretreatment of myocytes with U-II receptor antagonist, Urantide (10−5 M); pertussis toxin (PTX, 2μg/ml, 36°C, 6h); and dibutyryl-cAMP (Db-cAMP, 10−4 M).
Results. Compared with normal myocytes, in HF myocytes, both U-II (93%, 0.081 vs 0.042) and UT (52%, 0.038 vs 0.025) protein levels were significantly increased. Importantly, these changes were associated with enhanced U-II -mediated negative modulation on LCa,L. Superfusion of U-II (10−8-10−5 M) caused a dose-dependent decrease in ICa,L in both normal and HF myocytes with maximal inhibition at 10−5 M. In normal myocytes, U-II (10−5 M) decreased ICa,L by 18% (5.0±0.29 vs 6.1±0.5 pA/pF, p<0.01). In HF myocytes, the baseline ICa,L was significantly lower (3.3±0.1 vs 5.9±0.3 pA/pF, p<0.01) and was further reduced by 27% with U-II 10−5 M (2.4±0.1 vs 3.3±0.1 pA/pF), that was significantly greater than U-II-induced changes in normal myocytes. U-II-induced reductions in ICa,L were significantly attenuated by U-II-antagonist. In both groups of rats, U-II-induced decreases of ICa,L were prevented with pretreatment myocytes with PTX (normal: 5.6±0.5 vs 5.5±0.4; HF: 3.4±0.4 vs 3.5±0.2 pA/pF) or Db-cAMP (normal: 5.6±0.5 vs 5.6±0.7; HF: 3.7±0.3 vs 3.8±0.4 pA/pF).
Conclusions. In HF rat cardiomyocytes, U-II and UT-coupled, Gi-mediated signaling pathway is upregulated, which amplifies U-II stimulation-induced inhibition of ICa,L. This may lead to exacerbation of the dysfunctional [Ca2+ ]i homeostasis and enhance cardiac depression of the failing myocardium.