Abstract 448: Modulation of the β-Adrenergic Stimulated Inotropy by PDE5a Inhibition is Blunted and Redox Sensitive in Myocytes From Chronically Hypertrophied or Failing Hearts
Background. We previously reported that inhibition of the cGMP-specific phospho-diesterase PDE5a suppresses β-adrenergic stimulated contractility in normal intact hearts and isolated myocytes. This effect was not observed, however, in a canine model of cardiac failure. Whether this loss of efficacy is intrinsic to the myocyte and/or β-receptor complex is unknown.
Methods. Adult murine cardiomyocytes were isolated from C57BL/6 mice subjected to 3 weeks of chronic transaortic constriction (TAC), and FVB/N mice over-expressing activated Gαq. Cells were suspended in Tyrode’s solution (1mM Ca2+) and field stimulated (0.5 Hz, 25°C). Sarcomere shortening (SS) was assessed by real-time image analysis, Ca2+ transients by Indo-1 fluorescence.
Results. Myocytes from both TAC and Gαq hearts had relatively normal unloaded shortening under rest conditions (3.55±0.3% for TAC, 3.31±0.2% for Gαq, 3.59±0.4% and 3.48±0.4% for controls, p=NS); however they exhibited a markedly reduced response to 10nM isoproterenol (ISO) compared to genotype-specific controls: for example after ISO infusion, the increase in SS was lower (p<.001) in TAC vs control cells (48±24% vs 155±13). In contrast, forskolin (FSK, 100nM) increased SS and Ca2+ transients in all groups. Co-infusion of the PDE5a inhibitor sildenafil (SIL, 100nM-1μM) blunted FSK inotropy in controls (−26±6% and −15±4% for C57BL/6 and FVB/N, respectively, both p<0.01 vs FSK alone) without altering the Ca2+ transients. However, SIL had less effect in TAC myocytes (−8±3%, p=.03 vs FSK) and no impact in Gαq cells. Pre-incubation of TAC or Gαq cells with 4mM GSH, restored SIL suppression of FSK responses to −22±6% and −10±3% for TAC and Gαq cells, respectively (p=.05 vs GSH-untreated cells).
Conclusions. PDE5a modulation of β-adrenergic contractility is suppressed in myocytes from chronic hypertrophied or failing hearts. The effect is downstream of the β-receptor but involves cAMP-mediated inotropy. Oxidative stress contributes to this behavior, perhaps by diminishing cGMP synthesis via nitric oxide-soluble guanylate cyclase. Rescue by antioxidants may be a useful strategy to enhance effects of PDE5 inhibitors to blunt cardiac adrenergic stress.