Abstract 1576: Contractile Defects In Cardiac Myocytes From sGCa1-Deficient Mice
Background: Soluble guanylate cyclase (sGCα1β1) is an obligate heterodimer which catalyzes the synthesis of the second messenger cGMP in response to NO. The role of NO/sGC/cGMP signaling in cardiac myocytes (CMs) is incompletely understood. In order to elucidate how NO/sGC/cGMP signaling regulates contractility at baseline and in response to β-adrenergic stimuli, we measured sarcomere function and calcium levels in ventricular CMs from WT and sGCα1-deficient mice.
Methods: CMs were loaded with the Ca2+ indicator Fura-2, and paced at 1, 2, 4, and 6 Hz. Ca2+ transients and sarcomere shortening (twitch amplitude) were measured under basal conditions and after incubation with the non-selective β-adrenergic agonist, isoproterenol (1 μM) or the β3-adrenergic agonist, BRL 34377 (10 nM).
Results: Compared with WT CMs, sGCα1−/− CMs exhibited decreased basal contractility and delayed relaxation, without altering Ca2+ transients. Specifically, the velocity of sarcomere shortening was greater in WT cells than in sGC α1−/− cells (5.8 ±0.08 vs. 4.1 ±0.8 μm/sec, p <0.01, 6Hz). The % shortening was also greater in WT cells than in sGC α1−/− cells (7.7±0.7 vs. 6.0 ±1.5%, p <0.01, 6Hz). Relaxation was faster in WT cells than in sGCα1−/− CMs, as measured by the exponential time constant, τ (35 ± 6 vs. 52±10 ms, p<0.001, 6Hz). Isoproterenol stimulation increased contractile function in both WT and sGCα1−/− CMs; however, % shortening remained significantly less in sGCα1−/− cells. With isoproterenol stimulation, the amplitude of the Ca2+ transient at 6Hz in sGCα1−/− CMs was 17±9% less than in WT CMs. Addition of the NO donor spermine NONOate (100 μM) decreased the isoproterenol-stimulated twitch amplitude by 27±10% in WT cells, but NO did not attenuate the isoproterenol-stimulated increase in sarcomere shortening in sGCα1−/− CMs. The β3 adrenoceptor agonist, BRL 34377 (10 nM) decreased cell shortening in WT but not in CMs from sGCα1−/− mice.
Conclusions: The deletion of sGCα1 resulted in decreased basal and α-adrenergic stimulated sarcomere shortening, and delayed relaxation in paced CMs, suggesting that cGMP can positively regulate cardiac contraction and relaxation. The ability of NO and β3-adrenergic signaling to attenuate contractility appears to require sGC and cGMP.