Intracellular mechanisms of cGMP-mediated regulation of myocardial contraction

The intracellular mechanisms of cGMP, a major intracellular mediator of nit ric oxide that regulates the contractility of cardiac muscle, are still to some extent unknown. To investigate these mechanisms, we observed the effec ts of 8-bromo-cyclic GMP (8br-cGMP) on myofibrillar Ca2+ sensitivity and Ca 2+ handling of the sarcoplasmic reticulum (SR) using beta -escin-skinned pr eparations from Wistar rat hearts. Both low (1 muM) and high doses (100 muM ) of 8br-cGMP significantly decreased the myofibrillar Ca2+ sensitivity obt ained from pCa-tension relationships to a similar extent (pCa(50); from 6.0 4 to 5.95 by 1 muM 8br-cGMP and 6.00 to 5.89 by 100 muM 8br-cGMP, respectiv ely, n = 9 each). Whereas this Ca2+ desensitization induced by 100 muM 8brc -GMP was blocked by 1 muM KT5823, a specific inhibitor of cGMP-dependent pr otein kinase (PKG), not induced by 1 muM 8br-cGMP was not effected by KT582 3. When the amount of Ca2+ released from the SR was estimated by the peak a mplitude of 25 mM caffeine-induced contractions after constant Ca2+-oading by pCa 6, both doses of 8br-cGMP significantly augmented the caffeine-induc ed peak force to a similar extent (125 +/- 5.8 % by 1 muM 8br-cGMP and 116 +/- 5.1 % by 100 muM 8br-cGMP, respectively, n = 6 each). The two observed effects of cGMP (a decrease in myofibrillar Ca2+ sensitivity and an increas e in Ca2+ uptake by the SR) may participate in regulating myocardial contra ction via nitric oxide. Low and high doses of cGMP seem to work mainly via PKG-independent and PKG-dependent pathways, respectively.