Positive functional effects of milrinone and methylene blue are not additive in control and hypertrophic canine hearts

This study was designed to test whether increased inotropy caused by raising intracellular cAMP would add to the positive inotropy caused by reducing cGMP and whether this relationship was affected by experimental hypertrophy. We used open chest anesthetized dogs with left ventricular hypertrophy (LVH) induced by valvular aortic stenosis and age matched controls (CON). Hearts were instrumented to measure local segment shortening, force, and regional work. Milrinone (MIL), a selective cyclic AMP- phosphodiesterase inhibitor, and methylene blue (MB), a guanylate cyclase inhibitor, were used to alter cAMP and cGMP levels. Ten CON and 11 LVH animals were randomly assigned to receive first either MIL (1 μg/kg/min) or MB (2 mg/kg/min) intracoronary (i.c.) infusion. After 10 min, simultaneous i.c. infusion of the other agent was begun. MIL increased regional minute work (g x mm/min) in both CON (1311 ± 207 to 2072 ± 285) and LVH (1052 ± 136 to 1371 ± 351) hearts. MB did not increase work significantly, but did increase contractile force. MIL + MB increased work from baseline; however, the combination did not increase work more than either agent alone (1961 ± 510 CON; 1390 ± 285 LVH). Myocardial cAMP levels (pmol/g) were significantly increased by MIL in both CON (329 ± 53 to 437 ± 13) and LVH hearts (351 ± 67 to 538 ± 32), and the addition of MB further raised cAMP (879 ± 115 CON; 741 ± 96 LVH). MB resulted in decreased myocardial cGMP (pmol/g) (3.20 ± 0.61 to 2.16 ± 0.92 CON; 5.21 ± 1.15 to 2.46 ± 0.56 LVH), while MIL increased cGMP (3.20 ± 0.61 to 6.24 ± 1.79 CON; 5.21 ± 1.15 to 6.53 ± 0.41 LVH). Both MIL and MB caused increases in O₂ consumption, with MIL + MB together increasing O₂ consumption further. The current findings demonstrate a potentiation of cAMP production in the presence of MIL + MB above either agent alone, but this did not lead to potentiation of positive functional effects. High levels of cGMP caused by milrinone may have limited the positive functional effects of cAMP.