Reduced Inotropic Reserve and Increased Susceptibility to Cardiac Ischemia/Reperfusion Injury in Phosphocreatine-Deficient Guanidinoacetate- N -Methyltransferase–Knockout Mice

Abstract

Background-The role of the creatine kinase (CK)/phosphocreatine (PCr) energy buffer and transport system in heart remains unclear. Guanidinoacetate-N-methyltransferase-knockout (GAMT Ϫ/Ϫ ) mice represent a new model of profoundly altered cardiac energetics, showing undetectable levels of PCr and creatine and accumulation of the precursor (phospho-)guanidinoacetate (P-GA). To characterize the role of a substantially impaired CK/PCr system in heart, we studied the cardiac phenotype of wild-type (WT) and GAMT Ϫ/Ϫ mice. Methods and Results-GAMTϪ/Ϫ mice did not show cardiac hypertrophy (myocyte cross-sectional areas, hypertrophy markers atrial natriuretic factor and ␤-myosin heavy chain). Systolic and diastolic function, measured invasively (left ventricular conductance catheter) and noninvasively (MRI), were similar for WT and GAMT Ϫ/Ϫ mice. However, during inotropic stimulation with dobutamine, preload-recruitable stroke work failed to reach maximal levels of performance in GAMT Ϫ/Ϫ hearts (101Ϯ8 mm Hg in WT versus 59Ϯ7 mm Hg in GAMT Ϫ/Ϫ ; PϽ0.05). 31 P-MR spectroscopy experiments showed that during inotropic stimulation, isolated WT hearts utilized PCr, whereas isolated GAMT

Inhibition of cardiac contractility by 5-hydroxydecanoate and tetraphenylphosphonium ion: a possible role of mitoKATP in response to inotropic stress

“…This energy transfer system is not required during normal workload conditions but is necessary to achieve high-work states. Indeed, the heart is unable to function in the upper 50% of its dynamic range when CK is inhibited (4,8,11,24,27). Thus energy transfers via ADP-to-ATP ratio (ADP/ATP), which are sufficient to match low rates of energy production and consumption, represent an energy "leak" at high rates of cardiac work.…”

Supranormal Myocardial Creatine and Phosphocreatine Concentrations Lead to Cardiac Hypertrophy and Heart Failure

“…7,8 A number of studies in both humans and animal models have all shown that phosphocreatine (PCr) and free creatine levels decrease by up to 60% to 70% in heart failure, independent of the underlying etiology but in relation to the severity of contrac-tile dysfunction. 9 -15 Impairment of high-energy phosphate availability in creatine kinase (CK)- 16 or in guanidinoacetate-N-methyltransferase 17 -knockout mice has adverse consequences for myocardial function. Failing hearts with an impaired CK system are unable to maintain low levels of free ADP and thus, high phosphorylation potential during inotropic stimulation.…”

“…These measurements were obtained, as previously described, 17 immediately after the 3D echocardiogram under the same general anesthesia, which was maintained with 2% isoflurane during cannulation. The right carotid artery was dissected and cannulated with a 1.4F microtipped pressure cannula (SPR-839, Millar Instruments), which was advanced retrogradely into the LV cavity under echocardiographic guidance.…”

“…Isolated, perfused heart experiments were performed as previously described 17 (constant pressure of 80 mm Hg at 37°C) with KrebsHenseleit buffer containing (in mmol/L) 11 D-glucose, 4.5 pyruvate, and 0.5 lactate as substrates. Experiments were performed in a vertical-bore, 11.7-T (500-MHz) MR system (Magnex Scientific) with a Bruker Avance console (Bruker Medical) as previously described, 17 a pulse-and-collect sequence at a repetition time of 2 seconds, and a flip angle of 60°.…”

Thematic review series: Systems Biology Approaches to Metabolic and Cardiovascular Disorders. Network perspectives of cardiovascular metabolism

“…However, they showed increased susceptibility to ischemic injury (46) and electrical instability during stress (47) as well as a markedly reduced work capacity during voluntary exercise (48). Mice with undetectable levels of creatine phosphate attributable to knock out of a critical enzyme required for creatine synthesis also had normal resting cardiac function but a markedly impaired inotropic reserve in response to adrenergic stimulation (49). In permeabilized cardiac muscle fibers from CK knockout mice, ATP generated endogenously by mitochondria was found to be much more effective at supporting SR Ca uptake (6) than exogenous ATP delivered via the bath, indicating that competition for exogenous ATP by other ATPases limited its availability to Ca ATPases in the SR.…”