Bioenergetic consequences of cardiac phosphocreatine depletion induced by creatine analogue feeding.

To further evaluate the bioenergetic role of phosphocreatine, we assessed several parameters in normal and depleted rat hearts. Rats were fed (8 weeks) a diet containing either 1% beta-guanidinoproprionic acid or 2% beta-guanidinobutyric acid (beta-GBA), resulting in an 80% phosphocreatine depletion compared to controls. Left ventricular pressure-volume curves were obtained to determine contractile function. At any volume, the developed pressure in depleted hearts was lower than in controls. At the plateau, the rate-pressure product was between 37-45% lower: 34,000 (beta-GBA), 30,174 (beta-guanidinoproprionic acid) versus 54,400 (control). 31P NMR spectroscopy on beta-GBA-treated hearts obtained the [ATP] and [phosphocreatine], which with saturation transfer estimated the rates of creatine kinase and ATP production. In depleted hearts, the rate constant for ATP synthesis from phosphocreatine was increased 33%. However, the flux was 72% lower. ATP production from ADP and Pi were similar under normal conditions, in spite of higher rates of oxygen consumption in the depleted hearts. The addition of 50 mM creatine to control perfusate had no effect on function or high energy phosphates. In contrast, a 28% increase in function and a 52% increase in [phosphocreatine] was seen in beta-GBA hearts. There was a marked increase in free [ADP] in beta-GBA hearts, resulting in a lower estimated ATP phosphorylation potential. Overall, the results suggest that phosphocreatine may play an important function by optimizing the thermodynamics of cardiac high energy phosphate utilization.

Effect of creatine depletion on myocardial mechanics.

The physiological significance of energy transport by means of shuttling creatine (C) and creatine phosphate (CP) between mitochondria and the energy utilizing sites was examined in C-depleted rat hearts. Feeding a diet containing structural analogues of C [either 1% guanidinoproprionic acid (GPA) or 2% guanidinobutyric acid (GBA)] led to the decrease of total myocardial C (C + CP) by 78% and 75% after seven weeks and by 89% and 82% after ten weeks. Mechanics of isolated papillary muscles were examined under isometric conditions at different temperatures, muscle lengths and frequencies of stimulation. No differences were found in the basic characteristics of contraction and relaxation among the hearts of normal, GPA- and GBA-treated rats at low workloads. However, when paired stimulation was applied, the interval at which fusion occurred was significantly longer in C-depleted than in controls. At high workloads, the developed force (DF) in C-depleted decreased in the same way as in controls, but its subsequent recovery took significantly longer and the recovered DF in C-depleted muscles was smaller than in controls. We conclude that C depletion has greater effects on the excitation to contraction coupling and recovery rather than on the DF during stimulation at high frequencies.