Trimethylamine-N-oxide acutely increases cardiac muscle contractility

We demonstrate for the first time that elevated concentrations of TMAO acutely augment myocardial contractile force ex vivo in both murine and human cardiac tissue. To gain mechanistic insight into the processes that led to this potentiation in cardiac contraction, we used two-photon microscopy to evaluate intracellular calcium in ex vivo whole hearts loaded with the calcium indicator dye Fluo-4. Acute treatment with TMAO resulted in increased Fluo-4 fluorescence, indicating that augmented cytosolic calcium plays a role in the effects of TMAO on force production. Lastly, TMAO did not show an effect on aortic smooth muscle contraction or relaxation properties. Our results demonstrate novel, acute, and direct actions of TMAO on cardiac function and help lay the groundwork for future translational studies investigating the complex multiorgan interplay involved in cardiovascular pathogenesis during CKD.

Downregulation of ventricular contractile function during early ischemia is flow but not pressure dependent

The mechanism responsible for the abrupt fall in myocardial contractile function following coronary artery obstruction is unknown. The “vascular collapse theory” hypothesizes that the fall in coronary perfusion pressure after coronary artery obstruction is responsible for contractile failure during early ischemia. To test the role of vascular collapse in downregulating myocardial contractile force at the onset of ischemia, coronary flow of isolated rat hearts was abruptly decreased by 50, 70, 85, and 100% of baseline, and subsequent changes in coronary perfusion pressure and ventricular function were recorded at 0.5-s intervals. At 1.5 s after flow reductions ranging from 50 to 100%, decreases in contractile function did not differ, although perfusion pressure varied significantly from 45 ± 1 to 20 ± 2 mmHg. When function fell to 50% of baseline, perfusion pressures ranged from 35 ± 0.5 to 2.5 ± 1 mmHg for flow reductions ranging from 50 to 100%. Identical contractile function at widely differing coronary perfusion pressures is incompatible with the vascular collapse theory.

Correlation between myocardial contractile force and cytosolic inorganic phosphate during early ischemia

To test the role of inorganic phosphate (Pi) in downregulation of myocardial contractile force at the onset of ischemia, Pi of rat hearts was determined with 31P nuclear magnetic resonance spectroscopy. Forty cycles of brief hypoperfusion (30% of baseline flow for 33 s) were used to achieve a time resolution of 0.512 s for comparing dynamic changes in Pi and contractile force. Initial control values of left ventricular developed pressure (LVP), heart rate, and oxygen consumption were 136 +/- 11 mmHg, 236 +/- 4 beats/min, and 95 +/- 3 microl O2 x min(-1) x g(-1); these values were unchanged at the end of the experiment. During the first 10 s of hypoperfusion, Pi increased at a rate (percentage of the total observed change) faster than the decrease in LVP; Pi and LVP then changed at the same rate during the remainder of the hypoperfusion. ADP did not change in advance of LVP. Intracellular pH did not change. The results indicate that Pi plays an important role in initiating the downregulation of myocardial contractile force at the onset of ischemia. Perfusion pressure also declined faster than LVP at the onset of ischemia, indicating potential importance of vascular collapse in contractile downregulation during early ischemia.

Role of extracellular calcium on heart muscle energetics: effects of verapamil

The mechanical and energetic effects of verapamil (VER) and reduction of extracellular Ca concentration ([Ca]o) were studied in the interventricular rabbit septa and the dog papillary muscle. Even though the negative inotropic effects of VER [i.e., decrease in developed tension (T), maximal rates of contraction (+T) and relaxation (-T), and tension time integral] qualitatively resemble [Ca]o reduction, VER also elicited an anti-relaxant effect (decrease in -T/T and prolongation of the last phase of relaxation) that was not found with [Ca]o reduction. Resting heat production was similar in both preparations and remained unaffected either by changes in [Ca]o or by the presence of VER. The ratio between T and active heat production per beat (H'a) under constant fiber length decreased with VER, and this decreased economy of contraction was more marked with the increase in contraction frequency. Conversely, the T/H'a remained unaltered with changes in [Ca]o. Tension-independent heat decreased in the presence of VER and, although muscle economy can be improved by increasing muscle length in a VER-treated muscle, it is not possible to achieve either the maximal T or the maximal contraction economy that can be obtained by stretching a nontreated muscle. It may be concluded that at constant fiber length and frequency of contraction VER decreases myocardial contractile force, impairs relaxation, and decreases contraction economy. Neither the mechanical nor the energetic effects of VER can be explained solely on the basis of a reduced extracellular Ca availability, so that either the density of the Ca that enters through the channel is different from that of other sources of Ca or VER has an effect on the cross-bridge cycling mechanism.