Energetic compartmentation plays a significant role in regulating cellular ionic homeostasis; for example, ATP derived from glycolysis may preferentially regulate Ca²⁺ channels of the sarcolemma, whereas ATP from mitochondrial oxidative phosphorylation is required for contractile protein regulation (Losito et al. 1998). Pyruvate has been reported to increase contraction in myocytes and hearts, but the mechanisms are not understood. In this study the effects of pyruvate and/or glucose on cell contraction and intracellular calcium concentration ([Ca²⁺]_i) were investigated using isolated rat cardiomyocytes.

Male rats were humanely killed by cervical dislocation and ventricular myocytes isolated by collagenase digestion. [Ca²⁺]_i was measured using indo-1, and the dye loaded into cells under conditions that promote a cytosolic location (Griffiths, 1999). Cell contraction was determined using an edge-tracking device. Cells were superfused with Hepes-based buffer containing 1 mM Ca²⁺ at 30 °C. Glucose (16 mM) and/or pyruvate (5 mM) were present as indicated. Results are expressed as means ± S.D., and statistical significance determined using Student’s t test (paired where appropriate).

Superfusion of cells with glucose buffer caused no significant change in the contractile amplitude (cell shortening) over a 30 min superfusion period. In contrast, when the buffer was switched to one containing both glucose and pyruvate, there was a rapid depression of contraction by about 45 %, from an initial value of 12.5 ± 3.5 to 7.4 ± 3.9 % (n = 6, P < 0.001). No change in [Ca²⁺]_i occurred during this time. Continued superfusion with pyruvate resulted in a gradual increase in contraction which peaked between 15 and 20 min, and was significantly higher than initial contraction (16.4 ± 5.0 %, P < 0.05), and then a return to initial values after 30 min (12.3 ± 2.5 %). [Ca²⁺]_i followed a similar pattern, i.e. an increase in the amplitude of the Ca²⁺ transient followed by return to initial values. Superfusion with pyruvate alone resulted in a similar initial decrease in cell contraction, which then gradually returned to initial values. However, there was no positive inotropic effect with pyruvate alone.

The initial depression of contraction using either glucose and pyruvate or pyruvate alone is likely to be due to intracellular acidification caused by pyruvate (Wang et al. 1994), since [Ca²⁺]_c was unchanged. The overshoot in contraction with glucose and pyruvate can, however, be explained by an increased [Ca²⁺]_c. The cell is thus able to overcome the negative inotropic effect of pyruvate caused by intracellular acidification by compensatory mechanisms that involve an increase in [Ca²⁺]_i. However, the positive inotropic effect of pyruvate was seen only in the presence of glucose, and was not maintained.This work was supported by the British Heart Foundation and Garfield Weston Trust.

Griffiths, E.J. (1999). FEBS Lett. 453, 400-404.

Losito, V.A., Tsushima, R.G., Diaz, R.J., Wilson, G.J. & Backx, P.H. (1998). J. Physiol. 511, 67-78. abstract

Wang, X., Levi, A.J. & Halestrap, A.P. (1994). Am. J. Physiol. 267, H1759-1769.