Pc. Dooley et Ir. Wendt, METABOLISM OF TOAD VENTRICLE DURING ALTERATIONS TO THE INOTROPIC STATE, Journal of Molecular and Cellular Cardiology, 26(10), 1994, pp. 1357-1370
The heat produced by toad ventricle during manipulations of the inotro
pic state was measured using thermopiles, and some comparisons made to
rat ventricle. The tension-independent heat, peak stress, and the ten
sion-dependent heat increased when [Ca2+](o) increased from 0.25 to 2
mM in Ringer. In 2 mM [Ca2+](o), tension-independent heat, peak stress
, and tension-dependent heat were 3.1 +/- 0.4 mJ/g, 38.4 +/- 5.5 mN/mm
(2), and 0.49 +/- 0.06 units; about 25% of the tension-independent hea
t may relate to the Na+-K+ pump. At similar [Ca2+](o), rat ventricle p
roduced a smaller tension-independent heat (1.6 +/- 0.2 mJ/g), and act
ive heat per unit stress (0.22 +/- 0.01 units) than toad. Tension-inde
pendent heat, stress, and tension-dependent heat were increased by orc
iprenaline, and decreased by BDM. Ouabain increased the stress and ten
sion-dependent heat but not the tension-independent heat. Five millimo
lar [Ca2+](o) in HEPES buffer decreased the stress but increased the t
ension-dependent heat compared to 2 mM [Ca2+](o) in Ringer. Ryanodine
and CPA caused major reductions in force and tension-independent heat
in rat, but had little effect on toad ventricle. In conclusion, our re
sults suggest that in toad ventricle (a) the sarcoplasmic reticulum pl
ays only a minor role in activation and relaxation, (b) the Na+-K+ pum
p contributes substantially to activation metabolism, (c) active metab
olism is stimulated by increases in [Ca2+](o) and (d) there is a large
r tension-independent heat, a larger active metabolism per unit stress
, and a lower basal metabolism than in rat papillary muscle. The energ
y cost of removing intracellular Ca2+ through the sarcolemma appears t
o be greater than uptake into sarcoplasmic reticulum.