Simultaneous heat and fluorescence changes in cardiac muscle at high rates of energy expenditure: Effects of caffeine and isoprenaline

Abstract

The relationship between the fluorescence-time integral and heat production was investigated in isolated rabbit papillary muscles at room temperature. Four treatments were used: (1) control; (2) 10⁻³ m caffeine; (3) 5×10⁻³m caffeine; (4) 5×10⁻³ m caffeine +2×10⁻⁷m isoprenaline. The first two treatments were associated with low rates of energy expenditure for a given rate of stimulation whereas high rates of energy turnover occurred with the last two treatments. The enthalpy versus load relationship was studied in each treatment as a means for obtaining plots of fluorescence-time integral versus active heat production. All the plots showed high correlations between the two parameters but the linearity of the relationship broke down at high rates of energy expenditure, associated with treatments (3) and (4), as complexities arose in the fluorescence waveforms. No significant changes in resting heat production were induced by any of the drug treatments. Although the mechanical work output was much higher in treatments (3) and (4), the associated extra-active heat production was sufficient to cause a marked drop in mechanical efficiency. Possible biochemical mechanisms underlying the results are discussed.