Tension‐independent heat in rabbit papillary muscle.

1. Heat and force were measured from isometrically contracting (0.2 Hz) rabbit papillary muscles at 21 degrees C during a single contraction‐relaxation cycle using antimony‐bismuth thermopiles and a capacitance force transducer. 2. Tension‐independent heat (TIH) associated with excitation‐contraction coupling was isolated from the initial heat by eliminating tension and tension‐dependent heat with a Krebs‐Ringer solution containing 2,3‐butanedione monoxime (BDM) and mannitol. 3. A strategy for testing the validity of this new method for measuring TIH in heart muscle is described and the test confirms that the BDM‐hypertonic solution partitioning method properly estimates the magnitude of the TIH component of initial heat. 4. TIH at the time of complete mechanical relaxation is 1.00 +/‐ 0.17 mJ/g wet weight and the data suggest that calcium cycling is complete by this time. Conversion of TIH to calcium cycled, assuming that 87% of TIH is due to calcium pumping by the sarcoplasmic reticulum, indicates that approximately 52 nmol calcium/g wet weight are required to support a single cycle of mechanical activity (0.2 Hz, 21 degrees C). 5. The length and frequency dependence of excitation‐contraction coupling were demonstrated. TIH is reduced by shortening muscle length and by increasing the interval between stimuli. These steady‐state data suggest that only a portion (approximately 40%) of TIH is directly related to activation of the contractile apparatus. 6. TIH in the first twitch following a 45 min rest period is significantly reduced by approximately 30%. 7. With subsequent twitches in the positive treppe following the rest period, TIH does not increase as steeply as expected suggesting that tension rise in twitches 1‐10 may be modulated by competitive binding of calcium rather than increased calcium delivery.