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.
Alpert
N R
NR
Department of Physiology and Biophysics, University of Vermont College of Medicine, Burlington 05405.
Blanchard
E M
EM
Mulieri
L A
LA
D000818:Animals; D002863:Chromogenic Compounds; D003931:Diacetyl / Q000031:analogs & derivatives / Q000494:pharmacology; D004558:Electric Stimulation / Q000379:methods; D005069:Evaluation Studies as Topic; D006358:Hot Temperature; D006982:Hypertonic Solutions / Q000494:pharmacology; D007552:Isotonic Solutions / Q000494:pharmacology; D008297:Male; D008722:Methods; D009200:Myocardial Contraction; D010210:Papillary Muscles / Q000033:anatomy & histology / Q000502:physiology*; D011817:Rabbits; D012146:Rest; D013997:Time Factors
D002863:Chromogenic Compounds; D006982:Hypertonic Solutions; D007552:Isotonic Solutions; C051865:Krebs-Ringer solution; C004717:diacetylmonoxime; D003931:Diacetyl