SMOOTH-MUSCLE ENERGETIC IN THE PIG CORONARY-ARTERY DURING A CALCIUM-DEPENDENT AND A CALCIUM-INDEPENDENT ISOMETRIC CONTRACTION

Heat production by resting smooth muscle, was measured with a heat-flu x micro calorimeter on cut-open segments of pig coronary artery superf used at 30 degrees C, was 0.93+/-0.06 (n=16) mW/g wet weight. Time cou rses of the increases in isometric tension and heat production with re spect to basal during sustained supra maximal acetylcholine stimulatio n were qualitatively similar: initial peak tapering down to a supra ba sal plateau. Mean tension-associated heat production over 1 h was 0.16 J/g. During sustained exposure to phorbol 12,13-dibutyrate, supra bas al tension and with a 5-10 min initial delay - supra basal heat progre ssively increased to a plateau in about 40 min. Mean tension-associate d heat production over 1 h was only 0.02 J/g with normal extracellular and intracellular mobilizable Ca2+ pools, and it was further reduced to 0.01 J/g with depleted Ca2+ pools. These results show that the main tenance - if not necessarily the building up - of tension under phorbo l 12,13-dibutyrate does not entail any large dissipation of energy and is not dependent on the presence of normal Ca2+ pools.