This work shows that vesicles derived from the blue marlin heater organ retain a sarcoplasmic reticulum (SR) Ca(2+)-ATPase that can interconvert different forms of energy. During the hydrolysis of ATP part of the energy is always converted into heat, and the other part can be converted into work (Ca(2+) transport) or heat, depending on the temperature and the presence of KCl in the reaction medium. At 15 degrees C, where KCl stimulates the activity approximately threefold, measurements of the amount of heat released per mole of ATP hydrolyzed (DeltaH(cal)) show similar values (approximately -11 kcal/mol) in the presence or absence of a Ca(2+) gradient. At 25 degrees C, KCl activates the enzyme to the same extent as at 15 degrees C, but inhibits the production of extra heat by SR Ca(2+)-ATPase when a Ca(2+) gradient is built up across the membrane. The DeltaH(cal) values found in the presence of a Ca(2+)-gradient were -26.2 +/- 2.9 kcal/mol (n = 7) in control experiments and -16.1 +/- 1.5 (n = 14) in the presence of 100 mM KCl. At 35 degrees C, KCl has a smaller effect ( approximately 1.5-fold) on activating the enzyme. Similar to SR Ca(2+)-ATPase from mammals, at this temperature the enzyme produces almost twice the amount of heat per mole of ATP hydrolyzed in the presence of a Ca(2+) gradient and KCl has no effect at all on this increment. These data suggest that the marlin SR Ca(2+)-ATPase may play an important role in heater organ thermogenesis and that KCl has the potential for regulating the heat production catalyzed by the enzyme.