Ca2(+)-induced Ca2+ release as examined by photolysis of caged Ca2+ in single ventricular myocytes

In cardiac muscle, entry of Ca2+ through the voltage-gated Ca2+ channel and its interaction with an intracellular site are thought to trigger the release of the intracellular Ca2+ pools and to activate contraction. The availability of a novel "caged calcium" compound, and its effective use in neuronal and heart cells to modulate Ca2+ channel and contraction, made it possible to examine directly the Ca2(+)-induced Ca2+ release hypothesis in intact mammalian cardiac myocytes. We used the caged Ca2+ compound DM-nitrophen, which on photolysis, rapidly (less than 200 microseconds) changes its Ca2(+)-binding affinity from 3 X 10(-9) to 2 X 10(-3) M at pH 7.0. In isolated whole cell clamped guinea pig ventricular myocytes dialyzed with unphotolyzed DM-nitrophen (Ca2+ buffered to values less than 10(-7) M), we found that a 160-microseconds light pulse photoreleased sufficient Ca2+ to activate contraction. Photorelease of Ca2+ failed to activate significant contraction in myocytes pretreated with caffeine, supporting the idea that the release of Ca2+ from intracellular pools was necessary to generate tension. However, photorelease of Ca2+ after the depolarization-induced Ca2+ release failed to suppress contraction, as predicted from the Ca2(+)-induced inactivation hypothesis. The failure to suppress contraction was not sufficient to definitively reject the Ca2(+)-induced inactivation hypothesis, since the intracellular Ca2+ concentration may not have risen sufficiently to inactivate the release channel.