Oscillations of intracellular Ca2+ in mammalian cardiac muscle.
Contraction of cardiac muscle depends on a transient rise of intracellular calcium concentration ([Ca2+]i) which is initiated by the action potential. It has, however, also been suggested that [Ca2+]i can fluctuate in the absence of changes in membrane potential. The evidence for this is indirect and comes from observations of (1) fluctuations of contractile force in intact cells, (2) spontaneous cellular movements, and (3) spontaneous contractions in cells which have been skinned to remove the surface membrane. The fluctuations in force are particularly prominent when the cell is Ca2+-loaded, and have been attributed to a Ca2+-induced Ca2+ release from the sarcoplasmic reticulum. In these conditions of Ca2+-loading the normal cardiac contraction is followed by an aftercontraction which has been attributed to the synchronization of the fluctuations. The rise of [Ca2+]i which is thought to underlie the aftercontraction also produces a transient inward current. This current, which probably results from a Ca2+-activated nonspecific cation conductance, has been implicated in the genesis of various cardiac arrhythmias. However, despite the potential importance of such fluctuations of [Ca2+]i their existence has, so far, only been inferred from tension measurements. Here we present direct measurements of such oscillations of [Ca2+]i.