Cellular origins of the transient inward current in cardiac myocytes. Role of fluctuations and waves of elevated intracellular calcium.

Activation of the transient inward current (ITI) by a rise in intracellular calcium concentration (Ca2+i) is believed to be responsible for generating triggered cardiac arrhythmias. In this study, the cellular basis of the rise in Ca2+i that activates ITI and aftercontractions in single rat ventricular myocytes was examined. Ca2+i was measured both indirectly by cell contraction and directly with fura-2. Under conditions that caused steady-state Ca2+i to increase (i.e., calcium overload) membrane repolarization after a voltage-clamp depolarization resulted in the appearance of ITI that was similar in many respects to that observed in multicellular preparations. This ITI occurred at the same time that Ca2+i spontaneously increased and preceded the aftercontraction by 60-90 msec. However, ITI recorded from a single cell was variable in time course and amplitude (unlike that observed in multicellular preparations). Examination of cell contraction and digital imaging of fura-2 fluorescence showed that ITI was often associated with propagating regions of increased Ca2+i, which arose from discrete sites of origin within the cell. Apparently synchronous aftercontractions could also be associated with multiple propagating waves of Ca2+i. The variation in the time course and amplitude of ITI in single cells appeared to be due to changes in the location and number of sites of origin for the waves of Ca2+i. After the first aftercontraction and ITI, desynchronization of the sites of origin of increased Ca2+i occurred, and this resulted in a decrease in the amplitude of ITI and an increase in its duration. We conclude that the variability seen in single cells arises from changes in the pattern of spontaneous Ca2+ release. Such phenomena will seriously complicate interpretation of multicellular data, even when Ca2+i is measured directly.