Changes in intracellular Ca2+ activity with stimulation in sheep cardiac Purkinje strands

Abstract

One hypothesis to explain increased contraction strength with increased stimulation rate in heart muscle requires that stimulation increase intracellular Na+ activity .**GRAPHIC**. This apparently increases intracellular Ca2+ activity .**GRAPHIC**. via the Na-Ca exchange mechanism. Several indirect studies supported the idea that .**GRAPHIC**. is increased with stimulation, and more recently .**GRAPHIC**. elevated with Na+-sensitive microelectrodes was demonstrated. After trains of stimuli at different rates .**GRAPHIC**. was elevated in sheep cardiac Purkinje strands. The resting level of .**GRAPHIC**. in 6 strands was 93 .+-. 15 (SE) nM, and it increased to 162 .+-. 20 nM after stimulation at 3 Hz. The recovery of .**GRAPHIC**. was exponential, and the time constants of 80-120 s were similar to those previously found for .**GRAPHIC**. A positive force-frequency relation was found in sheep Purkinje fibers that correlated with the increased .**GRAPHIC**. Evidently the Na-Ca exchange system plays an important role in the force-frequency phenomenon.