AJP - Heart and Circulatory Physiology, Vol 243, Issue 1 133-H137, Copyright © 1982 by American Physiological Society
Changes in intracellular Ca2+ activity with stimulation in sheep cardiac Purkinje strands
M. G. Lado, S. S. Sheu and H. A. Fozzard
One hypothesis to explain increased contraction strength with increased
stimulation rate in heart muscle requires that stimulation increase
intracellular Na+ activity(aiNa). This is proposed to increase
intracellular Ca2+ activity (aiCa) via the Na-Ca exchange mechanism.
Several indirect studies have supported the idea that aiNa is increased
with stimulation, and more recently we have directly demonstrated aiNa
elevation with Na+-sensitive microelectrodes. We now report aiCa to be
elevated after trains of stimuli at different rates in sheep cardiac
Purkinje strands. The resting level of aiCa in six strands was 93 +/- 15
(SE) nM, and it increased to 162 +/- 20 nM after stimulation at 3 Hz. The
recovery of aiCa was exponential, and the time constants of 80-120 s were
similar to those previously found for aiNa. A positive force-frequency
relation was found in sheep Purkinje fibers that correlated with the
increased aiCa. The results support the hypothesis that the Na-Ca exchange
system plays an important role in the force-frequency phenomenon.
