AJP - Heart and Circulatory Physiology, Vol 256, Issue 3 916-H920, Copyright © 1989 by American Physiological Society
Modulation of Ca2+ channel selectivity and cardiac contraction by photorelease of Ca2+
M. Nabauer, G. C. Ellis-Davies, J. H. Kaplan and M. Morad
Department of Physiology, University of Pennsylvania, Philadelphia 19104-6085.
The effect of rapid increases of extracellular Ca2+ concentration on
tension development and Ca2+ channel selectivity has been investigated in
frog (Rana pipiens) ventricular myocardium using a novel light-sensitive
Ca2+ chelator, dimethoxy-nitrophen. Dimethoxy-nitrophen is a photolabile
tetracarboxylate Ca2+ chelator that on photolysis to dicarboxylate
fragments alters its affinity (Kd) for Ca2+ from 5 X 10(-9) to 2 X 10(-3)
M. A single 160-microseconds ultraviolet light pulse induced the release of
approximately 80-100 microM Ca2+ with a half-time of 200 microseconds. In
low extracellular Ca2+ concentration, Na+ current through the Ca2+ channel
was blocked by photorelease of extracellular Ca2+ in less than 500
microseconds; nevertheless, 60-80 ms were required to activate or enhance
tension. Enhancement of tension was more effective when Ca2+ was released
during the activation of the Ca2+ channel than during its inactivation. The
voltage dependence of enhanced tension, caused by photorelease of Ca2+, was
bell shaped and was similar to that of Ca2+ current. These findings suggest
that Ca2+ transport through the Ca2+ channel is the primary mechanism for
the transport of Ca2+ to activate tension in the frog heart. The use of
dimethoxy-nitrophen makes it possible to examine the rapid kinetics of the
Ca-dependent processes involved in regulation of channel function and
contraction.
