AJP - Cell Physiology, Vol 255, Issue 3 C393-C400, Copyright © 1988 by American Physiological Society
Dependence of force, velocity, and O2 consumption on [Ca2+]o in porcine carotid artery
J. M. Krisanda and R. J. Paul
Department of Physiology, University of Cincinnati College of Medicine, Ohio 45267-0576.
The relationship between extracellular calcium concentration ([Ca2+]o),
isometric force (Fo), unloaded shortening velocity (Vus), and the rate of
ATP utilization (JATP) were studied in vascular smooth muscle (VSM) during
the steady state of an isometric contraction at 37 degrees C. Experiments
were conducted on porcine carotid artery media strip and ring preparations
that were stimulated with 109 mM KCl substituted for NaCl. Unloaded
shortening velocity was estimated by the "slack" test method. Both Fo and
Vus were dependent on [Ca2+]o. Vus at 7.5 mM [Ca2+]o was 1.7 times greater
than Vus at 1.6 mM [Ca2+]o. The difference in force at these two Ca2+
concentrations was more variable than Vus, but in general was less than the
change in Vus. The rate of ATP utilization was assessed from steady-state
measurements of tissue O2 consumption. Increasing [Ca2+]o in the range of
0.15-7.5 mM resulted in an increase in both JATP and Fo. The relation
between JATP and Fo was nonlinear with JATP increasing proportionately more
than Fo between 1.6 and 7.5 mM Ca2+. The calcium-dependent increase in JATP
appears to be primarily related to contractile protein interaction, since
the effect of [Ca2+]o on JATP was substantially reduced during stimulation
at short muscle lengths (Lmin), where tension development is absent. The
economy of tension maintenance was 3.3. times greater when measured in the
[Ca2+]o range of 0.15 and 1.6 mM than when measured at 1.6 to 7.5 mM
[Ca2+]o. These data indicate that [Ca2+]o may regulate both the number and
the rate of cycling of cross bridges in porcine carotid artery.
