AJP - Heart and Circulatory Physiology, Vol 249, Issue 2 328-H336, Copyright © 1985 by American Physiological Society
Saxitoxin binding and "fast" sodium channel inhibition in sheep heart plasma membrane
D. D. Doyle, D. M. Brill, J. A. Wasserstrom, T. Karrison and E. Page
We compared specific [3H]saxitoxin (STX) binding to isolated sheep
ventricular sarcolemmal vesicles with inhibition of maximal action
potential upstroke velocity (V max) by STX and tetrodotoxin (TTX) in sheep
trabeculae carneae. In sarcolemmal vesicles purified 30 to 40 times over
cardiac homogenate, STX binding at 0 degrees C in Na-free solution
exhibited both high-affinity sites (KD = 0.22 +/- 0.05 nM, n = 85 +/- 13
fmol/mg protein) and low-affinity sites (KD = 11 +/- 4 nM, n = 360 +/- 42).
The STX-inhibition constant for V max in Tyrode solution at 37 degrees C
was 280 nM. TTX was approximately 10% as effective as STX in displacing
bound [3H]STX and inhibiting V max. Allowing for different experimental
conditions during [3H]STX binding and V max measurements, we suggest that
the low-affinity sites are physiologically relevant "fast" Na+ channels of
myocardial cells. Combining morphometric data for plasmalemmal area of
mammalian cardiac myocytes with n for low-affinity sites, we estimate
3.6-7.6 fast Na+ channels/micron2 plasmalemma.
