Saxitoxin binding and 'fast' sodium channel inhibition in sheep heart plasma membrane

We compared specific [³H]saxitoxin (STX) binding to isolated sheep ventricular sarcolemmal vesicles with inhibition of maximal action potential upstroke velocity (V̇(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°C in Na-free solution exhibited both high-affinity sites (K(D) = 0.22 ± 0.05 nM, n = 85 ± 13 fmol/mg proteins) and low-affinity sites (K(D) = 11 ± 4 nM, n = 360 ± 42). The STX-inhibition constant for V̇(max) in Tyrode solution at 37° C was 280 nM. TTX was ~ 10% as effective as STX in displacing bound (³H]STX and inhibiting V̇(max). Allowing for different experimental conditions during [³H]STX binding and V̇(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/μm² plasmalemma.