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.