Pharmacological modification of sodium channels from the human heart atrium in planar lipid bilayers: electrophysiological characterization of responses to batrachotoxin and pentobarbital.

BACKGROUND AND OBJECTIVE To investigate the effects of barbiturates on batrachotoxin-modified sodium channels from different regions of the human heart. Single sodium channels from human atria were studied and compared with existing data from the human ventricle and from the central nervous system. METHODS Sodium channels from preparations of human atrial muscle were incorporated into planar lipid bilayers in the presence of batrachotoxin, a sodium channel activator. The steady-state behaviour of single sodium channels was recorded in symmetrical 500 mmol NaCl before and after the addition of pentobarbital 0.34-1.34 mmol. RESULTS The batrachotoxin-treated human atrial sodium channel had an average single-channel conductance of 23.8 +/- 1.6 pS in symmetrical 500 mmol NaCl and a channel fractional open time of 0.83 +/- 0.06. The activation mid-point potential was -98.0 +/- 2.3 mV. Extracellular tetrodotoxin (a specific sodium channel blocking agent) blocked these channels with a k(1/2) = 0.53 micromol at 0 mV. Pentobarbital reduced the time average conductance of single atrial sodium channels in a concentration-dependent manner (ID50 = 0.71 mmol). In the same way, the steady-state activation was shifted to more hyperpolarized potentials (-10.6 mV at 0.67 mmol pentobarbital). CONCLUSIONS The properties of batrachotoxin-modified sodium channels from human atrial tissue did not differ greatly from those described for ventricular sodium channels in the literature. Our data yielded no explanation for the observed functional diversity. However, cardiac sodium channels differ from those found in the central nervous system.