COMPARISON OF PENTOBARBITAL EFFECTS ON HUMAN SODIUM CHANNELS FROM BRAIN, HEART AND STRIATED MUSCLE

Abstract

S520 INTRODUCTION: Voltage-dependent sodium channels are essential for neuronal communication and muscle contraction. In spite of prevalent structural homologies, sodium channels from different tissues vary with respect to their functional properties [1]. To investigate the response to general anesthetics of different sodium channel subtypes, we examined the effects of pentobarbital (PTB), a close thiopental analog, on single sodium channels from human ventricular muscle and human striated muscle and compared them to existing data from human brain channels [2]. METHODS: With the approval of the local Committee on Human Rights in Research, sodium channels from preparations of human ventricular muscle or human skeletal muscle were incorporated into planar lipid bilayers in the presence of batrachotoxin (BTX), a sodium channel activator. Single channel currents were recorded before and after the addition of pentobarbital (PTB; 0.34 - 1.34 mM); our standard steady-state voltage-clamp conditions were used. Statistical significance was tested by t-test, p<0.05 was considered significant, unless indicated otherwise. RESULTS: Under control condition sodium channels displayed a tissue-specific behavior typical of BTX-modified sodium channels. The cardiac sodium channel was different from other sodium channels in open probability (see Table 1) and tetrodotoxin sensitivity (k1/2=416 nM, compared to brain: 40 nM and striated muscle: 60 nM); the striated muscle sodium channel had a lower single channel conductance than the other two human sodium channels (see Table 1). The effect of PTB on the open probability was smallest with striated muscle channels (block of 35%; brain: 51% and heart: 48%, data at 670 mM pentobarbital).Table 1The steady-state activation of all three channels was shifted to hyperpolarized potentials by PTB. This effect was significantly lower with striated muscle channel (shift of -6.2 mV) or cardiac channels (-8.0 mV) than with CNS sodium channels (-14.1 mV; data at 670 mM PTB). All of the shifts in activation are significantly different from their respective control. CONCLUSIONS: The observed variations in the electrophysiological properties of human sodium channels from different tissues suggest both significant structural homologies and diversities between the channel subtypes. A low tetrodotoxin sensitivity of cardiac channels and a reduced ion conductivity of striated muscle channels had been reported before for other species [3]. Each of the sodium channel subtypes differed from the other two in at least one of their properties under control conditions. Each of the channel subtypes showed at least two different responses to pentobarbital. However, each subtype differed in at least one of their anesthetic responses. Differences in control behavior did not necessarily correlate with differences in anesthetic sensitivity. This is the first demonstration that each of the three subtypes of sodium channels from different human tissue have their own characteristic response to a general anesthetic.