Sodium channel heterogeneity in the apical membrane of porcine thyroid epithelial cells

Porcine thyroid epithelial cells cultured as a monolayer with their apical membranes facing the medium are known to absorb Na and secrete Cl. Two types of Na channels were found in cell-attached patches of apical membrane. A low conductance Na channel (conductance g = 4 picosiemens (pS)) remained open for seconds and showed a high selectivity for Na compared with K. In contrast, a high conductance Na channel (g = 10 pS) flickered rapidly and had reduced selectivity. Both types of Na channel became more prevalent when the cells were exposed to Na-free medium, though only the high conductance channel increased in prevalence on addition of prostaglandin E, a stimulator of adenylate cyclase which increases Na absorption in this cultured epithelium. Two minority types of channel were slso found: a non-selective small conductance cation channel which had been reported previously, and an intermediate conductance channel found only in Na-free medium. It was concluded that passage of Na across the apical membrane of thyroid cells is mediated by typical epithelial Na channels, but that the two types of channel are differentially regulated