SODIUM-CHANNEL HETEROGENEITY IN THE APICAL MEMBRANE OF PORCINE THYROID EPITHELIAL-CELLS

Porcine thyroid epithelial cells cultured as a monolayer with their ap ical membranes facing the medium are known to absorb Na+ and secrete C l-. Two types of Na+ channels were found in cell-attached patches of a pical membrane. A low conductance Na+ channel (conductance g=4 picosie mens (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+-fre e medium, though only the high conductance channel increased in preval ence on addition of prostaglandin E(2), a Stimulator of adenylate cycl ase which increases Na+ absorption in this cultured epithelium. Two mi nority types of channel were also found: a non-selective small conduct ance cation channel which had been reported previously, and an interme diate conductance channel found only in Na+-free medium. It was conclu ded that passage of Na+ across the apical membrane of thyroid cells is mediated by typical epithelial Na+ channels, but that the two types o f channel are differentially regulated.