CLUSTERED DISTRIBUTION OF CALCIUM SENSITIVITIES - AN INDICATION OF HETERO-TETRAMERIC GATING COMPONENTS IN CA2-ACTIVATED K+ CHANNELS RECONSTITUTED FROM AVIAN NASAL GLAND-CELLS()

Calcium-activated potassium channels (maxi K+ channels) isolated from avian nasal salt gland cells were reconstituted into lipid bilayers an d characterized. The 266 pS channel is blocked discretely by charybdot oxin from the external solution at nanomolar concentrations and by Ba2 + from the cytosolic side at micromolar concentrations. Fast tetraethy lammonium (TEA) block is seen as apparent reductions in amplitude of t he unitary currents. From the extent of the reductions, TEA binding af finity was calculated to be 0.16 mM from the external solution and 37 mM from internal solution. The overall channel properties conform to t hose of maxi KC channels in other epithelial tissues. The calcium sens itivity of the channel was found to be variable from channel to channe l, extending over a wide range of concentrations from 1 to 1,000 mu M. Examination of the pooled calcium titration curves, revealed that the se curves are grouped into five clusters, and the probability distribu tion of the clusters matches a binomial distribution. The Hill coeffic ient derived from the titration curves varies from 1 to 5 and is linea rly correlated to calcium binding with a slope of 1 per 10-fold change in K-d. Clustered titration curves with such a characteristic suggest that the gating components and the calcium binding sites of the maxi K+ channels in the avian nasal gland are heterotetrameric and may resu lt from random mixing of two distinct subunits possessing high and low calcium sensitivities, respectively.