Ion channels have generally been found to have two predominant conductance levels thought to be associated with 'open' and 'closed' states, but intermediate (subconductance) states have also been reported. We have now found that a large conductance, anion-selective channel in pulmonary alveolar epithelial cells can adopt any of six open levels of conductance that are integer multiples of 60-70 pS. The channel is usually either fully open or fully closed. The frequencies of the different conductance levels are inconsistent with the notion that there are six independent channels. We suggest that the channel consists of six conducting pathways in parallel, 'co-channels', with a shared gating mechanism that can synchronously render all of them non-conducting. Other channels with lower maximum conductance may operate in a similar way but multiple conductance levels would not easily be detected because of a less favourable signal-to-noise ratio.

SUMMARY1. The post-synaptic effects of the aliphatic alcohols, ethanol to hexanol, were investigated at the neuromuscular junctions of toads, with particular emphasis on the effects of ethanol.2. The alcohols increased the amplitude and duration of miniature endplate potentials. It is shown that this effect was due to the prolongation of the decay phase of miniature end-plate currents (m.e.p.c.s). There was no effect of alcohols on the growth phase of m.e.p.c.s.3. The prolonged decay of m.e.p.c.s in ethanol remained exponential and was normally sensitive to membrane potential. Prolonged m.e.p.c.s were associated with an equivalent prolongation of the mean duration of elementary events, as determined from power spectra of acetylcholine noise in 0-5 M ethanol.4. The relationship between the time constant of decay of m.e.p.c.s (T) and the concentration of an alcohol of carbon chain length N (UN) was exponential, conforming to the equation T = T8 exp (BN .CN), in which T8 is the decay time constant in standard solution and BN is a constant, different for each alcohol.5. There was also an exponential relationship between BN and N, which closely followed the relationship between membrane-buffer partition coefficient and carbon chain length for the different alcohols, indicating that the alcohols are active in the lipid phase of the post-synaptic membrane.6. It is suggested that the alcohols act by causing a change in the dielectric constant of the post-synaptic membrane which forms the 410 PETER W. GAGE AND OTHERS environment of the rate-limiting reaction responsible for the decay of the end-plate conductance. On the assumption that this reaction involves dipoles, it is shown that the small changes in dielectric constant, calculated from the partition coefficients of the alcohols and by assuming an initial lipid dielectric constant of 3, would give an exponential relationship between the time constant of decay of m.e.p.c.s and alcohol concentration.7. The results support the hypothesis that the decay (but not the onset) of acetylcholine-induced conductance changes is rate-limited by a firstorder reaction which involves dipoles and occurs in the lipid environment of the post-synaptic membrane.

The properties of ion channels in patches from the luminal membrane of cultured monolayers of type II alveolar epithelial cells from adult rat lung have been studied. In excised patches, prepared as described to yield the "outside-out" configuration (with cesium ions in the bathing solutions to suppress currents arising from potassium channels), we observed anion-selective channels with a conductance of 350-400 pS, and burst lengths lasting seconds. When patches were bathed in solutions with equal chloride concentrations, channels opened and closed spontaneously at membrane voltages close to zero, but tended to close when the potential was shifted by +/- 10 mV, particularly in the negative direction. Other anions could pass through these channels with a permeability sequence of I(1.5) greater than Br(1.02) = greater than Cl(1.00) greater than NO3(0.9) greater than gluconate(0.6) but there was a very low permeability for sodium (PNa/PCl = 0.015 +/- 0.006). Calcium ions reduced channel conductance. The channels probably have a role to play in salt absorption by this epithelium, but insufficient information is available about how sodium crosses it for the role of the anion channels in transport to be defined with confidence.

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A B S T R A C T A partial testicular defect in testosterone secretion has been documented in a pubertal male with a congenital adrenal hyperplasia due to hereditary deficiency of the A5-isomerase-3fi-hydroxysteroid dehydrogenase enzyme complex (AY-3j-HSD). Diagnosis of the enzymatic defect is based on the clinical picture of ambiguous genitalia and salt-losing crisis in infancy, together with high urinary A5-pregnenetriol and plasma dehydroepiandrosterone when the patient was taken off replacement corticoid treatment. No hormonal response to ACTH or salt deprivation was demonstrable. In addition, in vivo studies revealed a partial enzymatic defect in the testis. Although plasma testosterone was low-normal (250 ng/100 ml), plasma A5-androstenediol was markedly elevated and rose to a greater extent than testosterone after human chorionic gonadotropin administration. In vitro testicular incubation studies suggested a testicular A5-3%-HSD enzyme defect with less A4 products formed from A5 precursors than in a control testis. Histochemical studies of the testis were also consistent with this defect. Testicular biopsy revealed spermatoAn abstract of a portion of this work has appeared in Pediatr. Res.