Permeation Properties of a C a 2+- blockable Monovalent Cation Channel in the Ectoderm of the Chick Embryo: Pore Size and Multioccupancy Probed with Organic Cations

A Ca2+-blockable monova len t cat ion channel is p resen t in the apical m e m b r a n e o f the ec tode rm of the gastrulat ing chick embryo. We used the patch c lamp technique to study several s ingle-channel pe rmea t ion proper t ies o f this channel . In symmetrical condi t ions without Ca z+, the Na + cur ren t car r ied by the channe l rectifies inwardly. The channe l has an a p p a r e n t dissociation cons tant for extracel lular Na § o f 115 mM at 0 mV and a low density o f negative surface charge ( 0 . 0 3 e / n m 2) at its ext racel lu lar entrance. The min imal pore d iamete r is "-'5.8 ~ as calcula ted f rom the relative permeabi l i t ies o f 10 small organic cations. Extracel lular appl ica t ion of six large organic cat ions decreased the inward Na + cur ren t in a vo l t age-dependen t manner , which strongly suggests an in t rachanne l block. The presence of at least two ion b ind ing sites inside the pore is infer red f rom the Na + d e p e n d e n c e o f the block by the organic cations. This hypothesis is s t r eng thened by the fact that the extracel lular Ca 2+ block is also modi f ied by the Na § concent ra t ion . In part icular , the rise of the unb lock ing rate with increased Na + concent ra t ions clearly suggests the presence o f an in terac t ion between Ca 2+ and Na § inside the channel . A low probabi l i ty o f double occupancy at physiologica] ionic condi t ions is impl ied f rom the absence of an anomalous mole fract ion effect with mixtures o f extracel lular Li § and K § Finally, the absence o f inward cu r ren t at very s t rong hyperpolar iza t ions and in the presence of 10 mM extracellular Ca z+ demonst ra tes the absence of significant Ca 2+ cur ren t th rough this channel . I t is a rgued that this embryonic epi thel ia l Ca~+-blockable monova len t cat ion channel is re la ted to bo th L-type Ca 2+ channel and cyclic nucleot ide-gated channels . Dr. Li's present address is U-318 INSERM-UJFG,CHU Michallon, BP 217, F 38043 Grenoble, France, CEDEX. Address correspondence to Blaise Prod'hom, rue du Bugnon 7, Institut de Physiologie, Facult~ de M~decine, Universit~ de Lausanne, 1005 Lausanne, Switzerland. J. GEN. PHYSIOL. 9 The Rockefeller University Press. 0022-1295/95/08/149/26 $2.00 Volume 106 August 1995 149-174 149 on Jne 9, 2017 D ow nladed fom Published August 1, 1995