66
mul iple ypes o K· channels. Below, we
conside he sepa a e ionic cu en s o ype
I cells, and compa e esul s ob ained om
di e en p epa a ions in di e en labo a o
ies.
NA+ CURRENTS
Using s anda d p ocedu es o blocking
K· cu en s (i.e., in acellula Cs+) and ea2+
cu en s (i.e., ex acellula Cd2+), Na+ cu 
en s (INa) can be ou inely eco ded in iso
la ion in adul abbi ype I cellsl.4 (Fig.
4. lA). INa is sensi i e o e odo oxin (TTX),
has an appa en
ac i a ion h eshold a ound
-40 m
V and a as ac i a ion ime cou se,
eaé:hing i s peak in less han 1 ms. The peak
ampli ude a O m V is app oxima ely 0.4 nA
on a e age. The hal s eady-s a e inac i a
ion occu s a a memb ane po en ial o
-50
A
�-ª
0.2 .... L
s ..
B
e
The Ca o id Body Chemo ecep o s
m
V. Inac i a ion ollows a monoexponen ial
ime cou se, wi h a ime cons an o 0.67 ms
a O m V. All hese cha ac e is ics a e ypical
o Na• cu en s ound in o he neu osec e
o y cells.
Na+ cu en s ha e no been ound in ab
bi emb yos5 and hei p esence is no ubiq
ui ously epo ed in a ype I cells. So ne
g oups ha e epo ed a lack o Na+·cu en s
in eshly dissocia ed ype I cells om adul 6
and young a s.7 Howe e , o he g oups ha e
epo ed apidly ac i a ing and inac i a ing
TTX-sensi i e Na+ cu en s p esen in cells
om young a s main ained in long- e m
cul u esª o in eshly dissocia ed cells om
adul 9 and neona al a s.10 The wide ange
o epo ed pe cen ages o cells ha ing Na+
cu en s in a cells ( om noné7 o 46% 10)
may e lec age- ela ed di e ences o mo e
u�.
·-
VliNl(mV)
�..0-200204090
l(Ml
Fig. 4.1. Whole-cell inwa d cu en s eco ded in adul abbi (A & B) and
neona al a (C) cells. (A) Family o Na• cu en s eco ded in he p esence o
0.1 mM Cd2+. The 1-V ela ionship o he peak cu en is also shown. (B) 1-V
ela ionship o Ca2+ cu en s ob ained in an adul abbi chemo ecep o
cell using 1 O mM Ca2• ( illed ci cles) o 1 O mM Ba2• (open ci cles) as cha ge
ca ie s. Na• cu en s a e blocked wi h 0.1 mM TTX. The cu en s ob ained
wi h Ca2• and Ba2• a + 1 O mV a e shown supe imposed. (C) Ca2+ channel
cu en s eco ded in a neona al a ype I cell using 1 O mM Ba2+ as cha ge
ca ie . No e he lack o apidly ac i a ing and inac i a ing Na• cu en ,
despi e he ac ha TTX was no p esen . Ca2• channel cu en s could be
ully blocked by 100 mM Cd2+ and enhanced by he DHP agonis Bay K
8644 (S mM). The 1-V ela ionship om he same cell is shown in he ab
sence ( illed ci cles) and p esence (open ci cles) o Bay K 8644.
Gonzalez-lex 66 5/29197, 3:58 PM