Different modes of electrogenic Na+ absorption in the coprodeum of the chicken embryo: role of extracellular Ca2+ |
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Authors: | M Heinz R Krattenmacher B Hoffmann W Clauss |
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Institution: | (1) Institut für Veterinärphysiologie, Freie Universität Berlin, Koserstr. 20, W-1000 Berlin 33, Federal Republic of Germany |
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Abstract: | Summary Transepithelial electrogenic Na+ transport (INa) was investigated in the coprodeum of 20-days-old chicken embryos in Ussing chambers. Short circuit current (Isc) and transepithelial resistance (Rt) were 14.7±4.8 A · cm-2 (n=12) and 0.53±0.09 k · cm-2 (n=12), respectively. INa was calculated from changes in Isc by substitution of mucosal Na+ by (N-methyl-d-glucamine) (NMDG). Isc inversed during Na+ removal, and INa was found to be 27.8±4.7 A · cm-2 (n=12). Amiloride (100 mol · l-1) inhibited only about 60% of INa. Analysis of Isc fluctuations revealed a Lorentzian component in the power density spectrum with a corner frequency of about 57 Hz. This component was not correlated to INa, and its origin is still unclear. Removal of mucosal Ca2+ increased INa about 2.5-fold due to an increase of the amiloride-insensitive component of INa in additionally investigated adult tissues. The results clearly show that this is due to a non-selective cation channel with an apparent order of selectivity Cs+>Na+=K+>Rb+>Li+. The Ca2+ concentration required to block 50% of the Isc was about 18 mol · l-1. The I
sc
Ca
could also be supressed by other divalent cations such as Mg2+ and Ba2+. Additionally, an INa-linked Lorentzian component occurred which dominated the control spectrum with a significantly higher corner frequency (about 88 Hz). The results indicate that Na+ absorption in the coprodeum of the chicken embryo is more complex than in adult hens. However, the Ca2+ sensitivity of INa is similar to comparable effects described for other epithelia. This possibly reflects the existence of two types of amiloride-insensitive apical cation channels as pathways for Na+ absorption, which may be involved to differing degrees in ontogenetic developments of nonselective channels to Na+-specific ion channels.Abbreviations
DPL
direct-linear-plot method
-
slope of the back-ground noise component
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EGTA
ethylene glycol-bi(2-amino-ethylether)-N,N,N,N-tetraacetic acid
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f
frequency
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f
c
corner frequency of the Lorentzian noise component
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G
t
transepithelial conductance
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HEPES
N-hydroxyethylpiperazine-N-ethanesulfonic acid
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I
sc
short-circuit current
-
I
Na
transepithelial sodium current
-
I
sc
Ca
Ca2+-sensitive short-circuit current
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K
m
Ca
Michaelis-Menten constant for Ca2+
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K
B
power density of the background noise component at f=1Hz
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m
mucosal
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NMDG
N-methyl-D-glucamine
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R
t
transepithelial resistance
-
s
serosal
-
SEM
standard error of mean
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S(f)
power density of the Lorentzian noise component
-
S
o
plateau value of the Lorentzian noise component |
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Keywords: | Noise analysis Chicken embryo Coprodeum Sodium absorption Calcium sensitivity |
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