HYPERTONIA ÉS NEPHROLOGIA - eLitMed.hu

2002; 6 (1):3–15. ONTOGENY OF POTASSIUM TRANSPORT IN THE DISTAL NEPHRON 7
mean number
of open channels
(NP )
o
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
0 1 2 3 4 5 6
9; 10) and urinary K + excretion (9). This response reflects, at
least in part, an increase in delivery to and reabsorption of Na +
by principal cells (1; 23), a process that enhances the
lumen-negative potential and rate of basolateral K + uptake,
thereby increasing the driving force for passive K + efflux
across the apical membrane.
The rate-limiting step for transcellular Na + absorption
appears to be localized to the apical epithelial Na + channel
(ENaC). This amiloride-sensitive channel, characterized by a
single-channel conductance in the range of 8-13 pS, has been
most thoroughly studied in the adult rat and cultured rabbit
principal cell (24; 25). Patch clamp analysis of the maturing
rabbit principal cell identified few conducting apical
amiloride-sensitive Na + channels in principal cells immediately
after birth (Fig. 5) (26). However, by 2 weeks of age,
the number of open Na + channels per patch reached a value
similar to that observed in the 5-wk-old animals (Fig. 5)(26).
This increase in number of conducting channels appeared to
be due to both an increase in the number of channels present
per patch as well as an increase in open probability (Po) (26).
ENaC, cloned from rat distal colon, exists as a multimeric
channel comprised of , and subunits (27; 28).
Coexpression of the three subunits in oocytes reconstitutes a
channel with ion selective permeability, gating properties and
pharmacologic profile similar to the native channel (28).
Expression of message encoding the three ENaC subunits
increases in the near-term fetal rat kidney (29-31), due at least
in part, to a 5-fold increase in abundance of mRNA in the
ureteric bud and CCD (29). Mature levels of mRNA
expression are achieved within the first few days of birth (30;
*
*
SK
*
0 1 2 3 4 5 6
age (wks) age (wks)
Figure 5. Postnatal changes in mean number of open SK (left) and amiloride-sensitive Na + (right) channels in cellattached
patches of the apical membrane of rabbit principal cells. Age of animals is given in weeks. CCDs from rabbits
>5 weeks of age were not examined because the rate of successful seal formation was too low. The number of conducting
SK channels increased significantly after birth (p