Zusammenfassung
1. Muscarinic K+ current (IK(ACh)) was measured in cultured atrial
myocytes from hearts of adult guinea-pigs using whole-cell voltage
clamp. IK(ACh) was activated by superfusion with solutions containing
either acetylcholine (ACh) or adenosine (Ado), in saturating concentrations
of 2 microM (ACh) and 1 mM (Ado), respectively. 2. In freshly isolated
cells the amplitude of the current activated by Ado (IK(Ado)) was
58% (mean) of the current that was induced by ACh. In serum-free
culture this relation, but also the absolute density of IK(ACh),
remained fairly constant for up to 8 days. 3. If the culture medium
was supplemented with fetal calf serum (FCS, 5%) the relation IK(Ado)/IK(ACh)
gradually decayed, reaching a value of less than 0.1 on days 7-8,
whereas the response to ACh remained stable over this period of time.
4. After treatment of cells with FCS-containing medium, no recovery
was observed upon FCS withdrawal for up to 4 days. 5. The effect
of FCS on responsiveness to Ado was half-maximal at about 1% (v/v).
The active principle can be dialysed (mol. mass exclusion: 10 kDa).
It is not identical with an albumin-associated factor that has been
shown to be a potent activator of atrial IK(ACh) upon acute superfusion.
Loss of responsiveness to Ado was paralleled by a reduction of binding
sites to the A1 adenosine receptor-specific radioligand 8-cyclopentyl-1,3-dipropylxanthine
(3HCPX). 6. It is concluded that FCS contains a factor that causes
down-regulation of A1 Ado receptors. The signalling pathway that
leads to an increased opening activity of IK(ACh) channels and other
receptors, such as the M2 muscarinic receptor, linked to this signalling
pathway are not affected by this factor.
Nutzer