Abstract
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 ([3H]CPX). 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.