DOWN-REGULATION OF A(1) ADENOSINE RECEPTORS COUPLED TO MUSCARINIC K+ CURRENT IN CULTURED GUINEA-PIG ATRIAL MYOCYTES

1. Muscarinic K+ current (I-K(ACh)) was measured in cultured atrial myocytes from hearts of adult guinea-pigs using whole-cell voltage clamp. I-K(ACh) was activated by superfusion with solutions containing either acetylcholine (ACh) or adenosine (Ado), in saturating concentrations of 2 mu M (ACh) and 1 mM (Ado), respectively. 2. In freshly isolated cells the amplitude of the current activated by Ado (I-K(Ado)) was 58% (mean) of the current that was induced by ACh. In serum-free culture this relation, but also the absolute density of I-K(ACh), remained fairly constant for up to 8 days. 3. If the culture medium was supplemented with fetal calf serum (FCS, 5%) the relation I-K(Ado)/I-K(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 PCS-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 I-K(ACh) upon acute superfusion. Loss of responsiveness to Ado was paralleled by a reduction of binding sites to the A(1) adenosine receptor-specific radioligand 8-cyclopentyl-1,3-dipropylxanthine ([H-3]CPX). 6. It is concluded that FCS contains a factor that causes down-regulation of A(1) Ado receptors. The signalling pathway that leads to an increased opening activity of I-K(ACh) channels and other receptors, such as the M(2) muscarinic receptor, linked to this signalling pathway are not affected by this factor.