In vivo downregulation of M-2 receptors revealed by measurement of muscarinic K+ current in cultured guinea-pig atrial myocytes

1. Muscarinic K+ current (I-K(ACh)) elicited by acetylcholine (ACh) was measured in guinea-pig atrial myocytes, which were either freshly isolated or cultured for up to 8 days. 2. The half-time of activation of inward I-K(ACh) by a saturating concentration (10 mu M) of ACh decreased from similar to 400 ms (in freshly isolated cells) to 250 ms after 6 days in culture. This was paralleled by an increase in the fast desensitizing component of I-K(ACh). The density of steady-state currents was not changed. Downregulation of M-2 receptors by long-term treatment of isolated myocytes with carbachol in vitro had opposite effects. 3. The EC50 of ACh for the activation of steady-state I-K(ACh) was reduced from 5 x 10(-7) M (day 0) to 8 x 10(-8) M (day 6). The shift in EC50 occurred with a half-time of about 2 days, similar to the recovery from downregulation induced by treating atrial myocytes with carbachol in vitro. 4. The increase in sensitivity to ACh can be accounted for by an similar to 6-fold increase in the density of M-2 receptors. 5. It is concluded that sensitization in culture reflects recovery from downregulation of M-2 receptors due to the tonic vagal input to the heart in the intact animal.