DOWN-REGULATION OF MUSCARINIC M(2) RECEPTORS LINKED TO K-PIG ATRIAL MYOCYTES( CURRENT IN CULTURED GUINEA)

1. Desensitization of muscarinic K+ current (I-K(ACh)) was studied in cultured atrial myocytes from guinea-pig hearts using whole-cell volta ge clamp. 2. Three different types of desensitization could be identif ied. A fast component which upon rapid superfusion with ACh resulted i n a partial relaxation of I-K(ACh) within a few seconds to a plateau w hich was maintained in the presence of ACh. Recovery from this type of desensitization paralleled the decay of I-K(ACh) after washout of the agonist. A second type of desensitization was observed within minutes . This was reversed around 10 min after washout of ACh. Both types wer e heterologous with regard to the A(1) receptor and the novel phosphol ipid (PI) receptor, both of which activate I-K(ACh) via the same signa lling pathway. 3. A third type of desensitization (downregulation) occ urred upon exposure of the cultures for 24-48 h to the muscarinic agon ist carbachol (CCh). The level of downregulation depended on the conce ntration of CCh (0.1 mu M less than or equal to [CCh] less than or equ al to 10 mu M). NO recovery was observed within 5 h after washout of C Ch. Thereafter sensitivity to ACh slowly returned (half-time (t(1/2)), similar to 20 h). 4. Downregulation by CCh (0.1-5 mu M) was character ized by an increase in EC(50) for ACh with no reduction in maximum I-K (ACh). With 5 mu M CCh, EC(50) was increased from 0.1 to 3.7 mu M. At 10 mu M CCh EC(50) was increased to 15 mu M and maximal current that c ould be evoked by ACh was reduced to 15%. 5. Downregulation by CCh was homologous with regard to A(1) and PI receptors. Maximum I-K(ACh), as sayed by a saturating concentration of PI, was not reduced in downregu lated cells, suggesting a mechanism localized at the M(2) receptor. 6. The changes in the concentration-response curves can be accounted for by assuming an excess of M(2) receptors relative to the subsequent co mponent of the signalling pathway. 7. As the intact heart is under ton ic vagal control, downregulation is likely to contribute to controllin g the sensitivity of the heart to vagal activity in situ.