Zusammenfassung
1. Desensitization of muscarinic K+ current (IK(ACh)) was studied
in cultured atrial myocytes from guinea-pig hearts using whole-cell
voltage clamp. 2. Three different types of desensitization could
be identified. A fast component which upon rapid superfusion with
ACh resulted in a partial relaxation of IK(ACh) within a few seconds
to a plateau which was maintained in the presence of ACh. Recovery
from this type of desensitization paralleled the decay of IK(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 were heterologous with regard to the A1 receptor
and the novel phospholipid (Pl) receptor, both of which activate
IK(ACh) via the same signalling pathway. 3. A third type of desensitization
(downregulation) occurred upon exposure of the cultures for 24-48
h to the muscarinic agonist carbachol (CCh). The level of downregulation
depended on the concentration of CCh (0.1 microM < or = CCh < or
= 10 microM). No recovery was observed within 5 h after washout of
CCh. Thereafter sensitivity to ACh slowly returned (half-time (t1/2),
approximately 20 h). 4. Downregulation by CCh (0.1-5 microM) was
characterized by an increase in EC50 for ACh with no reduction in
maximum IK(ACh). With 5 microM CCh, EC50 was increased from 0.1 to
3.7 microM. At 10 microM CCh EC50 was increased to 15 microM and
maximal current that could be evoked by ACh was reduced to 15%. 5.
Downregulation by CCh was homologous with regard to A1 and Pl receptors.
Maximum IK(ACh), assayed by a saturating concentration of Pl, was
not reduced in downregulated cells, suggesting a mechanism localized
at the M2 receptor. 6. The changes in the concentration-response
curves can be accounted for by assuming an excess of M2 receptors
relative to the subsequent component of the signalling pathway. 7.
As the intact heart is under tonic vagal control, downregulation
is likely to contribute to controlling the sensitivity of the heart
to vagal activity in situ.
Nutzer