Acetylcholine (ACh) binds to the m2 muscarinic receptor in the heart, causing activation of G_i protein and, subsequently, the muscarinic K⁺ channel. During a continuous application of ACh, muscarinic K⁺ current declines as a result of desensitization (Fig. 1, left). Recent studies have suggested that the desensitization is the result of ACh also binding to the m3 muscarinic receptor (Jan & Jan, 2000; Kobrinsky et al. 2000). This is expected to activate G_q and, subsequently, phospholipase C (PLC), causing hydrolysis of phosphatidylinositol bisphosphate (PIP₂) and, consequently, a decrease in the activity of the muscarinic K⁺ channel. This hypothesis is partly based on the observation that 4-DAMP (an m3 receptor antagonist) greatly reduces the desensitization of the muscarinic K⁺ current (Jan & Jan, 2000; Kobrinsky et al. 2000). To examine the involvement of the m3 receptor in the desensitization process, we recorded whole-cell muscarinic K⁺ current in neonatal rat atrial cells with and without m3 and m2 receptor antagonists (rats were killed by stunning and cervical dislocation). When 4-DAMP was applied at the same time as ACh, peak current was reduced by 18 ± 11 % (mean ± S.E.M., n = 8) and the subsequent decline of current (‘desensitization’) was apparently increased rather than decreased (Fig. 1A, middle). When 4-DAMP was applied before as well as during the application of ACh, peak current was reduced by 54 ± 10 % (n = 3) and the subsequent decline of current was apparently decreased (Fig. 1A, right). The effects of 4-DAMP may not be the result of block of the m3 receptor and instead may be the result of non-specific block of the m2 receptor (Caulfield & Birdsall, 1998). Figure 1B shows that atropine (an m2 receptor antagonist) can mimic the effects of 4-DAMP on the muscarinic K⁺ current. When 0.5 nM atropine was applied at the same time as ACh, peak current was reduced slightly and the subsequent decline of current was apparently increased (Fig. 1B, middle), whereas when it was applied before as well as during the application of ACh, peak current was greatly reduced and the subsequent decline of current was apparently decreased (Fig. 1B, right). Similar results to that in Fig. 1B were obtained from four cells. We conclude that it is unlikely that the m3 receptor is involved in desensitization of the cardiac muscarinic K⁺ current.This work was supported by the British Heart Foundation.

Figure 1. Typical traces of whole-cell current in atrial cells isolated from 3- to 5-day-old neonatal rats. 100 µM ACh, 0.5 µM 4-DAMP (4-DAMP mustard hydro-chloride) and 0.5 nM atropine were applied as indicated by the bars. Holding potential, -60 mV.

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