Muscarinic regulation of cardiac ion channels

Abstract

The parasympathetic component of the autonomic nervous system plays an important role in the physiological regulation of cardiac function by exerting significant influence over the initiation as well as propagation of electrical impulses, in addition to being able to regulate contractile force. These effects are mediated in whole or in part through changes in ion channel activity that occur in response to activation of M(2) muscarinic cholinergic receptors following release of the neurotransmitter acetylcholine. The coupling of M(2) receptor activation to most changes in cardiac ion channel function can be explained by one of two general paradigms. The first involves direct G protein-dependent regulation of ion channel activity. The second involves indirect regulation of ion channel activity through modulation of cAMP-dependent responses. This review focuses on recent advances in our understanding of the mechanisms by which M(2) muscarinic receptor activation both inhibits and facilitates cAMP-dependent ion channel responses in the heart.

Figure 1

Muscarinic inhibition and stimulation of the L-type Ca²⁺ current. Top panel: Time course of changes in the magnitude of the L-type Ca²⁺ current recorded from an isolated guinea-pig ventricular myocyte following exposure to the muscarinic receptor agonist ACh in the presence of a submaximally stimulating concentration of the β-adrenergic receptor agonist isoprenaline (Iso). Currents were elicited by applying 100 ms depolarizing voltage clamp steps to a test potential of 0 mV following a 40 ms conditioning pulse to −30 mV from a holding potential of −80 mV. The Ca²⁺ current was measured as the absolute magnitude of the peak inward current elicited during the test pulse. Muscarinic inhibition of the β-adrenergic response (I) is observed in the presence of ACh. Washout of ACh in the continued presence of β-adrenergic stimulation reveals the rebound stimulatory response (S) to muscarinic receptor activation. Bottom panel: Current traces recorded at time points indicated in top panel. (1) Steady-state response to submaximally stimulating concentration of Iso, (2) steady-state inhibitory effect of ACh in the presence of Iso, and (3) peak rebound stimulatory effect following washout of ACh in the continued presence of Iso. Changes in the steady-state pre- and post-test pulse currents are due to parallel activation of the cAMP regulated, time-independent Cl⁻ current. The Cl⁻ current did not affect measurement of the Ca²⁺ current at the test potential since the Cl⁻ equilibrium potential was set at 0 mV. Further experimental details are as described in Belevych et al. (2001).

Figure 2

Proposed signaling pathways responsible for M₂ muscarinic receptor inhibition of cAMP-dependent ion channel responses. Muscarinic inhibitory responses may be mediated by directly inhibiting AC via the α subunit (α_i/o) of the PTX-sensitive G proteins G_i or G_o or by stimulating PDE2 via production of NO and cGMP. In ventricular myocytes, muscarinic responses are only observed in the presence of agonists that stimulate cAMP production. β₁-adrenergic receptors stimulate cAMP synthesis by directly activating AC via the α subunit (α_s) of the stimulatory G protein G_s. See text for details; sGC, soluble guanylyl cyclase.

Figure 3

Proposed pathways responsible for M₂ muscarinic receptor stimulation of cAMP-dependent ion channel responses. Muscarinic stimulatory responses may be mediated by direct activation of AC via the βγ subunits of a PTX-sensitive G protein G_i or G_o or by inhibition of PDE3 via production of nitric oxide (NO) and cGMP. Direct stimulation of AC by βγ subunits is only observed under conditions, such as the presence of a β₁-adrenergic receptor agonist, that can activate AC via direct interaction of the α subunit (α_s) of the stimulatory G protein G_s. See text for details; sGC, soluble guanylyl cyclase.

Figure 4

Proposed mechanism by which M₂ muscarinic receptor activation can produce both inhibitory and stimulatory responses through the differential regulation of specific AC isoforms. Muscarinic inhibitory effects can be produced by inhibition of AC types 5 and/or 6 (AC5/6) via direct interaction with the α subunit (α_i/o) of a PTX-sensitive G protein G_i or Go. Muscarinic stimulatory effects can be produced by activation of AC types 4 and/or 7 (AC4/7) via direct interaction with the βγ subunits of G_i or Go. See text for details.