Effects of muscarinic receptor stimulation on Ca²⁺ transient, cAMP production and pacemaker frequency of rabbit sinoatrial node cells

We investigated the contribution of the intracellular calcium (Ca _i ²⁺ ) transient to acetylcholine (ACh)-mediated reduction of pacemaker frequency and cAMP content in rabbit sinoatrial nodal (SAN) cells. Action potentials (whole cell perforated patch clamp) and Ca _i ²⁺ transients (Indo-1 fluorescence) were recorded from single isolated rabbit SAN cells, whereas intracellular cAMP content was measured in SAN cell suspensions using a cAMP assay (LANCE^®). Our data show that the Ca _i ²⁺ transient, like the hyperpolarization-activated “funny current” (I _f) and the ACh-sensitive potassium current (I _K,ACh), is an important determinant of ACh-mediated pacemaker slowing. When I _f and I _K,ACh were both inhibited, by cesium (2 mM) and tertiapin (100 nM), respectively, 1 μM ACh was still able to reduce pacemaker frequency by 72%. In these I _f and I _K,ACh-inhibited SAN cells, good correlations were found between the ACh-mediated change in interbeat interval and the ACh-mediated change in Ca _i ²⁺ transient decay (r ² = 0.98) and slow diastolic Ca _i ²⁺ rise (r ² = 0.73). Inhibition of the Ca _i ²⁺ transient by ryanodine (3 μM) or BAPTA-AM (5 μM) facilitated ACh-mediated pacemaker slowing. Furthermore, ACh depressed the Ca _i ²⁺ transient and reduced the sarcoplasmic reticulum (SR) Ca²⁺ content, all in a concentration-dependent fashion. At 1 μM ACh, the spontaneous activity and Ca _i ²⁺ transient were abolished, but completely recovered when cAMP production was stimulated by forskolin (10 μM) and I _K,ACh was inhibited by tertiapin (100 nM). Also, inhibition of the Ca _i ²⁺ transient by ryanodine (3 μM) or BAPTA-AM (25 μM) exaggerated the ACh-mediated inhibition of cAMP content, indicating that Ca _i ²⁺ affects cAMP production in SAN cells. In conclusion, muscarinic receptor stimulation inhibits the Ca _i ²⁺ transient via a cAMP-dependent signaling pathway. Inhibition of the Ca _i ²⁺ transient contributes to pacemaker slowing and inhibits Ca _i ²⁺ -stimulated cAMP production. Thus, we provide functional evidence for the contribution of the Ca _i ²⁺ transient to ACh-induced inhibition of pacemaker activity and cAMP content in rabbit SAN cells.