36 research outputs found
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 (Cai2+) 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 Cai2+ 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 (LANCEÂź). Our data show that the Cai2+ transient, like the hyperpolarization-activated âfunny currentâ (If) and the ACh-sensitive potassium current (IK,ACh), is an important determinant of ACh-mediated pacemaker slowing. When If and IK,ACh were both inhibited, by cesium (2Â mM) and tertiapin (100Â nM), respectively, 1Â ÎŒM ACh was still able to reduce pacemaker frequency by 72%. In these If and IK,ACh-inhibited SAN cells, good correlations were found between the ACh-mediated change in interbeat interval and the ACh-mediated change in Cai2+ transient decay (r2Â =Â 0.98) and slow diastolic Cai2+ rise (r2Â =Â 0.73). Inhibition of the Cai2+ transient by ryanodine (3Â ÎŒM) or BAPTA-AM (5Â ÎŒM) facilitated ACh-mediated pacemaker slowing. Furthermore, ACh depressed the Cai2+ transient and reduced the sarcoplasmic reticulum (SR) Ca2+ content, all in a concentration-dependent fashion. At 1Â ÎŒM ACh, the spontaneous activity and Cai2+ transient were abolished, but completely recovered when cAMP production was stimulated by forskolin (10Â ÎŒM) and IK,ACh was inhibited by tertiapin (100Â nM). Also, inhibition of the Cai2+ transient by ryanodine (3Â ÎŒM) or BAPTA-AM (25Â ÎŒM) exaggerated the ACh-mediated inhibition of cAMP content, indicating that Cai2+ affects cAMP production in SAN cells. In conclusion, muscarinic receptor stimulation inhibits the Cai2+ transient via a cAMP-dependent signaling pathway. Inhibition of the Cai2+ transient contributes to pacemaker slowing and inhibits Cai2+-stimulated cAMP production. Thus, we provide functional evidence for the contribution of the Cai2+ transient to ACh-induced inhibition of pacemaker activity and cAMP content in rabbit SAN cells
MT1 melatonin receptor mRNA expressing cells in the pars tuberalis of the european hamster: Effect of photoperiod
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