Different signal molecules involved in the muscarinic modulation of pacemaker current I f on the heart of mouse embryo in different developmental stages

We isolated mouse embryonic cardiomyocytes derived from timed-pregnant females at different periods and used patch- clamp technique to investigate the muscarinic cholinergic modulation of pacemaker current I f in different developmental stages. In early development stage (EDS), muscarinic agonist carbachol (CCh) significantly decreased the magnitude of the pacemaker current I f but had no effect in late development stage (LDS). Forskolin (a direct adenylate cyclase activator) and IBMX (a non-selective phosphodi- esterase inhibitor) increased I f in both EDS and LDS cells. Interestingly, although both forskolin and IBMX increased basal I f , their effects on CCh-inhibited I f were different. Forskolin did not reverse the inhibitory action of CCh until intermediate development stage (IDS). In contrast, IBMX reversed the inhibitory action of CCh on I f in EDS but not in IDS. It is suggested that a decrease in intracellular cAMP is a possible mechanism for CCh to modulate I f . During the EDS and IDS CCh controls the cytoplasmic cAMP level by different pathways: In EDS, CCh modulates I f possibly by activating PDE which accelerates the breakdown of cAMP, but in IDS possibly by inhibiting adenylate cyclase (AC) which then reduces the synthesis of cAMP.